HomeMy WebLinkAbout10C - Surface Water Mgmnt Plan
16200 Eagle Creek Avenue S.E.
Prior Lake, MN 55372-1714
CITY COUNCIL AGENDA REPORT
MEETING DATE:
AGENDA #:
PREPARED BY:
JUNE 26, 2006
10 C
ROSS BINTNER, WATER RESOURCES ENGINEER
AGENDA ITEM:
CONSIDER APPROVAL OF THE CITY OF PRIOR LAKE LOCAL SURFACE
WATER MANAGEMENT PLAN AND PUBLIC WORKS DESIGN MANUAL
HYDROLOGY APPENDIX.
DISCUSSION:
Introduction
The purpose of this agenda item is for the City Council to approve the Local
Surface Water Management Plan and Public Works Design Manual Hydrology
Appendix.
The City of Prior Lake has been a leader in water resource management
throughout its history. Through cooperation and partnership with the Prior Lake
/ Spring Lake Watershed District, and careful planning and monitoring of water
quality, the City continues to take an active role in the management and
protection of its water resources.
Within the municipal boundary of the City of Prior Lake are two water
management entities, the Prior Lake Spring Lake Watershed District (WD) and
the Scott Water Management Organization (WMO). Each of these
organizations has authority for management of water resources; however the
rules and approaches of the WMO and WD vary.
In an effort to streamline water resource management within the municipality
and maintain local control of water resource management, the City of Prior
Lake has undertaken an effort to update its rules and regulations to meet the
requirements of both WMO and WD. In addition to streamlining, State Statute
requires the City of Prior Lake to maintain a Local Water Management Plan
that is consistent and equivalent with WMO plans.
This process of revising City planning documents, rules and regulations is
consistent with the active role the City has pursued in managing its water
resources and will ensure compliance with State Statute. Through partnership
with the WMO and WD, application of its modern rules, and its prudent
planning efforts, the City of Prior Lake will remain a leader in the management
of its water resources.
History
Since the WD began permitting, development that fell within both the City and
WD has had to apply for permits under both entities. This "dual track" system
can be a difficult thing to navigate at times, with a developer being pushed
back and forth between two review agencies and sets of standards. When the
WMO was created and enacted their own rules, the City was given the choice
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of allowing another "dual track" to begin in WMO areas, or to update its own
plan and rules to become equivalent with the WMO. With this impetus, the
City began to update its own rules to become equivalent with both the WMO
and WD in the effort consolidate review authority and streamline the process,
eliminating the "dual track."
Two documents are used by the City to plan and enforce water resource
management issues within the City; revisions to these two documents are
considered. The Local Surface Water Management Plan (LSWMP or Plan)
sets forth a framework for the management of water resources; serving as a
guide, establishing policy, highlighting and detailing the overall water system,
and providing a plan for implementation, the LSWMP serves as a
comprehensive planning document. The Public Works Design Manual
Hydrology Appendix (PWDM or Rules) gives specific engineering standards
that govern the development and redevelopment of land consistent with the
policy spelled out by the WMO, WD and City.
In a practical sense, the PWDM will have a direct effect on development
issues, while the LSWMP sets the policy and planning framework behind the
rules.
Public Comment Process
Both the Plan and Rules documents were submitted for review to the WMO
and WD on two separate occasions. Two iterations of review were provided to
the WMO and WD and many constructive comments came from both
organizations. Changes were made as a result of these reviews to gain
equivalency. At this time the documents are in form where only minor
differences remain.
During the time that the WMO and WD review process was taking place the
documents were opened for public review and comment from May 1 to May
31, 2006. The documents were available for download on the City website and
available for review at City offices. The public was invited to comment in
writing or in person at a public hearing held at the Planning Commission on
May 22. During the public comment process, staff received calls inquiring
about the rules; however, no comments were received. The Builders
Association of the Twin Cities was sent a copy of the Plan and Rules and was
invited to provide public comment.
The Planning Commission recommended approval to the City Council at its
May 22 meeting.
Details of Plan and Rules
During the public hearing, the Planning Commission was presented with a
detailed summary chapter by chapter in both Plan and Rule documents. The
following is a summary of only the major differences of the Plan and Rule from
current City standards:
Volume Control: These Plan and Rules represent the first time the City
proposes to regulate volume flowing offsite from development. Previously the
requirement of volume control was a requirement of only the WD. The rule
was structured such that a development must include designs to abstract a
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volume of water equal to %" over all new impervious surfaces. This standard is
equal to that of the WD; however, the implementation method the City
proposes includes an incentive system to promote, but not require, the use of
sensitive development techniques.
Landlocked Basins and Drainage Alterations: An area that previously did not
exist in the scope of City standards is that of drainage alteration and
watershed connectivity. Because of grade changes associated with
development, the potential exists to change flow patterns watershed wide with
potential impacts to downstream drainage systems. These rules propose that
development be required to plan for and assess the downstream impact due to
all sizes of drainage alterations. A tiered system is proposed that will treat
larger drainage alterations with increasing rigor in design, with the top tier of
greater than 25 acres requiring the approval of the Watershed District.
Stormwater Management Overlay District: The need has arisen to have
separate standards for areas of the City that have unique watershed features.
Under the proposed set of rules an overlay district is created for all areas that
drain to the Prior Lake outlet channel. The rate control standard for this area
is increased, meaning a larger amount of water must be held back under large
storm events. The overlay district is necessary because of the unique flow
characteristics of the outlet channel, and the need for a future peak design
flow.
Wetland Standards: The proposed Plan identifies prime wetland areas and
proposes a higher standard of preservation. The DNR statewide guidance for
the utilization and protection of wetlands was followed, meaning higher quality
wetlands are allowed less stormwater impacts, while low quality wetlands are
allowed to be utilized for storm storage. This system of utilized wetlands
provides both stormwater and water quality benefit to the City.
2030 Vision Elements
Five goals and objectives of the Natural Resources Vision Element of the 2030
Vision and Strategic Plan are furthered through the update and implementation
of the new Plan and Rules; they are:
Five Year Goals:
1. Adopt and implement plans to monitor and improve surface water
quality (Le., lakes, ponds, wetlands, streams, storm water runoff and
non-point runoff).
2. Protect unique natural areas in the City and annexation areas by
promoting environmentally sensitive development.
Two Year Objectives:
1. Assure subdivision storm water plans conform to rate, volume and
particulate criteria.
2. Identify and protect prime natural areas for preservation (Le., unique
water, forest or topography).
3. Assure that all developments are supportive of watershed and DNR
priorities.
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Water Resources Manacement
The approval of this Plan and Rules fits into a larger group of rules, regulations
and planning efforts. This section is meant to put the LSWMP and PWDM into
the greater context.
The approval of the Plan and Rules follows the approval of a Stormwater Rate
Study. The Stormwater Rate Study was included in the plan and formed the
basis of the conceptual model presented in the plan. The Plan also formed the
basis for Chapter 7 of the 2030 Comprehensive Plan, which has been
approved by Council. Chapter 7 of the 2030 Comprehensive Plan is basically
an executive summary of the LSWMP.
The Rules come before an expected update to the entire Public Works Design
Manual. The Rules are attached as an Appendix to this document in the
expectation that they will have to be modified in the future with the approval of
the WMO and WD. In attaching this set of rule as an Appendix, the
documents can be reviewed and approved under separate processes and
timeframes without broadening the scope to the entire set of engineering
standards that are contained in the Design Manual itself.
The Rules contain a provision for a rate control district for areas draining to the
Prior Lake outlet channel. This provision will be part of a Joint Powers
Agreement (JPA) with the WD dealing with the operations and maintenance of
the outlet channel.
Conclusion
The proposed revisions to the LSWMP and PWDM provide the City with a
Comprehensive Water Management Plan and corresponding design standards
equivalent to Scott WMO and the PLSLWD plans. Updates to the PWDM and
LSWMP are needed both to comply with the watershed planning framework
set forth in State Statute and to modernize local controls to remain a leader in
the field of water resource management. These revisions are undertaken to
preserve and protect the water resources of the City of Prior Lake and meet
and further goals and objectives of the 2030 Vision and Strategic Plan.
Adoption of these revisions will trigger changes to some sections of the
Subdivision Ordinance. These changes will be reviewed at a future public
hearing.
Adoption of these revisions will also trigger work to begin on Memorandums of
Agreement with the WMO and WD on the implementation of equivalent rules.
The final goal of that process will be the elimination of the "dual track"
permitting. Prior to the Agreements being finalized, approval will be gained for
the Plan and Rules at both the WMO and WD boards.
ISSUES:
With the addition of new rules from the WMO, new requirements exist that will
have a cost increase to development; however, those increases are difficult to
quantify given the variation in site conditions. Approximately 75% of the City is
within the WD; those areas already operate under rules requiring volume
control provisions. The streamlining of the review system should save these
developing areas money in the design process. An exception to this is in
areas with rare wetlands where additional setbacks may be required. The
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City's experience in these areas, however, indicates that for the most part
developers have voluntarily stayed away from sensitive wetlands. The number
of rare wetlands that this applies to is limited, and the use of the PUD process
on these sites should mitigate financial impacts to development.
The impact to development in the remaining 25% of the City that is in WMO
areas is unknown. There are expected to be costs increases due to the new
volume control requirements. Staff will work with these developers to educate
and develop projects that minimize these additional costs.
FINANCIAL
IMPACT:
The approval of the Plan and Rules is meant to be revenue neutral for the City
to implement; however, some additional staff time is expected in the transition.
In the effort to provide a high level of customer service, it is anticipated that
additional meetings and correspondence with developers and their engineers
will be required. After the completion of a Memorandum of Agreement (MOA)
with the Watershed District sole, permitting authority will be transferred to the
City. The MOA will also propose to modify some of the workload on water
quality issues; for example, the District may take over wetland buffer
encroachment compliance from the City.
ALTERNATIVES:
1. Approve a resolution approving the Local Surface Water Management Plan
and Public Works Design Manual Hydrology Appendix.
2. Deny this item for a specific reason and provide staff with direction.
3. Table this item until some date in the future.
RECOMMENDED
MOTION:
Alternative #1.
Reviewed by:
J<1ifT..
Steve Albrecht, Public Works Director/City Eng.
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16200 Eagle Creek Avenue S.E.
Prior Lake, MN 55372-1714
RESOLUTION 06-xx
A RESOLUTION APPROVING THE CITY OF PRIOR LAKE LOCAL SURFACE WATER
MANAGEMENT PLAN AND PUBLIC WORKS DESIGN MANUAL HYDROLOGY
APPENDIX.
Motion By:
Second By:
WHEREAS, A public and stakeholder comment period was conducted; and,
WHEREAS, The City of Prior Lake is committed to protecting its water resources through prudent
planning and implementation of modern rules; and
WHEREAS, Minor amendment to the Public Works Design Manual Hydrology Appendix will be
competed by the City Engineer; and
WHEREAS, Major amendments will be competed only with the approval of the Watershed District,
Watershed Management Organization and the City Council; and
WHEREAS, All complete preliminary plat applications submitted after the effective date of the
Rules must comply with the rules laid out in the Public Works Design Manual
Hydrology Appendix; and
WHEREAS, The Rules will be effective on July 1, 2006.
NOW THEREFORE, BE IT HEREBY RESOLVED BY THE CITY COUNCIL OF PRIOR LAKE,
MINNESOTA as follows:
1. The recitals set forth above are incorporated herein.
2. The Prior Lake Local Surface Water Management Plan and Public Works Design Manual
Hydrology Appendix are hereby approved.
PASSED AND ADOPTED THIS 26th DAY OF JUNE 2006.
Haugen Haugen
Dornbush Dornbush
Erickson Erickson
LeMalr LeMair
Millar Millar
YES
NO
Frank Boyles, City Manager
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City of Prior Lake
Public Works Design Manual - Hydrology Appendix
Public Works Design Manual - Hydrology Appendix
BACKGROUND INFORMATION
Site hydrology must follow the rules specified herein. These rules are based on the policy set forth
in the City of Prior Lake Local Surface Water Management Plan (LSWMP), the Water Resource
Management Plan of the Prior Lake Spring Lake Watershed District (PLSL WD), and the
Comprehensive Water Resource Management Plan and Rules of the Scott Water Management
Organization (Scott WMO).
By enforcing the rules spelled out in this appendix the City of Prior Lake is fulfilling requirements
spelled out under State Statute Chapter 103B, 103D, and Minnesota Rules Chapter 8410. The City
of Prior Lake enforces these rules under terms spelled out in a Memorandum of Understanding or
Memorandum of Agreement (MOU or MOA) with both the PLSLWD and Scott WMO.
In the future, when major amendments are needed in this Appendix, the Scott WMO Board and the
PLSLWD Board must be consulted and grant approval for said amendments as required by the
WMO, WD, State Statute and Rules.
Index of Sections
Section 1 - Definitions
Section 2 - Format and Standards
Section 3 - Grading, Erosions and Sediment Control
Section 4 - Site Hydrology and Stormwater & Volume Management
Section 5 - Drainage Alterations and Floodplain Management
Section 6 - Wetlands
Section 7 - Pond Design Criteria
Section 8 - Stormsewer Design Criteria
Section 9 - Miscellaneous / CN Reductions
SECTION 1:
DEFINITIONS
Best ManaQement Practices lBMPs): Techniques that are proven to be effective in the
management of stormwater, including those documented in the Minnesota Stormwater Manual
(MPCA, 2005), Protecting Water Quality in Urban Areas (MPCA 2000), and others as amended.
Buffer: An area of natural, non-invasive, permanently undisturbed, vegetated ground cover
adjoining and surrounding a wetland measured from the delineated edge of the wetland.
Buffer Averaoino: A buffer of variable width around a wetland equal in area to a corresponding fixed
width buffer around the same wetland, set at the average width.
EOF: Emergency Overflow
Exoandable Pondino: Ponds built in low areas common to multiple developments that can be easily
expanded when neighboring areas develop.
HWL: 100-yr High water level
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Public Works Design Manual - Hydrology Appendix
Landlocked Basin: Any drainage area greater than 1 acre that does not have a natural surface
outflow below the level of the 100 year storm, or below its OHW.
LSWMP: Local Surface Water Management Plan.
Net Acre: Total land area minus any wetland, lake, or bluff acreage.
No-arade zone: An area around a wetland that no change in grade is allowed
OE: Outlet elevation
OHW: Ordinary High Water level.
Reaional Dondinc: Permanent stormwater facility used to provide rate control and water quality
treatment for an area that encompasses two or more entities (including but not limited to;
developments, subdivisions, building additions, and conditional uses.)
Stormwater Manacement Overlav District: An area within the City that has a separate standard,
generally defined by a tributary feature. (Example; Any area draining to the Outlet Channel)
Tributary Acre: Total land area tributary to a Pond or Wetland on site in the existing condition, prior
to any drainage alterations or landlocked basin connections.
Volume Abstraction: Policy of encouraging infiltration, evaporation and transpiration to mitigate the
volume increasing effects of urbanization.
Volume Manaaement: Policy of limiting volume and rate entering lakes by impounding water for
extended durations in the upper reaches of the Tributary in stormwater ponds and wetlands to
mitigate the increased water volume effects of urbanization on the watershed.
Volume Storace: Volume set aside for stormwater below a natural or created outfall that during
hydrologic variation mitigates effects of increased stormwater volume.
SECTION 2:
FORMAT AND STANDARDS
Construction Drawings
Show the OE, HWL for ponds and OHW for water bodies on the plans.
Show garage floor, low floor, and rear pad elevations and housing style for each unit on the grading
plan. Include a schematic describing each housing style typical grading.
Show limits of clearing and limits of grading on grading plan and tree preservation plan. Show
removal of all trees and brush below the controlled water level that will be impacted from existing
and newly created ponding areas.
Show emergency overflow routes using arrows from all low points and show elevation of high point
along emergency overflow route. All emergency overflow routes shall be graded and the easement
area sodded prior to building permit issuance.
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City of Prior Lake
Public Works Design Manual - Hydrology Appendix
Show or define access routes for maintenance purposes to all inlets or outlets at ponding areas
(must be maximum of 8% grade, 2% cross slope and 10' wide). Paving or pavers on the access
routes is required with a design load to able support maintenance vehicles.
Stormwater Management Report
Calculations shall be submitted showing proposed design elements that meet requirements spelled
out in this Appendix. A narrative describing the proposed system shall accompany this collection of
calculations. The stormwater narrative shall be a brief and clear description of the stormwater
system that summarizes and reference figures, tables and plan sheets. The following are the
minimum summary/narrative elements:
. Narrative describing the proposed system; referencing requirements in this appendix.
. Summary of proposed & existing offsite runoff rates and volumes.
. Summary of volume control requirement & CN reductions claimed.
. Narrative describing volume control method & why it was chosen.
. Summary of volume control system showing that it meets requirements.
. Existing and Proposed Drainage Maps.
. Walker Method Calculations.
SECTION 3:
GRADING, EROSION & SEDIMENT CONTROL
Site Erosion and Sediment Control described here augments the Stormwater Pollution Prevention
Plan (SWPPP) as required by the MPCA NPDES Construction Site Permit. Even when not party to
a NPDES Construction Permit the City of Prior Lake remains involved and serves as a monitor to
confirm that the NPDES Construction Site Permit is being followed.
Grading Standards
Maximum 4: 1 slopes are allowed in "maintained" areas except approved by the City Engineer.
Maximum 3:1 slopes are allowed for road fill sections adjacent to water bodies or natural resource
preservation areas.
Minimum grade for drainage swales and lot grading shall be 2% or greater. Maximum length for
drainage swales shall be 300 feet or a total of eight lots draining to a point, or as approved by the
City Engineer. Backyard drainage structures should be avoided. Drainage swales shall be graded
and stabilized (drainage blanket, seed and mulch, or sod) prior to the issuance of building permits.
A minimum of 15 feet beyond the house pad shall have a slope less than 10: 1.
Show or define paved access routes for maintenance purposes to all manholes outside the public
right-of-way and inlets or outlets at ponding areas (8% maximum grade, 2% cross slope, and 10'
wide). Access easements shall be dedicated at the time of final platting to provide this access.
Verify locations and design of all overland drainage routes for capacity and erosion potential. All
low points in streets must have E.O.F's designed for the 100 year storm event.
Erosion and Sediment Control/ SWPPP Standards.
All development that disturbs greater than 1 ac (or 10,000 sf in a shoreland) area must apply for
and comply with the requirements of a constructions site NPDES permit. The SWPPP must be
reviewed and approved by the City before a grading permit is issued.
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Public Works Design Manual - Hydrology Appendix
All provisions of a NPDES constructions site permit & SWPPP must be adhered to for the duration
of the permit.
Provide name of company, contact person, and phone number for person responsible for erosion
and sediment control plan preparation, implementation and maintenance.
Provide note on the plans specifying that all erosion control best management practices shall be
installed by the Contractor and inspected by the City prior to any site work.
Slopes greater than or equal to 4:1 shall have erosion control blanket installed immediately after
finished grading.
Area coming out of agricultural production must be seeded with a cover crop prior to development.
SECTION 4:
SITE HYDROLOGY, STORMWA TER & VOLUME MANAGEMENT
General Standards
A hydrologic method, based on sound hydrologic theory must be used to analyze runoff for the
design of stormwater conveyance systems and permanent stormwater facilities. Curve numbers
shall follow recommendations of SCS Technical Release 55, Second Edition (TR-55, 1986).
Rate and volume control will be required for all development, redevelopment or change in use that
creates more than 3,500 SF of new impervious area and disturbs more than 10,000 SF of land.
Rate Control
2, 10 and 100 year (24hour NRCS Type II) events shall be modeled. Events for Prior Lake are 2.8",
4.2" and 6.0" for the 2, 10 and 100 year storms respectively. (See Section 1, Definitions for
italicized terms)
Rate Control Standard:
Rate control shall hold total offsite peak runoff at or below the following schedule:
2 year: 0.05 cfs per net acre
10 year: 0.30 cfs per net acre
100 year: Existing peak flow
Stormwater Manacement Overlav District #1 - Prior Lake Outlet Channel: (Figure 1)
Rate control for areas tributary to the Prior Lake outlet channel shall hold total offsite peak runoff at
or below the following schedule:
2 year: 0.25 cfs per net acre
10 year: 0.25 cfs per net acre
100 year: 0.25 cfs per net acre
Alternate Rate Control Standard for Wetlands Utilized for Volume Manacement:
A portion of the 10 and 100 year events from a development may utilize wetlands for stormwater
rate control. The wetland must be eligible based on the requirements of Section 6. The following
schedule shall apply to the rate control a wetland provides.
The Alternate Standard for Wetlands shall hold discharge out of a utilized wetland at or below the
following rate schedule:
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Public Works Design Manual - Hydrology Appendix
2 year: 0.05 cfs per tributary acre
10 year: 0.15 cfs per tributary acre *
100 year: Existing peak flow *
* May be overridden by a stormwater management overlay district.
* In the case that the wetland cannot provide the volume for active storage in the 10 and 100 year
storms, the upstream stormwater system must be sized such that the system meets this
requirement.
Any proposed improvements utilizing wetlands for portions of the 10 or 100 year event storage must
consider the build out condition of the watershed draining to that wetland. Developments will be
allowed to utilize a wetland proportional to their share of the tributary area. Rate control must be
constructed to serve the build-out condition of the entire subwatershed based on current zoning.
Information on the utilization of wetlands for volume storage can be found in Section 6 of this
appendix. Wetlands may not be eligible for utilization if land ownership or easements for
stormwater uses cannot be obtained.
Additional requirements for rate control may be set by the Engineering Department. The utilization
of wetlands for active storage or volume storage may only be done with the approval of the
Engineering department.
Volume Control
Volume Control Standard:
In an effort to mitigate the effects of increased volume discharged from urbanization, site runoff
volume shall be reduced in the proposed condition by a volume equal to or greater than 0.5 inch
over all new impervious surfaces, unless that standard is modified by a Stormwater Management
Overlay District.
Methods for Volume Control:
CN Reduction Credit:
All sites shall consider the use curve number (CN) reductions as a portion of the volume control
requirement. These methods include tree plantings, native grass buffers, porous pavements,
impervious disconnections, green roofs, constructed wetlands, and soil amendments.
Credit for each method is given on an area basis at the following depth: An example of the credit
reduction can be found in Section 9.
. tree plantings 0.05 inch
. native grass buffers 0.05 inch
. natural area preservation 0.05 inch
. soil amendments 0.05 inch
. impervious disconnection 0.10 inch
. porous pavements 0.50 inch + not counted in impervious calculation
. green roofs 1.00 inch + not counted in impervious calculation
Each CN reduction has additional requirements described in Section 9. Additional CN reduction
techniques may be proposed, credits will be at the discretion of the Engineer.
Greywater/Stormwater Recycling:
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Public Works Design Manual - Hydrology Appendix
For many applications, use of pond water for irrigation can be a very cost effective method of
volume control (reduces monthly water bills and construction cost for infiltration areas). Since these
systems do not directly function on a storm by storm basis an applicant is asked to work with the
Engineer to determine the required volume standard.
Infiltration:
If infiltration is used the following is required:
1. Requirements and recommendations laid out in the Minnesota Stormwater Manual shall be
followed.
2. Infiltration tests shall be used to provide a base infiltration rate of surrounding soils. The
base rate shall be reduced to a conservative rate for the design. The Infiltration tests shall
be conducted at the location and elevation of the proposed infiltration system.
Bioretention:
If Bioretention is used the following is required:
1. Requirements and recommendations laid out in the Minnesota Stormwater Manual shall be
followed.
2. Infiltration tests shall be used to provide a base infiltration rate of surrounding soils. The
base rate shall be reduced to a conservative rate for the design.
Stormwater Wetlands! Constructed Wetlands:
Constructed wetlands may be utilized to abstract volume. Since these systems do not directly
function on a storm by storm basis an applicant is asked to work with the Engineer to determine the
required volume standard.
1. Requirements and recommendations laid out in the Minnesota Stormwater Manual shall be
followed.
Off site Volume Control:
Any of the above credits can be used at an offsite location to meet the volume control requirement
with the approval of the City Engineer. The offsite location must be within the same watershed.
Off site volume control should be permanently protected through an easement, development
agreement and/or maintenance agreement, to help ensure the volume reduction feature continues
to function as designed.
Stormwater Manaaement Overlav District #2 - All Landlocked Basins
If a development is tributary to a landlocked basin, the following restricted volume control is
required for that tributary portion:
. Volume shall be reduced in the proposed condition by a volume equal to or greater than 1.0
inches over all new impervious surfaces.
. Extended duration detention is required such that volume discharging offsite in the proposed
condition not exceed the volume discharging offsite in the existing conditions in the 24 hour
period following the peak of the 100-year 24 hour NRCS Type II storm.
Additional Volume Control required under this rule is not waived if an outlet or emergency overflow
is provided.
Limitations on Volume Control Method:
The method or location of Volume Control used may be limited by the following:
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Public Works Design Manual - Hydrology Appendix
Groundwater table within 3' of soil infiltration feature:
. Infiltration features should be sited with the aid of soil boring information and infiltration tests
on soils representative of those in the vicinity of the proposed basin.
Wellhead protection area:
. Soil infiltration features must comply with requirements and limitations of wellhead
protections plans.
Impervious Soils:
. Surface infiltration features must have appropriate soils. In the case of type D/clay soils
Surface infiltration is not allowed.
Low Floor / Low Opening Elevations
Low Floor Elevations and Low Opening Elevations are to be designed to the following standards:
Low floor elevations shall be at least:
. 3' Above OHW or Highest Known (whichever is greater)
. 2' Above HWL
Low opening elevations shall be at least:
. 2' Above E.O.F
In the case of a landlocked basin, low floor elevations shall be at least:
. 3' above the basin overflow elevation or 3' above the back to back 100 year flood.
Additional Requirements
In the development of any subdivision or ponding area, the developer and/or property owner is
responsible for the removal of all significant vegetation (trees, stumps, brush, debris, etc.) from any
and all areas which would be inundated by the designated controlled outlet elevation (OE) of any
required ponding areas as well as the removal of all dead trees, vegetation, etc., to the high water
level (HWL) of the pond.
Upon the completion of the construction of a designated ponding area, developer is required to
submit an as-built record plan of the ponding area certifying that the pond constructed meets all
design parameters. The Developer can over-excavate the bottom of the water quality ponds to
compensate for erosion that will occur. The Developer will be responsible for verifying, at the end
of the Warranty Period, that the ponds are providing the required volume.
Water Quality
The water quality treatment standard is 60% reduction in Phosphorus and 90% reduction in Total
Suspended Solids. Section 7 details standard wet pool detention pond design criteria.
Alternatives to water quality ponds can be proposed but must meet water quality treatment
standards. If alternatives are proposed, documentation must be submitted by the applicant based
on literature values or independent laboratory work to demonstrate the performance of the
alternative being proposed.
SECTION 5:
DRAINAGE ALTERATIONS AND FLOODPLAIN MANAGMENT
Landlocked Basins
If a landlocked basin (tributary area < 25 acres) is proposed to be connected to a downstream
system the following is required:
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Public Works Design Manual - Hydrology Appendix
. If the landlocked basin contains a wetland, the rate of outflow from that wetland must follow
provisions for the "Alternate Rate Control Standard for Wetlands utilized for Volume
Management" detailed in Section 4.
. If the landlocked basin contains a wetland, Stormwater Utilization must occur to the
maximum extent allowable under provisions for wetlands detailed in Section 6.
. If the landlocked basin does not contain a wetland, or the wetland is legally proposed to be
filled under the Wetland Conservation act, the volume storage lost to connection must be
mitigated by increasing volume storage of a pond or wetland equal to the lost volume of the
landlocked basin utilized in the 100 year storm event.
. All Provisions described in Section 5, Drainage Alterations must be followed.
If a landlocked basin (tributary area> 25 acres) is proposed to be connected to a downstream
system the following is required.
. All requirements listed above apply.
. The PLSLWD or Scott WMO must review the proposed connection. All conditions of Scott
WMO or PLSLWD approval must be met.
Drainage Alterations
If the applicant proposes to artificially drain, connect a landlocked basin, obstruct, or redirect the
natural flow of runoff the following is required:
Drainaae alteration (tributary area < 5 acres) reauirements:
Demonstrate:
. That overall change in flow volumes to each subwatershed do not burden downstream
infrastructure.
Requirements:
. Considered a minor alteration provided applicant demonstrates that downstream burdens do
not exist.
. If there is evidence to suggest there is or will be a flooding problem immediately
downstream of the proposed alteration, the requirements for drainage alteration of between
5 and 25 acres shall apply.
Drainaae alteration (tributary area> 5 and < 25 acres):
. All requirements listed above apply.
Demonstrate:
. There is a necessity for such a change.
. Reasonable care has been taken to avoid impact to upstream or downstream land.
. Efforts have been taken to mitigate changes in downstream volume and rates.
Requirements:
. City approval of drainage alterations is required.
. Peak rate to the gaining subwatershed of the drainage alteration must be held to rate control
standard per tributary acre included in the development.
DrainaQe alteration (tributary area> 25 acres) reauirements:
. All requirements listed above apply.
. The PLSLWD or Scott WMO must approve the proposed alteration. All conditions of Scott
WMO or PLSLWD approval must be met.
G:\Water ResourcesILSWMP\2006 LSWMPIDESIGN MANUAL - HYDROLOGY APPENDIX.doc
8
City of Prior Lake
Public Works Design Manual - Hydrology Appendix
Floodplain Alterations
If grading takes place within the floodplain (below the predicted 1 OO-year flood elevation for a public
water) no net decrease in flood storage is allowed.
SECTION 6:
WETLANDS
Procedure
For developments with wetland on or near, the following procedure is followed.
The applicant must submit a wetland delineation and must choose whether to submit a MnRAM 3.0
wetland assessment.
Figure 2 and 3 of the hydrology appendix show the results of a City wide wetland survey.
The results of this survey are valid until fall 2010. The applicant may use the stormwater
susceptibility and functional classification from these figures.
If an applicant does not wish to use the findings of the wetland survey, or if no data is
available on a wetland, or the data is no longer valid, a MnRAM will be required in addition
to the wetland delineation. The results from the MnRAM wildlife and floral diversity will be
compared to the flow chart described in Figure 4 to determine the wetland's functional
classification. The function classification will determine the size of a native vegetative buffer
and a no grade zone as described below.
The MnRAM and Delineation will also be used to determine the hydrologic sensitivity of
wetland vegetation as well as the wetland's functional classification based on the criteria
presented under "requirements."
After a delineation is submitted and the wetland is given a stormwater susceptibility and functional
classification, these documents are reviewed by the City.
If impacts are proposed to any wetland, the procedure follows the requirements of the Wetland
Conservation Act and these rules.
Requirements
Any drainage, fill, excavation or other alteration of a public waters or wetlands is regulated by the
Wetland Conservation Act (WCA), State Statutes 1 03G.245 and regulations adopted thereunder.
Wetland replacement for impacts occurring within the PLSLWD must take place within the District at
a rate of 0.5:1 New Wetland Credit (NWC) per acre impacted.
The City must review and approve of the wetland delineation and Minnesota Routine Assessment
Version 3.0, (MnRAM) (as amended).
Stormwater Susceotibilitv.
Highly Susceptible: A wetland is considered highly susceptible if:
. Forty percent or more of the wetland complex has highly susceptible wetland communities
as shown in Table 6.1 and;
. Highly susceptible wetland communities have medium to exceptional floral diversity/integrity.
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9
City of Prior Lake
Public Works Design Manual - Hydrology Appendix
Moderately Susceptible: A wetland is considered moderately susceptible if:
. Forty percent or more of the wetland complex has a moderately susceptible wetland
communities shown in Table 6.1 and;
. Moderately susceptible wetland communities have medium to exceptional floral
diversity/integrity .
Least Susceptible: Wetlands with low floral diversity, as determined by MnRAM, were considered to
be least susceptible wetlands.
Slightly Susceptible: Wetlands that do no fall under the high, moderate or least susceptible
categories are considered slightly susceptible.
to Stormwater 1m acts
Moderately Susceptible
Wetland Communities.
Shrub-Carrs
Alder Thickets
Fresh wet Meadows
Shallow Marsh
Calcareous Fens Dee Marsh
* Wetland communities determined using key provided in MnRAM Version 3.0.
Stormwater Utilization:
Hydroperiod Highly Moderately Slightly Least
Standard SusceDtible SusceDtible SusceDtible Susceptible
100-year Storm Existing Existing + 0.5 ft Existing + 1.0 ft No Limit
Bounce
Discharge Rate Existing Section 4: Section 4: Section 4:
Wetland Standard Wetland Standard Wetland Standard
(1 \ (1) (1 )
1 & 2 year NRCS Existing Existing + 1 Day Existing + 2 Days Existing + 7 Days
event Inundation
10 yr NRCS event Existing Existing + 7 Days Existing + 14 Existing + 21
Inundation Days Davs
(2) Outlet Control None: Note None: Note o - 2 ft additional o - 4 ft additional
Elevation OE/HWL on Map OE/HWL on MaD storace storage
Table 6.2
Stormwater Utilization
(1) Rates shall be held to the rate control spelled out in section 4, alternate rate control
standard for wetlands, unless obtaining these rates is prevented by inundation period
requirement.
(2) Outlet Control Elevation changes can be made to mitigate volume storage as required in
Section 5.
Buffer & No-Grade Zone ReQuirements
The following are the buffer and minimum no-grade zone requirements for each functional
classification. The tiered buffer requirement is based on a functional classification of; unique, high,
G:IWater ResourcesILSWMP\2006 LSWMPIDESIGN MANUAL - HYDROLOGY APPENDIX.doc
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City of Prior Lake
Public Works Design Manual - Hydrology Appendix
moderate or low, and can be found using the results of the MnRAM vegetation assessment as
described above.
Buffer Unique High Moderate Low
Reauirement
Average Buffer 60 30 30 30
Width (ft)
Minimum Buffer 30 20 20 20
Width (ft)
Minimum No- 30 20 15 10
Grade Zone (ft)
Table 6.3
Buffer and No-Grade Matrix
Foundations for proposed homes must be set back a minimum of 20' from the buffer edge.
Grade changes or other disturbances are not allowed in No-Grade Zones with the following
exceptions: Pipe Outlets, stilling pools and associated riprap, reseeding or soil amendments,
embankment and impacts associated with an approved CIP transportation corridor, grade changes
adjacent to approved WCA impacts to wetlands, approved wetland or flood storage mitigation
areas.
A buffer width may vary using "Buffer Averaging." Buffer width may be reduced to the minimum
buffer width, but the overall buffer area must be equal in area to a hypothetical fixed width average
buffer around the same wetland. This means that while one side of a buffer is reduced in width, the
buffer must be increased in width in another area to make up for the loss of area.
Pipe outlets into a wetland are required to incorporate a stilling pool and following design elements:
. Pipe outlet velocity should be less than 5fps and discharge to a stilling pool lined with riprap
to provide further velocity attenuation. (Similar to MnDOT Standard Plate 3133)
. Flow path from stilling pool to wetland must promote level spread of water.
. Pipe and riprap lined stilling pool must not encroach into wetland.
Buffer Vecetation ReQuirements:
Buffer must be seeded with a native mix with forbs. A two year maintenance period is required as
part of the developers agreement. If at the end of the two year maintenance period the seed has
not established, there is a prevalence of invasive species, or there are other encroachments, over-
seeding or reseeding may be required.
Land use within a buffer shall be subject to the following restrictions: Buffer vegetation shall not be
cropped, cultivated, hayed, mowed, fertilized, or subject to the placement of mulch or yard waste or
otherwise disturbed, except for the periodic cutting or burning that promotes the vegetative health of
the buffer or as needed to address invasive or noxious species. Buffers may be temporarily
disturbed when permitted by the City Engineer. No new structure or hard cover may be placed
within a buffer area. No fill, debris, or other material may be excavated from or placed in a buffer
area.
Buffer strips shall be required whether or not the wetland is on the same parcel as the proposed
development or on an adjacent parcel. Wetlands on adjacent parcels need not be delineated, but
an estimation using aerial photos or other methods will be required.
G:IWater ResourcesILSWMP\2006 LSWMPIDESIGN MANUAL - HYDROLOGY APPENDIX.doc
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City of Prior Lake
Public Works Design Manual - Hydrology Appendix
Buffer Monuments and Easement:
Buffer strips shall be identified within each parcel by permanent monumentation. A monument shall
be required at each parcel line where it crosses a buffer strip and shall have a maximum spacing of
200 feet along the edge of the buffer strip. An additional monument shall be placed at the midpoint
of each lot and/or as necessary to accurately define the edge of the buffer strip. (considering
curvature) A monument shall consist of a post and a buffer strip sign. The signs shall be 4.5" x 6.5"
inch vertical, have brown field with white lettering, and shall be securely mounted on a post to a
minimum height of 4 feet above grade. The signs shall include warnings about disturbing or
developing the buffer strip. The signs shall be installed prior to the issuance of a building permit
and should be shown on the approved plans. Buffer strip signs can be purchased at the
Engineering Department at a cost shown on the latest fee schedule.
All buffer strips must be covered by drainage and utility easement granted to the City of Prior Lake.
If the buffer is intended to be used for volume control credit under Section 4, a conservation
easement is required in addition to drainage and utility easements. Conservation easements shall
be dedicated to the PLSLWD in District areas or to the City in WMO areas.
SECTION 7:
POND DESIGN CRITERIA
Water Qualitv Volume
The permanent pool volume for water quality ponds shall be calculated using the Design
Calculations for Wet Detention Ponds by Dr. William W. Walker (1987) presented below.
Aw = Total Watershed area (acres)
4 = Area of impervious surfaces draining to stormwater conveyors (acres)
A.
F; = Impervious Fraction =---.!....
Aw
eN = area weighted mean NRCS curve number for pervious portion of watershed (Based on soil
type and land cover)
P= Design storm size =2.5 (inches)
S = ( 1 ~: ) -10 = Maximum soil retention (inches)
R = P x F; + (p - 0.2 x SY x (1- F;)= Runoff for design storm (inches)
P+0.8xS
RxA
V = w = Volume of permanent pool (acre-feet)
12
Rate Control Volume
The active volume (between the OE and the HWL) shall be sized to meet the rate control
requirements outlined in Section 4, Rate Control, using a maximum slope at 4:1 (H:V).
Additional Desian Criteria
G:IWaler ResourceslLSWMP\2006 LSWMPIDESIGN MANUAL - HYDROLOGY APPENDIX.doc
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City of Prior Lake
Public Works Design Manual - Hydrology Appendix
The use of regional ponding, stormwater wetlands or expandable ponding is encouraged by the
City.
A 10' wide aquatic vegetation bench is required below the OE of the pond, with the maximum slope
of10:1.
The invert elevations of pond inlet flared end sections shall match the OE of the pond. Submerged
outlets will only be allowed at the discretion of the City Engineer.
Outlet control structures from ponding areas are required as directed by the City. Location and
appearance of outlet structures shall be subject to City approval and may require landscape
screening.
The E.O.F of a pond should be at least 1 foot higher than the HWL. The top of dike elevation
should be at least 2 feet higher than the HWL.
Dikes used to create rate control ponds must at maximum use 4: 1 slopes and measure 10' wide at
top. At minimum, a clay core should be designed in consideration of groundwater flow. If head
difference between OE of pond and downstream land exceeds 3', soils data will be required in the
area and dike design must address maintenance of pond water level and dike stability. Outlet
pipes through engineered dikes with head differences greater than 6' should be designed with anti-
seep collars. Sand bedding shall not be used through dike section.
Pond outlets shall provide floatable debris skimming for the 10 year event. A manhole with a baffle
wall with orifice or notch is recommended to control rate. Outlet rate control manholes shall have a
top mitered to conform to fill. A 66" minimum diameter is required to provide access to both sides of
the weir wall.
SECTION 8:
STORM SEWER DESIGN CRITERIA
Storm water facilities shall use design criteria utilizing a rational or hydrograph method based on
sound hydrologic theory to analyze the storm water runoff and proposed development. (Such as the
Soil Conservation Service TR-55 Urban Hydrology for Small Watersheds)
Storm water facilities shall be designed for a 10-year frequency storm for local pipe design and a
100-year frequency storm for ponding detention basin design and trunk facilities. Pipe size and
grade shall be greater than 15" ID and 0.5% slope. Pipe class shall conform to design standards as
shown in "Exhibit D."
Drainage calculations shall be submitted to show the sizing of pipe, ponds, emergency overflow
spillways, and catch basin interception analysis. Assuming catch basins can receive a maximum of
3 cfs, multiple catch basins may be required at low points.
Provide for overflow routes to drain all street and backyard low points.
SECTION 9:
MISCELLANEOUS & CN REDUCTIONS
Chapter 4 detailed a credit system used for volume management; the following is additional
information on that credit system.
G:IWaler ResourcesILSWMP\2006 LSWMPIDESIGN MANUAL - HYDROLOGY APPENDIX.doc
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City of Prior Lake
Public Works Design Manual - Hydrology Appendix
Example calculation for CN Reduction: The following is an example of volume control met entirely
through the use of eN reductions, this will not be suitable for most sites but serves as an example
on how to use a variety of methods to meet the requirement.
50 acre site with 20% impervious surfaces, = 10 acres new impervious (actual calculation required)
10 acres impervious x 0.5in volume control requirement = 5ac-in or 18150cfvolume.
Reductions Claimed:
CN Reduction Claimed Area (acres) Area basis deoth (in) Credit (ac-in)
5 acres native Qrass buffers 5 0.05 0.25
1 acre of proposed pervious pavers 1 0.50 0.50
Same paver area no longer counted 1 0.50 0.50
in impervious calculation
5 acres tree plantinQs (436 trees) 5 0.05 0.25
30 acres of soil amendments 30 0.05 1.50
5 acres of impervious 5 0.10 0.50
disconnections
1 acre green roof 1 1.00 1.00
Same green roof no longer counted 1 0.50 0.50
in impervious calculation
Total Claimed: 5.00 ac-in
Additional Requirements for CN Reduction
Tree plantings:
Area determinations for tree plantings shall be assumed at 500sf per tree. (Based on an
conservative average water use of 1 cf per day per tree at 7 years in an open canopy condition, less
average tree mortality, 3days water use per storm O. 7*(11(.05/12)) *3 the beneficial effects of canopy
intercept are ignored). This reduction can be used in combination with the native grass buffer (ex.
Oak prairie restoration could count as tree planting, native grass and soil amendment reduction).
Native grass buffers:
Area determination shall only include buffers currently, or proposed to be, established in a native
species. Area must be included in a permanent conservation easement dedicated to the City of
Watershed District.
Natural area preservation:
Upland wooded or prairie areas proposed include only those areas not already prohibited from
development (buffers, bluffs, etc) and must be kept in their natural state through outlet dedication
and/or conservation easements. No grading is allowed in preservation areas. Preservation area
must be denoted through the use of decorative fencing, informational signing, or other methods.
Soil amendments:
Area determinations for soil amendments shall include only those areas from the back of building
pads to the grading limits in the rear. For areas in front and side, or under temporary constriction
access to be counted, soil amendments must take place after building constructions is compete,
due to the compactive nature of home construction.
Soil amendment shall be designed to mitigate the effects of compaction due to mass grading by
returning the soil to a loose, friable state able to transmit water.
G:IWater ResourceslLSWMP\2006 LSWMPIDESIGN MANUAL - HYDROLOGY APPENDIX.doc
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City of Prior Lake
Public Works Design Manual - Hydrology Appendix
All soil amendments must be designed by a registered engineer or professional soil scientist. Soil
design must promote deep loosening of the mass-graded soils strata, and improvement of
infiltration and moisture retention characteristics of the topsoil. (Example: 18" deep ripping or tilling
of base soils and compost mix after mass grading, followed by the application of a designed mix of
compost, peat, sand, and topsoil and spread at 6" depth).
Impervious disconnection:
Area determination for impervious disconnections will be that roof area or select pavement area that
is disconnected from the drainage system and allowed to flow over natural grounds that are
designed to promote infiltration and transpiration. Design shall not cause nuisance, wet lawn
conditions or basement seepage. Design must discourage reconnection to impervious surfaces by
providing a minimum of a 75' pervious flow path.
Porous pavements:
Area determination for porous pavements will be only that area meeting the following requirements:
Porous pavement systems must use permeable base material and promote infiltration. Porous
pavements must be designed by a professional engineer and approved by the City Engineer.
Green roofs:
Area determination for green roofs will be only that area meeting the following requirements:
Design must be must be designed by a professional architect and meet Building Code. The design
must be approved by the City Engineer
(More information on these and other CN reductions and site design measures can be found in
Chapters 4 and 11 or the MPCA stormwater manual).
G:\Water ResourceslLSWMP\2006 LSWMPIDESIGN MANUAL - HYDROLOGY APPENDIX.doc
15
0.5 0.25 0 0.5
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Miles
"'-' SUBWATERSHED
BOUNDARY
_ SW MANAGEMENT
OVERLAY DISTRICT
FIGURE 1
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.
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Yes
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Yes
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Yes
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Overall Functional Ranking Flow Chart
Surface Water Management Plan
Prior Lake, Minnesota
\S66\S6603114\cad\graphics\FuncRank_FlowChrt.ppt
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Low floral diversity/integrity
Overall Functional Ranking Flow Chart
Figure. ~
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Engineers & Architects
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Prior Lake, Minnesota
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FIGURE 5
W+E
S
TABLE OF CONTENTS
Table of Contents
Executive Summary
1. Introduction
1.1 Background
1.2 Purpose and Scope
1.3 Organization
2. Land and Water Resources
2.1 Land Use
2.2 Topography and Watersheds
2.3 Soils
2.4 Key Water Resources
2.5 Existing Flood Insurances Studies
2.6 Other Natural, Biologic, and Water Resources
3. Goals, Policies and Guidelines
3.1 Purpose
3.2 Background
3.2.1 2020 Vision and Strategic Plan
3.2.2 Comprehensive Plan 2020
3.3 City of Prior Lake SWMP Goals and Policies
3.3.1 Water Quantity
3.3.2 Water Quality
3.3.3 Recreation and Fish and Wildlife
3.3.4 Enhancement of Public Participation;
Information and Education
3.3.5 Public Ditches
3.3.6 Groundwater
3.3.7 Wetlands
3.3.8 Erosion and Sediment Control
3.3.9 Prior Lake NPDES Permit
3.3.10 Financial Management
3.4 County, State and Federal Agency Requirements
a City of Prior Lake
~
U Local Surface Water Management Plan
Vll
1-1
1-1
1-2
1-4
2-1
2-1
2-4
2-6
2-7
2-9
2-10
3-1
3-1
3-1
3-1
3-2
3-3
3-4
3-6
3-7
3-7
3-8
3-8
3-8
3-9
3-10
3-11
3-11
3.4.1 Minnesota Department of Natural Resources
3.4.2 U.S. Army Corps of Engineers
3.4.3 Board of Water and Soil Resources
3.4.4 Minnesota Pollution Control Agency
3.4.5 Environmental Protection Agency
3.4.6 Prior Lake Spring Lake Watershed District and Scott
County Watershed Management Organization
3.4.7 State and Federal Jurisdictional Boundaries for Public
Wetlands and Waters
3.5 Agency Contacts
3.6 Water Resources Management-related Agreements
3.7 Impacts of the Prior Lake LWSMP on other Units of Government
3.8 Watershed Goals and Strategies that Affect the City of Prior Lake
4. Wetland Management Plan
4.1 Wetland Inventory Goals
4.2 Wetland Identification
4.3 Wetland Mapping
4.4 Wetland Evaluation Methodology
4.4.1 Minnesota Routine Assessment Method
4.4.2 Database
4.5 Required Submittals at the Time of Development
4.6 Wetland Ranking Methodology
4.6.1 Habitat ProtectionlWetland Ranking
4.6.2 Stormwater Protection Ranking
4.7 Wetland Management Standards and Recommendations
4.7.1 Water Quality
4.7.2 Water Quantity
4.7.3 Wetland Buffer Strip and Setback Protection
4.8 Wetland Restoration/Enhancement Opportunities
4.9 Wetland Stewardship
4.9.1 Enhancement
4.9.2 Control ofInvasive Exotic Species
4.9.3 Habitat Structures
4.9.4 Learning Opportunities
5. System Assessment and Design
5.1 General
5.2 System Assessment
5.2.1 Water Quality Assessments
5.2.1.1 Clean Water Act Assessments
5.2.1.2 PLSLWD Assessments
5.2.1.3 Scott County Water Management Organization
5.2.2 Water Quantity Assessments
5.2.2.1 City Identified Problem Areas
5.2.2.2 Pike Lake and Jeffers Pond Districts
n City of Prior Lake
-=-
1J1 Local Surface Water Management Plan
3-11
3-12
3-12
3-12
3-13
3-13
3-13
3-16
3-17
3-17
3-18
4-1
4-1
4-1
4-2
4-2
4-2
4-3
4-3
4-3
4-3
4-4
4-7
4-7
4-8
4-8
4-11
4-11
4-12
4-12
4-12
4-13
5-1
5-1
5-2
5-2
5-2
5-4
5-8
5-8
5-8
5-8
ii
5.2.2.3 PLSLWD Volume Management
5.3 System Design
5.3.1 Hydrologic Modeling Discussion
5.3.2 Design Recommendations and Discussion
5.3.2.1 Conveyance and Storage System Concepts
5.3.2.2 Water Quality System Concepts
5.3.2.3 The Use of Wetlands in the Surface Water
System
5.4 System Description
6. Implementation Plan
6.1 General
6.2 Cost Analysis and Capital Improvement Plan
6.2.1 Cost Estimation Methods
6.2.1.1 Pipe Costs
6.2.1.2 Pond Costs
6.2.2 System Costs and Capital Improvement Plan
6.3 Financing and Cost Recovery
6.3.1 Area Charges and Cost Recovery Calculations
6.3.2 Area Charge Summary
6.4 NPDES Permit
6.5 Operation and Maintenance
6.5.1 Activities
6.5.2 Stormwater Basins
6.5.3 Sump Manholes and Sump Catch Basins
6.5.4 Storm Sewer Inlet Structures
6.5.5 Open Channels
6.5.6 Piping System
6.5.7 De-Icing Practices
6.5.8 Street Sweeping
6.5.9 Detection of Illicit Connections
6.6 Education
6.6.1 General
6.6.2 City Staff
6.6.3 City Residents
6.6.4 Development Community
6.7 Financing and the Storm Water Utility
6.7.1 Current Status - Summary
6.7.2 The Storm Water Utility into the Future
6.8 Design Standards
6.9 Watershed Implementation Priorities
6.10 City of Prior Lake Implementation Priorities
6.11 Amendment Procedures
6.11.11 Minor Amendments
6.12 Annual Report to Council
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7. Summary and Recommendations
7.1 Summary
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Aopendices
Appendix A
Appendix B
Appendix C
Appendix D
Appendix E
Drainage Areas
Trunk Storm Sewer Data
Pond Data
Stormwater System Costs
Wetland Ranking
Figures and Maos
Figure 1 Location Map
Figure 2 Land Use Plan
Figure 3 Annexation Phasing Plan
Figure 4 Public Waters: Water Basin
Figure 5 Public Water: Water Course
Figure 6 Overall Functional Ranking Flow Chart
Map 1 Surface Water System
Map 2 Surface Water System
Map 3 Surface Water System
Map 4 Surface Water System
Map 5 Surface Water System
Map 6 Surface Water System
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List of Tables
Table 1.1 Population and Households 1-2
Table 2.1 Land Use Based Peak and Volume Comparisons 2-4
Table 2.2 Soil Drainage Characteristics 2-6
Table 2.3 Hydric Soils of Scott County 2-7
Table 2.4 Flood Insurance Study Results 2-10
Table 4.1 Wetland Community Susceptibility to Stormwater Impacts 4-7
Table 4.2 Stormwater Protection Standards 4-7
Table 4.3 Wetland Quantity Standards 4-8
Table 4.4 Recommended Buffer Strip Features 4-10
Table 5.1 303(d) 2004 Final List of Impaired Waters Within the City
Of Prior Lake and its 2020 Growth Area 5-3
Table 5.2 Growing Season (May-Sept) Average Lake Monitoring Results,
2000-2003 From PLSL WD 2003 Annual Report 5-5
Table 5.3 Carlson's Trophic State Index (TSI) Explanation 5-6
Table 5.4 Relationship ofMCES Lake Grade to Trophic Status 5-6
Table 5.5 Tropic Status of District Lakes, 2003 5-7
Table 5.6 PLSLWD Memo Table #1: 2-YR Peak Flow Rates 5-10
Table 5.7 PLSL WD Memo Table #2: 100- YR Peak Flow Rates 5-10
Table 5.8 PLSL WD Volume Study Results Summary 5-11
Table 5.9 Runoff Coefficients 5-14
Table 5.10 Phosphorus Concentrations and Export Coefficients 5-22
Table 5.11 Benefits of Wet Detention Ponds 5-22
Table 5.12 Wetland Community Susceptibility to Stormwater Impacts 5-25
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Table 5.13 Drainage Districts 5-28
Table 5.14 Comparison of SWMP Modeled Flows to 2003 City of
Prior Lake/PLSL WD Agreement 5-35
Table 5.15 Potential Retention Volume 5-40
Table 6.1 Capital Improvement Plan 6-4
Table 6.2 Land Use Based Equivalency Factors 6-7
Table 6.3 Stormwater Area Charge 6-8
Table 6.4 Wet Pond Maintenance Schedule 6-10
Table 6.5 Stormwater Utility Revenue 6-17
Table 6.6 Future Stormwater Utility Funding 6-18
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EXECUTIVE SUMMARY
This report provides the City of Prior Lake with a Local Surface Water Management Plan
(LSWMP) that will serve as a policy basis for the management of the surface water
system throughout the City. The LSWMP is intended to complement the City's
Comprehensive Plan 2030 and official controls governing stormwater. The LSWMP will
carry the City through the end of 20 15. Periodic amendment to the Plan will occur in the
intervening 10 years so that the Plan remains current to watershed plan amendments and
revisions and current to the "state of the art" in surface water management.
The Prior Lake LSWMP will serve as a comprehensive planning document to guide the
City in conserving, protecting, and managing its surface water resources. The LSWMP
meets requirements as established in Minnesota Rules 8410. In addition, the participation
of other organizations, particularly Scott County WMO and Prior Lake Spring Lake
Watershed District, ensures the City's compliance with local and regional expectations.
The City will submit its LSWMP to Metropolitan Council, Scott County WMO, and
PLSL WD for review. These entities have 60 days for their review after written receipt of
the City Plan.
Three other activities complement the LSWMP. The Wetland Management Plan (WMP)
provides an assessment and management plan for numerous wetlands within the 2030
growth area. The WMP is based upon standard assessment methodology and is utilized,
in conjunction with the LSWMP hydrologic modeling, to determine future use of wetland
basins for storage, retention, and infiltration. The WMP constitutes section 4 of this
report. The second activity is the Upland Management Plan. Like its wetland
counterpart, this Plan provides an assessment and management plan for resources - this
time upland resources. The City will utilize this information in open space and park
planning. The third activity is the Public Works Design Manual (PWDM). The PWDM
summarizes the policy and recommendations set forth in the LSWMP and provides
design standards and a method of enforcing water resource management concepts
detailed in the LSWMP.
This report is a culmination of the activities described above and is organized as follows:
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. Section 2, Land and Water Resources Inventory, describes the physical
environment including watersheds and drainage patterns, dominant land uses, and
significant water bodies within the City.
. Section 3 - Goals, Policies and Guidelines -lists the City's goals and policies along
with public agency requirements affecting surface water management in the City.
. Section 4, Wetland Management Plan, presents the results of an assessment of the
City's larger wetlands within the 2020 growth areas. The Wetland Plan identifies
specific strategies for mitigating wetland impacts often associated with
development.
. Section 5, System Assessment and Design, presents an overview of all the major
watersheds in the City. This section describes in detail the affect rural drainage
has on municipal systems both now and in the future. Section 5 also provides
detail on a model of the storm water management system within the four focus
areas. The focus areas are soon to develop portions of the larger study area where
conceptual ponds and trunk pipes are sized and shown, where trunk alignments are
shown, and where volumes, discharge rates, and capital costs are analyzed.
. Section 6, Implementation Plan, covers regulatory responsibilities, priority
implementation items, educational programs, operation and maintenance, the
capital improvement program, and financing considerations. A plan amendment
process is also identified and the distinction between major and minor amendment
outlined.
. Section 7, Summary and Recommendations, contains a summary the SWMP and
makes recommendations for implementing the Plan.
It should be noted that the land use plan identifies future land use for areas within the
2030 growth boundary. Service areas for the sewer and water system can be effectively
defined by this boundary. In contrast, the surface water system is defined by topography
and the drainage that currently moves through the newly developing areas must continue
to be accommodated in the post development condition. For this reason, the modeling
and management strategies incorporated in the LSWMP must deal substantively with the
large rural and agricultural areas that will continue to drain through the City even after
build out of the 2030 Plan. For instance, drainage to Prior Lake extends as far south as
the PLSLWD boundary. This drainage extends almost to Cynthia Lake, three miles south
of the growth boundary and incorporates the discharge from Fish Lake, which is over 2.5
miles south of the growth boundary. So, while the 2030 Land Use Plan forms the basis
of the urban system outlined in this report, this urban system is also determined by these
large rural drainage areas discharging to Prior Lake.
The intent of the ponding system described in this report is to reduce the post
development peak to a rate more in line with natural conditions. This protects the city's
lakes, wetlands, and channels from erosion and flooding. Volume control, though not
specifically required by the LSWMP, is aimed at reducing the post development runoff
depth and is included as a requirement in the PWDM. By reducing the post development
runoff depth through volume reduction and infiltration - to something more akin to the
depths seen off the natural landscape - lake, wetland and channel protection is
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augmented. Infiltration will also help achieve the PLSL WD' s retention goals which are
outlined later in this report.
A change in land use from agricultural and natural to urban is the primary factor driving
the need for the Prior Lake Surface Water Management Plan. The goal ofthe plan is to
mitigate the impacts caused by urbanization.
Most of Prior Lake falls within the jurisdiction of the Prior Lake Spring Lake Watershed
District since, quite obviously, most the City's drainage - both current and within the
2030 growth area - ends up in either Prior Lake or in the Prior Lake outlet channel. A
portion of the City and City 2030 growth area falls within the Scott County WMO. This
area lies northwest of Spring Lake and generally drains toward Louisville Swamp, which
lies approximately 2 miles west of the 2030 growth area boundary.
The Prior Lake Spring Lake Watershed District encompasses approximately 42 square
miles of land in the jurisdictions of five local units of government: Prior Lake, Savage,
Shakopee, Sand Creek Township, and Spring Lake Township. Most of the district's land
area falls within Prior Lake's current limits and 2030 growth area. The primary water
resources within the district, which are discussed in detail below, include Spring Lake,
Upper and Lower Prior Lakes, Rice Lake, and Crystal Lake. Jeffers Pond and Pike Lake
are notable water resources that form a portion of the Prior Lake outlet channel.
Historically, three other watershed management organizations operated near the City of
Prior Lake. These were the Sand Creek, Credit River, and Shakopee Basin WMOs. All
three WMOs were determined to be "non-implementing" and subsequently they were
disbanded by the state Board of Soil and Water Resources. Scott County then assumed
the powers of these organizations through creation of the Scott County WMO. The Scott
WMO includes all of Scott County not currently managed by the PLSL WD, the Lower
Minnesota River Watershed District, the Vermillion WMO or the Black Dog WMO.
The primary Scott WMO hydrologic features within Prior Lake's existing or 2030
boundary include Campbell, Markley, Mystic and Howard Lakes.
This Goals and Policies section of the SWMP outlines goals and policies specific to
surface water management in Prior Lake and its environs. The goals and policies are
broad statements regarding the motivation and intent of the SWMP. The policies that
follow the individual goals are specific requirements that promote attainment of the goal.
The City of Prior Lake has maintained its natural drainage patterns throughout most of its
development. The City's goal is to foster continued optimum use of that natural drainage
system while enhancing the overall water quality entering the lakes. The intent is to
prevent flooding while using identified best management practices to enhance surface
water quality with minimal capital expenditures by the City.
Upon approval of this LSWMP by the two watersheds with jurisdiction over the City, it is
the City's intent to assume all permitting powers within it jurisdiction. Currently, the
Scott County WMO does not issue permits, so no impact to this organization would
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occur. The Prior Lake Spring Lake Watershed District does issue permits for any
planned activity that disturbs more than 10,000 square feet of land area. This threshold
rises to one acre if the activity is not near a lake, wetland, or the Prior Lake outlet
channel.
Since the watershed would still permit activities outside the City's jurisdiction its permit
process would remain in place. Within its jurisdiction, the City will use the permit
submittal requirements outlined in the watershed rules and updates. This will ensure
consistency of approach for all projects. Once the LSWMP is approved, the city will
enter into a memorandum of understanding, regarding the transfer of permit authority for
the PLSWD to the city.
The PLSL WD would continue in its role as a project review agency though it may defer
to the City review process for projects that don't have a direct impact on Prior Lake or
the Prior Lake outlet channel. The Prior Lake-Spring Lake Watershed District will also
continue to have responsibility for water quality monitoring.
The Prior Lake LSWMP envisions the City and its two watersheds as partners in
implementing this plan. In the PLSL WD lands, the City envisions the watershed taking
the lead on water quality and lake water quality issues. The City and watershed would be
equally responsible for implementation of the volume management targets discussed in
Section 5 of this Plan with the City taking the lead in the 2030 expansion areas and the
watershed taking the lead in areas outside the 2030 boundary.
The goal of this wetland inventory is the management of wetlands based on the functions
they perform and to determine appropriate protection strategies for stormwater discharge
to the wetlands if a land use change occurs that triggers a NPDES permit. Since smaller
wetlands are not typically used as major components in a stormwater storage system, we
focused our inventory on wetlands shown on the National Wetland Inventory (NWI) Map
that were over 0.5 acres in size.
The inventory and assessment of wetlands through the Wetland Management Plan
(section 4) allows the city to set up priorities based upon wetland functions and values.
This plan includes a wetland inventory and ranking system that will assist the city in
establishing priorities and focusing available resources for wetland protection,
enhancement and restoration. Because all wetlands have value, all are protected, to some
degree, in this plan.
The plan is designed to provide the following benefits:
· Provide wetland inventory, assessment, and management information:
· Aid in administration of the Wetland Conservation Act (WCA) by providing
information regarding the wetlands functions:
. Enhance wildlife values of wetlands:
. Provide and enhance recreational values:
· Designate wetland restoration/enhancement opportunities:
· Protect wetlands and adjacent resources that provide valuable ecological support:
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· Provide stormwater protection for wetlands.
It should be noted that the wetland inventory has been created for planning purposes
only. Regulation of activities potentially impacting individual wetlands will be based on
a site-specific delineation of the wetland boundary as part of a proposed project.
All of the inventoried wetlands within the study area were classified for Stormwater and
Habitat Protection. Stormwater Protection standards are listed in Table 4.2 and 4.3 and
Habitat Protection Recommendations are listed in Table 4.4. The Stormwater Protection
Standards include Water Quality and Quantity Protection. The Habitat Protection
Recommendations include Buffer Zones and No Grading Recommendations.
Water quality plays a significant role in the overall quality of a wetland. When the
quality of the incoming water declines, the wetland's plant community may change to
fewer numbers of species and retain only those species that are tolerant of high nutrient
and sediment loads. Once a wetland's plant community is changed, the wetland's
character and ecosystem will change, often to a less valuable system in terms of
biodiversity, habitat for wildlife, and aesthetic enjoyment. Pretreatment requirements
have been developed to maintain the character of the wetland. BMPs can be used to
accomplish the pretreatment requirements given in Table 4.2.
In the recent past, surface water management plans have protected wetlands from
nutrients but not water fluctuations or duration. In fact, it was common to use wetlands
to reduce flooding potential through sizing storm sewer pipes to maximize bounce and
detention time in wetlands.
This plan addresses stormwater quantity impacts to wetlands by providing protection
strategies to maintain the existing integrity of the wetland through special protection
strategies for highly, moderately, and slightly susceptible rankings and are described in
Table 4.3.
Wetland restoration/enhancement sites were identified during the field inventory and will
be further investigated at the time of development under the requirements spelled out in
the PWDM and ordinance. The wetland restoration portion ofthe filled out MnRAM
will be reviewed at the time of development to determine the potential for restoration of
wetlands on the property. The potential for wetland restoration will be determined based
on the ease with which the wetland could be restored, the number of landowners within
the historic wetland basin, the size of the potential restoration area, the potential for
establishing buffer areas or water quality ponding, and the extent and type of hydrologic
alteration.
Section 5 of the Plan serves two functions. The system assessment portion catalogues the
various assessments of problems that the Plan must address whether they relate to water
quality, wetland protection, flooding, volume management, or lakes management. The
intent is to identify the source of problems and, more importantly, specific actions the
City will take to address these problems either independently or in collaboration with
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some other organization - most commonly one of the watershed management
organizations. The purpose of the system design portion of this section is to identify and
quantify the infrastructure needed to allow continued development in Prior Lake while
avoiding the negative impacts, such as flooding and water resource degradation, often
associated with development.
The system design portion of this section describes the 2030 growth area surface water
management system and is intended to be used as a planning and analysis tool. This
system is shown in maps 1 through 5. The discussion ofthe system revolves around
answering the following questions:
· What are the general drainage patterns of the 2030 and existing system?
· What does the 2030 system entail in terms of storage, conveyance, volumes, and
discharge rates?
· Where does the proposed system discharge and what constraints in the existing
system limit discharge of the 2030 system?
· What is the impact of agricultural drainage, outside the 2030 growth area, on the
proposed and existing urban system?
· How have proposed wetland bounce, and duration of HWL, been determined by
management guidelines of the Wetland Management Plan, section 4 of the
LSWMP?
· What opportunities exist for obtaining the retention storage identified by the
PLSL WD both in the 2030 growth area and outside it?
· What is the impact of the City of Prior Lake's 2030 urban system on agricultural
areas and other municipalities?
· Are there any existing ponds where calculated HWL is a concern?
A number of water bodies within the existing City and its 2030 growth boundary are
listed in the state impaired waters list. Known as the 303(d) list from the applicable
section of the federal Clean Water Act, these waters are ones that do not currently meet
their designated use due to the impact of a particular pollutant or stressor. If monitoring
and assessment indicate that a water body is impaired by one or more pollutants, it is
placed on the list. At some point a strategy would be developed that would lead to
attainment of the applicable water quality standard. The process of developing this
strategy is commonly known as the Total Maximum Daily Load (TMDL) process.
When discussing nutrient impacts to lakes the nutrient most commonly identified is
phosphorus. Through its own monitoring efforts and those of the Citizen Assisted
Monitoring Program (CAMP) run by Metropolitan Council, the PLSLWD has been
collecting data on nutrient loading into the impaired waters, and others, identified above.
According to the PLSL WD 2003 Annual Report:
All of the lakes in the District are either eutrophic or hypereutrophic except for Cates Lake and
Lower Prior Lake, which are on the upper boundary of meso trophy. Review of Table 4.5 and
comparison with the TSI descriptions in Table 4.3 shows that both Cates Lake and Lower Prior
Lake are very close to the boundary for a eutrophic lake, and this boundary is where problems
really start to become evident. The western end of Lower Prior Lake is mesotrophic/eutrophic
largely because of water flowing through this end from Upper Prior Lake to the outlet. The rest of
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Lower Prior Lake has a limited watershed and is isolated from a majority ofthe inflowing water
from Upper Prior Lake.
The continued assessment of these lakes has led the PLSL WD to emphasize reduction in
phosphorus loading to the lakes. This will also be the focus ofa watershed-based TMDL,
when developed, for the impaired waters listed in table 5.1. Since the mercury TMDL
will be regional in nature, the City of Prior Lake and PLSL WD will focus their efforts on
reducing nutrient loading. According to the PLSL WD:
For noticeable improvements to occur in lake water quality, TSI values need to be reduced to 55 or
less. On the reverse, if these lakes are allowed to decline further, algae blooms will become worse
and fish kills are probable.
In addition to collecting and reporting on the above data, the PLSL WD has created a
model to quantify the internal and external phosphorus load for Spring and Upper Prior
Lakes. This modeling effort is summarized in the 2003 Annual Report:
In summary, sediment phosphorus release and recycling accounts for approximately 43 to 78% of
the total phosphorus load for Spring Lake and 49% of the total phosphorus load for Upper Prior
Lake. As a result, significant water quality improvements in each lake will require
implementation of lake improvement options that would greatly minimize the potential for
sediment phosphorus release. In addition, significant reductions in phosphorus from County Ditch
13 and Spring Lake should result in significant water quality improvements in Spring Lake and
Upper Prior Lake, respectively. To a lesser degree, senescing macrophytes and bottom-feeding
fish also affect the water quality of Spring and Upper Prior Lakes, since each of them contribute
approximately 5 to 15% of the total phosphorus load to each lake.
The Water Resources Management Plan for the PLSLWD, completed in 1999, identified
several planning efforts, that would occur subsequent to the Plan, to address issues with
the Prior Lake water levels and outlet operation. These included:
· Calibrating an hydrologic model for the watershed
. Designing improvements to the outlet channel for full-development conditions
· Addressing flood prone structures on Prior Lake
. Addressing increases in runoff volume as development occurs
The PLSL WD report Prior Lake Outlet Channel and Lake Volume Management Study
(May, 2003) addresses these issues in detail. The toO-year floodplain elevation for Prior
Lake established by FEMA is 908.9 MSL. There are 79 homes around the lake with low
openings lower than this floodplain elevation. Fifty-one of these have low openings
below 907.6 and ten have low openings below or within one foot of the lakes 904.0
OHW. According to PLSLWD information, this 904.0 elevation has been exceeded a
total of 259 days since 1983.
Since development tends to improve drainage pathways and increase runoff volume, the
impact of future development on Prior Lake could, without mitigation, increase the
frequency of water levels above the 904.0 OHW.
To assess the impact development might have on water levels in Prior Lake, the
PLSL WD created a calibrated model of the watershed. The calibration of this model
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started with standard curve numbers for the subwatersheds tributary to the lake and,
through the calibration process, modified these until modeled results matched monitored
lake levels for the 1998 to 2001 period. The hydrologic modeling for the LSWMP is
based upon this calibrated watershed model. The difference between the two, is that the
LSWMP model looks at the conditions that will exist when build out occurs in the 2030
growth area. Additionally, the LSWMP model includes more detail on the storage and
conveyance system necessary to serve the 2030 growth area.
Subsection 5.4 provides specific issues in the surface water management system for Prior
Lake's 2030 growth area. The study area has been broken into 21 major drainage
districts, which are further divided into subdistricts. The nomenclature for the major
drainage districts is based on the Prior Lake Spring Lake Watershed District (PLSLWD)
modeling and major tributary water bodies. Total acreage for major drainage districts
should roughly match the existing work completed by the Watershed District. Section
5.4 describes each drainage district in detail.
Section 6, Implementation Plan, of the Prior Lake SWMP describes those activities and
programs the City might develop toward improving its surface water management
program. Capital outlay for the surface water system (pipes, channels, and ponds) shown
on the system maps will be large. For this reason a financing mechanism, called an area
charge, is developed in this section. Based on the Capital Improvement Plan and the
developable acreage, an area charge is developed and application of this charge is
discussed.
The concept of an area charge to fmance expansion of the trunk stormwater management
system is not a new concept for the City. Since its report titled Trunk: Storm Sewer Fee
Determination Study (February, 2001) the City has quantified future trunk: and ponding
needs and developed an area charge based on actual costs of these needs spread across
the potential developable acreage. With the analysis contained within the SWMP the
City will update the fees for the 2030 growth area.
Section 6 also includes:
. An overview of the City's NPDES permit
. A discussion of operation and maintenance procedures and strategies
. An outline of an education program
. Financial considerations for the storm water utility
. A section referencing applicable design standards for stormwater management
. A section on Watershed implementation priorities
. Implementation priorities for the City
. A discussion of the process for amending this plan and an annual report to council
Appendix D summarizes the modeled system costs by element, by major watershed, and
for the system as a whole.
The potential system, as shown in system maps, carries an estimated cost of $14,858,788.
and serves as a basis for development fees in the City. This cost includes indirect costs of
30% on trunk: and pond construction and indirect costs of 10% on easement acquisition.
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Table 6.1 presents a financial model for the City of Prior Lake. The cost elements come
directly from the 2030 stormwater system design as described in the system maps and the
appendices to this report. The various trunk elements are organized by prospective year
of implementation as well as whether they constitute a pond cost or trunk pipe cost.
Total costs for the 2030 system are $10,836,957. It is important to note that the system
analysis was complete to estimate costs, and does not represent final design.
Sections 6.2 and 6.3 develop an area charge for the City of Prior Lake that can be applied
to future development within the City. The area charge has been constructed
methodically as follows:
1. Pond and trunk costs for near term development have been estimated. A
stormwater CIP has been created as shown in appendix D and table 6.1.
2. Net assessable acreage has been determined.
3. The base area charge has been modified into a land use based area charge through
the use of equivalent acres.
A storm water system is a major investment for the City of Prior Lake - both in terms of
initial capital cost and in terms of ongoing maintenance costs. The capital improvement
program outlines the costs for new trunk system construction which will be funded by
area charges. System maintenance is funded by the city's storm water utility.
The city's storm water system maintenance responsibilities include the following:
· Street sweeping
· Cleaning of sump manholes and catch basins
· Repair of catch basins and manholes
· Assessing pipe condition (typically by televising)
· Inspection of storm sewer inlet and outlet structures
· Pond mowing and other vegetation maintenance
· Excavation of accumulated sediments from ponds
· Maintenance or other structural BMPs owned by the City
The city has maintained its pipe system for decades and staff has a strong grasp on the
costs associated with this. As new development brings more ponds into the system, city
staff will find that pond maintenance becomes an increasingly large portion of both staff
time and maintenance budget. It is important to quantify the extent of this future
commitment so that the funds necessary for pond maintenance activities can be collected
via the city's storm water utility.
The City of Prior Lake implemented a stormwater utility in 1993. The current quarterly
residential charge is $6.00 per residential unit. Annual revenue from the stormwater
utility has grown as shown in table 6.5.
Generally, revenue has grown not because of increases in the charge (the charge has gone
from $5.63 in 1997 to $6.00 in 2005, an increase of 6.6%) but due to development
bringing in more properties over which to collect the charge. With this increased
revenue, though, has come an increase in the City's maintenance responsibilities.
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In the past the stormwater utility has funded a staff position, programs, and capital
expenditures. The 2002 capital projects totaled $140,000 and included a dredging
project, a lake bank stabilization project, and some storm drainage improvements.
In order that storm water utility (SWU) funding keeps pace with increase in municipal
maintenance responsibilities, the city should plan for the costs to conduct periodic pond
maintenance. Limited data on maintenance activities has been developed by watershed
management organizations. A review of this data suggests an annual maintenance budget
of $1 ,250 per acre-foot of wet volume or $4,350 per acre of surface at NWL. Either
parameter is relatively easy to track. This $1,250 per acre-foot maintenance item can be
translated into a per household cost by virtue of the fact that one acre-foot is sufficient
pond wet volume for 20 acres of residential development. Assuming 2.5 units per gross
acre, then $1,250 per year is spread among 50 units - $25 per unit per year.
The current residential rate is $24 per unit per year. The current charges provide
approximately $300,000 per year in revenue of which only about $20,000 to $40,000 has
been used for pond maintenance. As the city's maintenance responsibilities grow the
storm water utility funding also needs to grow to keep pace.
Prior Lake is a regulated MS4 under the Phase II NPDES Permit. There is a cost
associated with preparing an NPDES permit and the associated Storm Water Pollution
Prevention Plan (SWPPP). Some estimate cities the size of Prior Lake will spend
$50,000 every five years for permit preparation. For Prior Lake it is reasonable to
assume that $10 per household will be spent every five years - adding $2 per year to the
individual household's storm water utility bill.
The NPDES permit and SWPPP commit the city to certain activities, including capital
projects, for the purpose of improving the quality of the city's storm water discharge.
The U.S. EPA has estimated that the financial commitments that city's will make may
total $10 per household per year. Others place this figure at $20. Since many of the
activities identified by the SWPPP may already be funded (like street sweeping and pond
maintenance) the $20 figure is probably too high. For the purposes of planning increases
in SWU collection the $10 per year figure should be used. Table 6.5 summarizes the
additional storm water utility charges identified above.
The City of Prior Lake's implementation priorities include building the stormwater
management system described in this report. Other implementations priorities relate to
downtown redevelopment, retention storage, and adequate funding.
Originally, City Prior Lake and Prior Lake Spring Lake Watershed District staff
discussed the possibility of creating an overall stormwater management plan for the
downtown area that, once approved by the Watershed, would allow the City sole permit
authority for construction and reconstruction projects in the downtown area. In 2003, as
the City moved forward designing the rainwater gardens for Erie Street and City Hall and
the street reconstruction project for downtown it became apparent that how downtown
redevelops, and what sort of water quality and quantity retrofits can be implemented,
depends on factors that cannot be adequately quantified at this time. Consequently, at
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that time the City opted not to submit such a plan but rather would have the Watershed
permit the specific projects.
In 2007 the City will undertake a downtown stormwater management study in
cooperation with the district. Provisions for downtown permitting authority will be
spelled out in a MOA with the District. The results of the 1007 study will begin to be
implemented in 2011 or when the downtown area reconstruction begins. In the interim,
the City will continue to look for opportunities to retrofit small site BMPs, water quality
improvements, and rate control improvements as warranted by downtown redevelopment
activities.
Other implementation priorities for the City as it adopts this Plan and begins the
implementation phase of the Plan include:
1. Assisting the PLSL WD in implementing its retention storage program. Specific
areas with high potential for City implementation are indicated on the system
maps and within the body of this Plan.
2. Increasing Storm Water Utility Funding so that the City can meet its current and
future obligations toward pond maintenance, NPDES compliance, and mitigation
that may come out of the City's non-degradation analysis.
3. Application of the revised area charge outlined in this report and update ofthe
area charge based on increases in land value and construction costs. .
4. Implementation of the rate control targets as outlined in the appendices and
stormwater modeling that supports this plan.
5. Application of the wetland susceptibility criteria in determining how wetlands are
used for flood storage, retention, and rate control.
6. Working with the PLSLWD regarding the feasibility of augmenting storage in
Buck Lake.
7. Working with the City of Shako pee toward redefining rate control objectives from
their Sand Creek drainage which will ultimately enter the City of Prior Lake
system through its Louisville Swamp system.
The Prior Lake SWMP is intended to extend through the year 2015. For the plan to
remain dynamic, an avenue must be available to implement new information, ideas,
methods, standards, management practices and any other changes that may affect the
intent and/or results of the SWMP.
A brief annual report will be made by City staff summarizing development changes,
capital improvements, and other water management-related issues that have occurred
over the past year. The review will also include an update on available funding sources
for water resource issues. Grant programs are especially important to review since they
may change annually. These changes do not necessarily require individual amendments.
The report can, however, be considered when the plan is brought up to date. The annual
report should be completed by July I st to allow implementation items to be considered in
the normal budget process.
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The following recommendations are presented for the City Council's consideration based
upon the data compiled in this report:
1. The Surface Water Management Plan as presented herein be adopted by the City
of Prior Lake.
2. Standard review procedures be established to ensure all new development or
redevelopment within the City is in compliance with the grading and stormwater
management controls determined by this Plan.
3. Detailed hydrologic analyses be required or all development and redevelopment
activities.
4. Final high water levels governing building elevations adjacent to ponding areas
and floodplains be established as development occurs or when drainage facilities
are constructed.
5. Overflow routes be established and maintained to provide relief during extreme
storm conditions, which exceed design conditions.
6. A surface water system maintenance program be established to ensure the
successful operation of the system.
7. The erosion and sedimentation control criteria for new developments be enforced.
8. An education program for City residents, staff, and development community be
implemented.
9. Amendments to the plan be adopted and implemented as warranted by future
standards or regulations.
10. That the plan be updated in 2010 or earlier if needed.
11. Promote the use of small-site/distributed BMPs to help achieve water quality and
volume control goals.
12. Pursue partnerships with watershed management organizations and other agencies
to incorporate volume control BMPs into re-development projects, including City
projects.
13. Ordinances be revised to be consistent with rules detailed in the PWDM regarding
water resource management.
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1. INTRODUCTION
1.1 Background
This report provides the City of Prior Lake with a Local Surface Water Management Plan
(LSWMP) that will serve as a guide to managing the surface water system throughout the
City. The LSWMP is intended to complement the City's 2030 land use plan as
formalized in the Comprehensive Plan 2030. The LSWMP will carry the City through
the end of 20 15. Periodic amendment to the Plan will occur in the intervening 10 years
so that the Plan remains current to watershed plan amendments and revisions and current
to the "state of the art" in surface water management.
The City of Prior Lake is located in north central Scott County at the nexus of major
transportation corridors including Trunk Highway 13, Scott County Road 21 and Scott
County Road 42. According to some estimates, Scott County is the 15th fastest growing
county in the nation, and Prior Lake has seen a good portion of this growth.
The Village of Prior Lake was incorporated in 1891. In the period from the 1870's to the
1920's little growth occurred in the City. During this period, much of the activity in the
Village related to Prior Lake and its role as one of the preeminent recreation destinations
of that era. The lake remains a recreational focus and forms one part of the City's three-
fold recreational system:
1. Lakes and natural areas
2. Parks, trails and active recreational facilities
3. Venues such as Mystic Lake Casino and the City's golf clubs
It has been since the 1960's, and the City's emergence as a suburban community, that the
City's growth has escalated with the greatest growth occurring in the last 20 years. Table
1.1 provides City populations and population projections from 1980 through 2020. As
the city continues to grow, the importance of adequate surface water management
controls also grows. The intent of the Prior Lake LSWMP is to detail what these controls
are and make the connection between these controls and the overall city goal of
preserving and enhancing its natural resources and protecting its residents from flooding.
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Table 1.1
Population and Households
Year Population Number of Households
1980 7,284 2,313
1990 11,482 3,901
1995 13,427 4,630
2000 16,034 6,167
2010 21,000 8,077
2020 28,445 10,971
Lakes define the City since the core of the community developed between Upper and
Lower Prior Lake. The City's uniqueness is tied to the lake. Consequently, effective
surface water management cuts to the core of the City's vision for the future.
1.2 Purpose and Scope
The Prior Lake LSWMP will serve as a comprehensive planning document to guide the
City in conserving, protecting, and managing its surface water resources. The LSWMP
meets requirements as established in Minnesota Rules 8410. In addition, the participation
of other organizations, particularly Scott County WMO and Prior Lake Spring Lake
Watershed District, ensures the City's compliance with local and regional expectations.
According to the 1999 PLSL WD Plan local plans must do the following:
. Describe existing and proposed environment and land use
. Provide a narrative addressing stormwater infrastructure philosophy, which details
regulatory authority, and implementation and financial responsibilities.
. Define areas and elevations of stormwater storage adequate to meet performance
standards established in the watershed plan
. Identify quality and quantity protection methods which meet standards
. Identify regulated areas and potential easements or land acquisition areas
. Outline a procedure for submitting annual reports to agencies which document
Wetland Conservation Act and monitoring program data consistent with state
compatibility guidelines
. Set forth an implementation program, including a description of official controls,
inspection and maintenance, and capital improvement plan
. Describe official controls and the responsible unit of government in the following
areas: wetlands, erosion control, shoreland, floodplain, grading, and drainage
The City will submit its LSWMP to Metropolitan Council, Scott County WMO, and
PLSL WD for review. These entities have 60 days for their review after written receipt of
the City Plan.
In a four-part process, the Prior Lake LSWMP does the following:
. Collects and compiles the efforts of agencies and organizations including the City,
its departments and residents. This includes past reports and studies, management
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plans, monitoring studies, as well as completed and proposed improvement
projects.
. Reviews the current state of the City's surface water resources in the context of
goals and policies, ordinances, operations and maintenance, flood mitigation, and
achievement of targeted water quality levels in its surface water bodies.
. Establishes reasonable, achievable and affordable goals, and supports them by a
strong regulatory and management culture. Develops an implementation plan that
includes projects and processes that derive from a thorough assessment of current
City problem areas and current City surface water regulations and controls.
. Provides a blueprint for construction of new surface water systems as the City
expands into its 2030 growth area. Using advanced surface water modeling
software, a system of pond, wetlands and pipes is developed and costs applied to
these future systems. The costs give the City a framework for understanding the
impact development will have on City finances and applying these costs equitably
to development.
In order to arrive at a LSWMP that adequately addresses surface water related issues, the
emphasis has been to work with others to identify important issues through review and
meetings. City staff has participated in collecting data, providing feedback, and
contributing knowledge of local systems to aid in developing a strategy that encompasses
water quality and quantity issues. The City of Prior Lake is the organizer of the final
document though contributions from the watersheds have been substantial.
Two other activities complement the LSWMP. The Wetland Management Plan (WMP)
provides an assessment and management plan for numerous wetlands within the 2030
growth area. The WMP is based upon standard assessment methodology and is utilized,
in conjunction with the LSWMP hydrologic modeling, to determine future use of wetland
basins for storage, retention, and infiltration. The WMP constitutes section 4 of this
report. The second activity is the Upland Management Plan. Like its wetland
counterpart, this Plan provides an assessment and management plan for resources - this
time upland resources. The City will utilize this information in open space and park
planning.
Based on the guidance provided by the Prior Lake city council and staff, this report
addresses the city's current surface water management needs and provides a framework
for successful implementation of a comprehensive storm water management program. A
specific outline of the steps involved in the preparation of the SWMP is presented below:
1. System Inventory and Mapping - Analyze drainage patterns and develop a
trunk storm water system map for the 2030 drainage system.
2. Goals, Policies and Guidelines - Develop goals and policies that guide the
city's surface water management philosophy. Augment design guidelines for
development and redevelopment. This gives the City guidance for facilities
design and standards for reviewing development plans. Included in this
process is determining all regulatory agencies involved in the storm water
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management of the City and working with City staff to develop feasible goals,
policies and guidelines.
3. System Analysis and Design - Analyze the storm water system and develop a
recommended system. A system model was created that extended beyond the
city's current boundary into future development areas. This step also includes
specific recommendations for system upgrades and improvements due to
erosion and flooding issues.
4. Cost Estimates and Capital Improvement Program - For the recommended
system, develop itemized cost estimates of facilities. These are planning-level
estimates, suitable for budgeting and decision making. Feasibility studies will
be needed for more detailed costs. In addition, a 5-10 year CIP has been
developed to coordinate system construction with growth.
5. Storm water Ordinances - Recommend ordinances or revision to existing
ordinances.
6. Storm water System Management - Provide recommendations on operating
and maintaining the storm water system as well as best management practices
(BMPs) for water quality and erosion control. Information regarding
compliance with NPDES Phase II Storm water Permits is also included.
Prior Lake presents something of a contrast. In older parts of town, city staff must meet
the challenges of maintaining an older storm drainage system. In other areas, new
development is adding new infrastructure to the storm water system and thus increasing
the city's maintenance responsibilities. The SWMP is primarily aimed at this new
development and the goals, policies, guidelines, controls, and preliminary system design
reflect that emphasis.
1.3 Organization
This report is a culmination of the activities described above and is organized as follows:
. Section 2, Land and Water Resources Inventory, describes the physical
environment including watersheds and drainage patterns, dominant land uses, and
significant water bodies within the City.
. Section 3 - Goals, Policies and Guidelines -lists the City's goals and policies along
with public agency requirements affecting surface water management in the City.
. Section 4, Wetland Management Plan, presents the results of an assessment of the
City's larger wetlands within the 2030 growth areas. The Wetland Plan identifies
specific strategies for mitigating wetland impacts often associated with
development.
. Section 5, System Analysis and Design, presents an overview of all the major
watersheds in the City. This section describes in detail the affect rural drainage
has on municipal systems both now and in the future. Section 4 also provides
detail on the proposed storm water management system within the four focus
areas. The focus areas are soon to develop portions of the larger study area where
specific ponds and trunk pipes are sized and shown, where trunk alignments are
J1f City of Prior Lake
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1-4
shown, and where specific volumes, discharge rates, and capital costs are
calculated.
. Section 6, Implementation Plan, covers regulatory responsibilities, priority
implementation items, educational programs, operation and maintenance, the
capital improvement program, and financing considerations. A plan amendment
process is also identified and the distinction between major and minor amendment
outlined.
. Section 7, Summary and Recommendations, contains a summary the SWMP and
makes recommendations for implementing the Plan.
n City of Prior Lake
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2. LAND AND WATER RESOURCE INVENTORY
2.1 Land Use
Figure 2 provides the 2030 land use plan for the City of Prior Lake. Figure 3 provides a
guide to the timeframe over which annexation will occur. City growth will be
concentrated in the areas around Spring Lake and to the west of Spring Lake. Obviously,
growth requires planning for other utilities and transportation in addition to surface water
planning. These utilities are being studied under the auspices of the Comprehensive Plan
2030.
It should be noted that the land use plan identifies future land use for areas within the
2030 growth boundary. Service areas for the sewer and water system can be effectively
defined by this boundary. In contrast, the surface water system is defined by topography
and the drainage that currently moves through the newly developing areas must continue
to be accommodated in the post development condition. For this reason, the modeling
and management strategies incorporated in the LSWMP must deal substantively with the
large rural and agricultural areas that will continue to drain through the City even after
build out of the 2030 Plan. For instance, drainage to Prior Lake extends as far south as
the PLSLWD boundary. This drainage extends almost to Cynthia Lake, three miles south
of the growth boundary and incorporates the discharge from Fish Lake, which is over 2.5
miles south ofthe growth boundary. So, while the 2030 Land Use Plan forms the basis
of the urban system outlined in this report, this urban system is also determined by these
large rural drainage areas discharging to Prior Lake.
The comprehensive plan provides a significant amount of narrative and statistical detail
on existing and proposed land use and the reader is referred to that document for more
information on land use planning. The hydrologic modeling that supports the LSWMP
used the land use plan to determine hydrologic characteristics of the future landscape.
Elsewhere existing land use was assumed.
Changes from undeveloped land uses, like agricultural and natural, to more heavily
developed land uses like low, medium and high density residential, and commercial have
a pronounced affect on hydrology. The increased impervious surface associated with the
urban land uses leads to higher runoff peak flows and increased runoff volume. Table 2.1
shows how volume and peak increase for two typical rainfall events.
a City of Prior Lake
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1il1 Local Surface Water Management Plan
2-1
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that r-lur.. "eHftlH aa::"'.1J reflect "lie location. Tile
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Table 2.1
Land Use Based Peak and Volume Comparisons
2-year (2.8") SCS Type II, 24-hour 100-year (6.1") SCS Type II, 24-hour
Land Use Rainfall Rainfall
Runoff Peak Runoff Volume Runoff Peak Runoff Volume
cfs/ac inches cfs/ac Inches
Natural 0.03 0.2 0.4 1.8
Agricultural (row
crop in May and 0.3 0.8 1.3 3.4
June)
Low Density
Residential (35% 0.5 0.8 2.3 3.4
impervious)
Commercial
(85% 1.9 2.0 4.8 5.2
impervious)
The intent of the ponding system described in this report is to reduce the post
development peak to a rate more in line with natural conditions. This protects the city's
lakes, wetlands, and channels from erosion and flooding. The use of Volume control is
aimed at reducing the post development runoff depth. By reducing the post development
runoff depth through infiltration and other methods - to something more akin to the
depths seen off the natural landscape - lake, wetland and channel protection is
augmented. Volume control will also help achieve the PLSLWD's retention goals which
are outlined later in this report.
A change in land use from agricultural and natural to urban is the primary factor driving
the need for the Prior Lake Surface Water Management Plan. The goal ofthe plan is to
mitigate the impacts caused by urbanization.
2.2 Topography and Watersheds
The topography and geology of Prior Lake was influenced by several ice sheets that
advanced and retreated across southern Minnesota during the glacier age. The most
recent glacier deposited light yellowish-brown or light olive-brown, calcareous,
moderately fine textured material. The glacial action of the area resulted in the current
irregular topography, called an ice-stagnation and disintegration moraine. Some areas
contain deep unconsolidated'surficial material, up to several hundred feet deep.
The City of Prior Lake is located on the upland area of the south side of the Minnesota
River, in Scott County. The area has considerable relief, with steep slopes and potholes
common throughout the region. Many of the potholes are low wetland areas.
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The City of Prior Lake and the Prior Lake hydrologic system are part of the larger
Minnesota River watershed. The City of Prior Lake is approximately 4 miles south of the
Minnesota River and drainage within the Prior Lake system is to the north and northeast.
The City's Prior Lake system connects directly to the River through the operation of the
Prior Lake outlet, which is owned and maintained by the PLSLWD. A portion ofthe
City, northwest of Spring Lake and within the Scott County WMO jurisdiction, drains
northeast into Shakopee.
Prior to 1983, Prior Lake was a land-locked lake which had a natural outlet
approximately 18 feet above what was considered normal water level. In 1981 plans
were prepared for constructing a 36" RCP outlet pipe from Prior Lake which outletted on
the west side of County Road 21 into what is known as "Jeffers Pond." This project also
included upgrading the existing ditch and culvert system to drain the water all the way to
the Minnesota River. The water level to which Prior Lake is allowed to be drained is
902.5, 18" below the Ordinary High Water Level (OHWL) of 904.0 as established by the
Department of Natural Resources. According to DNR lake level records dating back to
1940, the lake level has varied from 893.48 feet to 905.68 feet, with a long-term average
of 90 1.97 feet.
Most of Prior Lake falls within the jurisdiction of the Prior Lake Spring Lake Watershed
District since, quite obviously, most the City's drainage - both current and within the
2030 growth area - ends up in either Prior Lake or in the Prior Lake outlet channel. A
portion of the City and City 2030 growth area falls within the Scott County WMO. This
area lies northwest of Spring Lake and generally drains toward Louisville Swamp, which
lies approximately 2 miles west of the 2030 growth area boundary.
The Prior Lake Spring Lake Watershed District encompasses approximately 42 square
miles of land in the jurisdictions of five local units of government: Prior Lake, Savage,
Shakopee, Sand Creek Township, and Spring Lake Township. Most of the district's land
area falls within Prior Lake's current limits and 2030 growth area. The primary water
resources within the district, which are discussed in detail below, include Spring Lake,
Upper and Lower Prior Lakes, Rice Lake, and Crystal Lake. Jeffers Pond and Pike Lake
are notable water resources that form a portion of the Prior Lake outlet channel.
Historically, three other watershed management organizations operated near the City of
Prior Lake. These were the Sand Creek, Credit River, and Shakopee Basin WMOs. All
three WMOs were determined to be "non-implementing" and subsequently they were
disbanded by the state Board of Soil and Water Resources. Scott County then assumed
the powers of these organizations through creation of the Scott County WMO. The Scott
WMO includes all of Scott County not currently managed by the PLSL WD, the Lower
Minnesota River Watershed District, the Vermillion WMO or the Black Dog WMO.
The primary Scott WMO hydrologic features within Prior Lake's existing or 2030
boundary include Mystic, Campbell, Markley and Howard Lakes.
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2.3 Soils
Soils of the Prior Lake area are classified into three associations:
1. Lester, Webster, Glencoe Association
2. Hayden and Lester soils and Peat bogs Association, and
3. Bumsville, Hayden, Kingsley, Scandia Association.
Information about each of the soil series listed above is available from the Scott County
Soil Survey. Table 2.2 shows the drainage characteristic of each soil series from the
above associations.
Table 2.2
Soil Drainage Characteristics
Soil Series Draining Characteristic Soil Type
Bumsville, Hayden, Kingsley well drained to excessively drained B
and Scandial
Glencoe very poorly drained D
Havden well drained B
Lester well drained B
Peat Bogs poorlv drained D
Webster poorly drained D
Note: Because the Bumsville, Hayden, Kingsley and Scandia series has formed from a mixing of two different kinds of
glacial drift, it is impractical to separate each into a separate series.
The drainage nature of the soil is important for determining the surface water runoff from
a given area. If the soil is well-drained, a significant portion of the precipitation will be
infiltrated into the ground, whereas if a soil is very poorly drained, most of the
precipitation will flow from the site of impact.
The hydrologic soil group (HSG) defines a soils propensity to generate runoff for a given
rainfall event. Four HSG groups area identified: A, B, C, D. HSG A soils have the
lowest potential to generate runoff and are typically sandy or gravelly soils. HSG D soils
have the highest potential to generate runoff and typically consist of muck, peaty muck,
and tight clay soils. The associations found within the Prior Lake LSWMP study area fall
into HSG B to D, indicating a moderate to high potential to generate runoff.
Hydric soils are those characteristic soils found in wetland areas. A wetland must
possess three technical criteria in order for it to be identified as a wetland. These three
are: 1) hydrophytic vegetation, 2) hydric soils, and 3) wetland hydrology. The definition
of a hydric soil is: "a soil that is saturated, flooded, or ponded long enough during the
growing season to develop anaerobic conditions in the upper part". A list of hydric soils
found in Scott County is shown in Table 2.3.
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Table 2.3
Hydric Soils of Scott County
Soil
Percent
Map Soil Of Map
Symbol Soil Map Unit Name Soil Name Acres Unit
Bc BLUE EARTH SILTY CLAY BLUE EARTH 1162 100
LOAM
Cc COM FREY SILTY CLAY COMFREY 1157 100
LOAM
De DUELM FINE SANDY LOAM DUELM 262 100
o to 3 Percent Slones
Df DUNDAS SILT LOAM DUNDAS 1061 100
o to 2 Percent Slopes
Fa FAXON SILTY CLAY LOAM FAXON 1193 100
o to 6 Percent Slopes
Ga GLENCOE SILTY CLAY LOAM GLENCOE 7939 100
Ia ISANTI FINE SANDY LOAM I SANTI 349 100
Oa OSHA WASIL TY CLAY LOAM OSHA W A 603 100
Pa PEAT AND MUCK, SHALLOW PALMS 3769 100
o to 2 Percent Slopes
PbA PEAT, DEEP HOUGHTON 13130 100
o to 2 Percent Slones
Ra RAUVILLE SILTY CLAY OSHA W A 969 100
LOAM
Wb WEBSTER-GLENCOE SILTY WEBSTER 11020 50
CLAY LOAMS GLENCOE 6754 30
Wc WEBSTER-LE SUEUR SILTY WEBSTER 6716 50
CLAY LOAMS
2.4 Key Water Resources
Upper and Lower Prior Lake
Upper Prior Lake lies between Lower Prior Lake and Spring Lake and is connected to
Lower Prior Lake through a channel under County Road 21. Both these lakes have high
watershed to lake area ratios and this, particularly is why they have problems due to
nutrient loading. Upper Prior Lake has a surface area of approximately 340 acres and a
total watershed area of 16,460 acres - a ratio of 48 to one. This ratio is extremely high
for any water body. Lower Prior Lake has a surface area of 827 acres and a total
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watershed area (including that tributary to Upper Prior Lake) of 19,560 acres - a ratio of
24 to one. Both lakes are relatively deep with a maximum depth of 56 feet in the Lower
and 43 feet in the Upper, although much of upper Prior Lake is shallow, and the lake has
an average depth of 8 feet. The Prior Lake outlet channel leaves Lower Prior Lake at its
far western shore near County Road 21. Both Upper and Lower Prior Lake have a public
water access.
Spring Lake
Spring Lake drains to Upper Prior Lake through a culvert and channel. Its 12,930 acre
watershed compares to a lake surface area of 630 acres. Spring Lake has a maximum
depth of 37 feet and an average depth of 18 feet, according to information provided by
the PLSLWD. Upper and Lower Prior Lakes and Spring Lake all have a substantial
amount of residential development around them. Spring Lake has a public water access.
Rice and Crystal Lakes
Rice and Crystal Lakes are connected though Rice Lake lies a little higher than Crystal.
The Rice Lake drainage includes 1,100 acres - mostly agricultural - versus a lake area of
approximately 30 acres. This equates to a watershed to lake ratio of37 to 1. The Rice
Lake ordinary high water level is 945. Crystal Lake also has a high ratio with 1,340 acres
draining to its 32 acre surface, a ratio of 42 to 1. Both lakes are relatively shallow with
Crystal having a maximum depth of approximately 26 feet. Current information
indicates that no part of Rice Lake exceeds the 10 foot depth. Neither Rice nor Crystal
Lakes have public water access.
Pike Lake
Pike Lake is a shallow lake through which the Prior Lake outlet channel passes. Pike
Lake has a surface area of 57 acres and a watershed area of21,770 acres - a ratio of382
to 1. While it is true that Pike Lake has a large tributary area due to the outlet channel,
Lower Prior Lake has much better quality than Pike Lake. An analysis completed for the
Outlet Channel EA W showed that Pike Lake tends to have higher water quality when the
outlet is open compared to when it is closed, presumably because the water from Lower
Prior Lake dilutes the high-nutrient levels in Pike Lake.
Jeffers Pond is another notable water resources within the PLSLWD portions of the City.
These two water bodies are classified as wetlands by the DNR and not as much
information is available for them as is available for the other lakes described above.
Mystic Lake has relatively low watershed to lake area ratios. Jeffers Pond also has a high
ratio due the Prior Lake outlet channel running through it.
Prior Lake Outlet Channel
In 1979 the Minnesota Department of Natural Resources (MnDNR) issued a permit to the
PLSL WD for construction of the Prior Lake Outlet Channel. The district entered into a
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joint powers agreement with Prior Lake and Shakopee in 1981 regarding the channel and
the outlet system was first used in 1983. The outlet box consists of a 36-inch RCP pipe
surrounded by a concrete structure with adjustable gates. The maximum discharge
capacity is determined to be 65 cfs though the discharge rate falls to about 40 cfs at the
902.7 elevation. Currently the PLSL WD, City of Prior Lake and other partners are
implementing a plan to restore and enhance the Prior Lake Outlet Channel.
Campbell Lake
Campbell Lake lies northeast of Spring Lake within the jurisdiction of the Scott County
WMO. Aerial photography and USGS mapping indicate that Campbell Lake discharges
to the northwest through a ditch. It is not known at what elevation this would occur. The
lake's OHW is 925.5'. Campbell Lake is also characterized by a well developed wetland
fringe and is relatively shallow for a lake though the actual maximum depth is not known.
All this lakes drainage area is agricultural. Campbell lake does not have a public access.
Howard Lake
This lake's OHW is 957.2'. Like Campbell it has large, shallow wetland areas around its
perimeter and it is not a deep lake though the actual maximum depth is not known.
Based on USGS mapping the lake apparently discharges west into Shakopee though it is
not known at what elevation, or how frequently, this occurs.
Markley Lake
Markley Lake straddles the City's eastern boundary toward the south part of town. The
lake is landlocked and wide fluctuation in water levels has historically been a problem.
Total lake area is approximately 21 acres and maximum depth is 22 feet. The
characteristic feature of Markley Lake is its steep and heavily wooded slopes. Since it is
landlocked the lake must be carefully managed. The City of Prior Lake Trunk Storm
Sewer Fee Determination Study (February, 2001) considered a pumping station and
forcemain from Markley Lake into the Credit River.
2.5 Existing Flood Insurance Studies
A Flood Insurance Study, dated March 1978, was completed for the City of Prior Lake by
the Federal Insurance Administration (FIA). This study updated a Flood Hazard
Boundary Map that was prepared by the FIA in 1975. The 1978 Flood Insurance Study
was based on Prior Lake being a land-locked lake, no outlet was considered in the
analysis. The method applied a water mass balance model to generate historic lake
levels. That study established a 100-year elevation of909.3 for Prior Lake and 914 for
Spring Lake, which is the current elevation the City regulates the lake at for flood
insurance purposes.
In September of 1994 a Flood Insurance, Interim Hydrology Report, was prepared by the
US Army Corp of Engineers for the FIA to see if the IOO-year flood elevation could be
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lowered for Prior Lake. Several models over the past have been run with the results of
each as shown in Table 2-4.
Table 2.4
Flood Insurance Study Results
Model Agency Event Elevation
SCS, 1988 Watershed 7.2" 100-Year lO-Day Runoff, TR-20 909.2
HEC-l, 1994 Corp 7.2" 100-Year 10-Dav Runoff 908.75
HEC-l, 1994 Corp 10.9" 100-Year lO-Dav Rainfall, new CN's 908.83
HEC-l, 1994 Corp 10.9" 100-Year lO-Day Rainfall, old CN's 908.4
FIS, 1978 Barr 100- Year, Water Mass Balance 909.3
The SCS TR-20 and the HEC-l models used a starting water surface elevation of902.0
for Prior Lake and assumed that the outlet structure was closed. The 1978 FIS model
resulted in the most critical elevation for Prior Lake. The adjusted critical elevation
adopted by the FIA is 909.3 for Prior Lake.
The United States Army Corps of Engineers and the Federal Emergency Management
Agency have mapped other floodplains within the City, 2030 growth area, and further
south. Floodplains have been determined over the following components of the drainage
system:
1. Fish Lake through the Buck Lake channel into Spring Lake
2. Rice Lake and Crystal Lake complex
3. Pike Lake
4. Spring Lake
5. Upper and Lower Prior Lakes
2.6 Other Natural, Biologic, and Water Resources
Upland and wetland water resources were thoroughly inventoried in the two studies that
ran parallel to the LSWMP effort. The Wetland Management Plan, included as section 4
of this report, uses an assessment of each wetland as a method of determining its
susceptibility to impacts from storm water runoff. This information is used to design a
surface water storage system that protects the 2030 growth area from flooding while
protecting valuable wetland resources.
The Natural Resources Inventory and Land Cover Mapping is a separate document. This
provides detail on pre-agricultural vegetation and existing vegetation and will serve the
City in its park and open space planning activities.
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3. GOALS AND POLICIES
3.1 Purpose
The primary goal of Prior Lake's Local Surface Water Management Plan (LSWMP) is to
plan for the orderly management of stormwater as development occurs in the city. The
plan provides clear guidance on how Prior Lake intends to manage surface water in terms
of both quantity and quality.
Much has changed since the city prepared its first LSWMP in 1973. Since that time the
city has seen a marked increase in residential and commercial development. Not
accounting for population growth due to annexation, city population has increased by
4,552 people from 1990 to 2000 (40%). Population growth combined with increased
regulation of stormwater at both the state and federal level necessitate that the city's
stormwater management goals evolve.
The goals and policies detailed in the LSWMP focus on future development as much as
they do on the existing state of things. This dual emphasis on existing and future ensures
that future development augments rather than diminishes the natural and built
environments.
3.2 Background
3.2.1 2020 Vision and Strategic Plan
In 2002 Prior Lake embarked on a strategic planning process intended to identify the
communities vision of itself in the year 2020. The committee convened for the visioning
process included over 60 community volunteers from all walks of life. This group
identified key issues facing the City:
. Growth and Land Management, including growth/land use, annexation, and
relationship between downtown and the lake.
. Economic Vitality, including commercial development, fmancial resources,
economic base and downtown redevelopment
. City/Community Quality and Amenities, including youth and senior citizen
activities, trails, general amenities, housing stock and City aesthetics.
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. City Services and Assets, including infrastructure and public safety.
. Preservation of Natural Resources, including environment and water quality.
. Intergovernmental Cooperation and Planning
. Community Leadership and Involvement
The Prior Lake LSWMP is a piece of a larger effort, conducted in partnership with the
City's two watershed organizations, toward addressing the 2020 vision of natural
resources preservation.
The key issues facing the City constitute its vision statements. The elements ofthat
vision become the City's implementation goals out into the future. The 2020 vision
includes goals in a variety of areas. Of specific importance in the context of the LSWMP
are the following:
City Financial Resources
4) Update and implement a system for development-related fees to support
community expansion and infrastructure needs.
Infrastructure
8) Continue City commitment to on-going infrastructure maintenance and
replacement.
Natural Resources
1) Adopt and implement a plan to improve surface water quality.
2) Adopt and implement a Comprehensive Lake Management Plan for Prior Lake
and other lakes within the City cooperatively with the watershed(s).
5) Develop and maintain City property, parks, and playfields in an
environmentally responsible and aesthetically pleasing manner.
Downtown Redevelopment
Install rainwater gardens and other needed storm water devices to facilitate
downtown improvements.
3.2.2 Comprehensive Plan 2020
The continued growth of Prior Lake has necessitated that its comprehensive plan be
updated. The primary purpose of this effort is determination of the land use plan
(included in the LSWMP as figure 2) which becomes the basis of the hydrologic
calculations summarized in the LSMWP. Prior Lake completed its first comprehensive
plan in 1973 and subsequent updates occurred in 1981 and 1996. These Comprehensive
Plans are mandated by the Metropolitan Land Planning Act. The goals of the Prior Lake
2020 Comprehensive Plan are:
. Housing Quality and Diversity
. Environmental and Natural Resource Protection
. Economic Vitality
. Security
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. Access
. Information and Technology
. Human Development
Specific to the goals and policies ofthis Local Surface Water Management Plan are the
following 2020 Comprehensive Plan objectives under the Environmental and Natural
Resource Protection goal.
OBJECTIVE No.1: Provide suitable passive open space for the
preservation ofthe natural environment and the enjoyment of residents.
POLICIES:
a. Retain natural ponding areas and wetlands, as appropriate.
b. Encourage platting of large planned unit developments.
OBJECTIVE No.2: Provide for conservation and protection of the lakes
and surface water.
a. Adopt and implement a plan to improve surface water quality.
b. Adopt and implement a Comprehensive Lake Management Plan
for Prior Lake and other lakes within the City cooperatively with
the Watershed District.
c. Adopt a program which ensures an acceptable level of lake
access parking and responsible lake utilization.
d. Implement a groundwater plan emphasizing production,
conservation, education, communication, and landscape
maintenance.
e. Participate with the Prior Lake-Spring lake watershed District in
developing and implementing a land management program for
upstream storage.
The Prior Lake LSWMP expands upon the goals and objectives provided in the 2020
vision and the 2020 Comprehensive Plan.
3.3 City of Prior Lake LSWMP Goals and Policies
This section of the LSWMP outlines goals and policies specific to surface water
management in Prior Lake and its environs. The goals and policies identified below are
broad statements regarding the motivation and intent of the LSWMP. The policies that
follow individual goals are specific requirements that promote attainment of the goal.
The City of Prior Lake has maintained its natural drainage patterns throughout most of its
development. The City's goal is to foster continued optimum use of that natural drainage
system while enhancing the overall water quality entering the lakes. The intent is to
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prevent flooding while using identified best management practices to enhance surface
water quality with minimal capital expenditures by the City.
The City of Prior Lake has adopted by ordinance the 1989 edition of MPCA publication
"Protecting Water Quality in Urban Areas" for implementing best management practices
for erosion control. The City of Prior Lake goals were established along the guidelines of
the goals developed by the Metropolitan Surface Water Management Act (M.S. 473.875
to 473.883).
"The purpose of the surface water management programs required by Sections
473.875 to 473.883 is to preserve and use natural water storage and retention
systems in order to (a) reduce to the greatest practical extent the public capital
expenditures necessary to control excessive volumes and rates of runoff, (b)
improve water quality, (c) prevent flooding and erosion from surface flows, (d)
promote ground water recharge, (e) protect and enhance fish and wildlife habitat
and water recreational facilities, and (f) secure the other benefits associated with
the proper management of surface water." (Ref. 20.)
3.3.1 Water Quantity
Goal 1 :
Protect, preserve, and use natural surface and groundwater storage and retention
systems to control excessive volumes and rates of runoff and flooding.
Policy 1.1:
Preserve and optimize where feasible the retention capaCItIes of the present
drainage systems by utilizing lakes, ponds, and wetlands for storing stormwater runoff.
Measures shall be taken in newly developing watersheds to limit the proposed runoff
rates to the existing rates or lower, or to the rates as specified in Policy 1.9 for the Jeffers
Pond District. The City will partner with the PLSL WD toward implementing its
retention storage goals within areas of the City that fall under municipal jurisdiction.
Policy 1.2:
Establish 100-year flood levels based on critical storm events.
Policy 1.3:
Alteration of wetlands is discouraged. Alteration may be allowed on individual
basis if the alteration can be accomplished within the regulations of all federal, state,
county, and local agencies that have jurisdiction over the particular wetland.
Policy 1.4:
Newly constructed detention areas shall meet the standards of the Public Works
Design Manual (PWDM).
Policy 1.5:
All minor storm sewer system design and analyses shall be based on the 10 year
rainfall event consistent with the standards ofthis plan and the City's PWDM.
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Policy 1.6:
Pond detention basin facilities shall be designed for the 100 year rainfall event,
consistent with the standards ofthis plan and the City's Public Works Design Manual.
Policy 1.7:
All hydrologic studies and drainage design shall be based on ultimate
development of the 2020 plan. In some cases near term conditions should also be
analyzed to determine whether unrestricted drainage from rural areas may lead to
construction of interim facilities, or management base upon interim HWLs or discharge
rates.
Policy 1.8:
There are numerous basins throughout Prior Lake which have no surface water
outlet and are considered to be "landlocked". It is Prior Lake's policy to require the
lowest building elevation opening be located 3 feet higher than the high water level of an
adjacent water body. In the case of a land-locked basin, the City may require the
openings to be set higher than the natural run-out elevation, depending on wetland
vegetation. Emergency overflows shall be provided to any areas in new development
without an overflow or outlet, however it is the policy of the City to include volume
storage and leave landlocked basins disconnected when possible.
The intent is to store two one-hundred year storms back-to-back within
landlocked basins and provide from this calculated elevation the required 3 feet of
freeboard because elevations in excess of I DO-year storms can be obtained through
incremental smaller events. The outlet elevation needs to be carefully scrutinized. In
cases of heavily wooded fringe areas around landlocked basins, it may be necessary to set
the outlet elevation lower to prevent the killing of trees.
Policy 1.9:
In order to mitigate future development flows from increasing erosion potential to
the outlet channel from Prior Lake the proposed two year event discharge rates will be
held to rates agreed to in the Joint Powers Agreement (JPA) with the PLSLWD.
Policy 1.10:
All developments shall, to an extent determined by the City, provide land,
funding, or a combination of both for developing regional detention sites to achieve the
existing rates as indicated in this plan.
Policy 1.11:
Implement volume control and encourage low impact development techniques in
developing and redeveloping areas to minimize runoff volumes that tend to increase with
. .. .
an mcrease m impervIOUS area.
Policy 1.12
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Regional detention basins are used to manage peak flow rates and provide flood
storage and flood retention. On-site detention basins are utilized when regional basins
are not in place or are not feasible. The city encourages the use of regional versus on-site
basins for rate control and flood protection. Where flood and rate control basins are not
feasible or desired.
Policy 1.13
Promote the use of overland versus pipe conveyance so that the benefits of natural
channels can be realized. These benefits include filtration, flow attenuation, infiltration,
and other water quality and quantity benefits. The city encourages the use of natural
vegetation within overland conveyance systems.
3.3.2 Water Quality
Goal 2:
Identify and plan for means to effectively protect and improve water quality.
Policy 2.1:
The City will work with the PLSLWD on implementation of TMDL(s) for
impaired water bodies in the City. The City will also complete a Nondegradation
analysis as required by the MPCA.
Policy 2.2:
Actively develop and implement a community education program relating to
preserving and improving water quality. This will principally be carried out through
articles contained in the quarterly newsletter sent to all residents within Prior Lake.
Policy 2.3:
Construct sediment basins at outlets of storm sewers meeting the requirements of
the Nationwide Urban Runoff Program (NURP) criteria, which serve to remove nutrients
and sediments from runoff.
Policy 2.4:
Construct skimmers on new pond outlets to retain floating debris and oils.
Policy 2.5:
Environmental manholes (3 ft. sumps) shall be placed at the last manhole
structure, which is road accessible, prior to discharge to remove sediments. Sump
manholes are scheduled to be cleaned three times per year by the Public Works
Department.
Policy 2.6:
Construct, where practicably feasible, storm water quality ponds which will serve
not only new development, but also existing development where the situation arises to
treat those areas that were established prior to detention pond criteria developed under
EPA's Nationwide Urban Runoff Program (NURP).
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Policy 2.7
Construct rainwater gardens or other smaller water quality retrofits within the
downtown area and as redevelopment occurs in other untreated areas.
Policy 2.8
On-site treatment is the preferred method of implementing water quality. The
more disperse the water quality system the longer lasting its performance. On-site
treatment refers to more than just water quality ponds. It also includes reduced
imperviousness, direct discharge of impervious surface onto pervious and not directly
into the storm sewer system, use of rainwater gardens and filtration devices, and other
such techniques that have the net result of reducing runoff volumes.
3.3.3 Recreation and Fish and Wildlife
Goal 3:
Protect and enhance fish and wildlife habitat and water recreational facilities.
Policy 3.1:
To the greatest possible extent, natural areas shall be preserved, especially when
adjacent to wetland areas.
Policy 3.2:
Buffer zones of natural vegetation shall be maintained around lakes, ponds and
wetlands as much as possible.
Policy 3.3:
Coordinate with the Department of Natural Resources (DNR) to protect rare and
endangered species.
Policy 3.4:
Explore with the DNR and the Prior Lake/Spring Lake Watershed District new
methods of eradicating or controlling eurasion water milfoil.
Policy 3.5:
Enforce the Wetland Conservation Act of 1991 in order to protect wetlands.
3.3.4 Enhancement of Public Participation; Information and Education
Goal 4:
Inform and educate the public concerning urban stormwater management and the
problems pollutants cause if allowed to enter into our water resources.
Policy 4.1 :
Enact a public education program based on the following objectives to reduce
storm water pollution:
1) Raise awareness of the problem and solutions,
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2) Promote community ownership of the lakes,
3) Recognize responsible parties and actions to date,
4) Merge public feedback into program execution.
3.3.5 Public Ditch Systems
Goal 5:
Organize a method in which to manage public ditch systems.
Policy 5.1:
No public ditches have been identified within the City of Prior Lake. If the need
arises, the City will organize a method to manage public ditches.
3.3.6 Groundwater
Goal 6:
Promote ground water recharge, unless prevented by wellhead protection plan.
Policy 6.1 :
Contribute to regional groundwater and source water protection planning.
Policy 6.2:
Provide a permanent ponding volume below the outlet or overflow in ponds and
wetlands to promote ground water recharge.
Policy 6.3:
Maximize infiltration with the use of bioretention basins, infiltration basins, and
other BMPs whenever possible, in open areas within all proposed developments
following the State Stormwater Manual guidelines.
3.3. 7 Wetlands
Goal 7:
Protect and preserve wetlands through administration of the Wetland
Conservation Act.
Policy 7.1
Act as the local government unit responsible for enforcing the Wetland
Conservation Act of 1991.
Policy 7.2
Discourage wetland disturbance. Wetlands must not be drained or filled, wholly
or partially, unless replaced by restoring or creating wetland areas of at equal public
value, as permitted by the Wetland Conservation Act. (Ord. 93-05, 3-15,93)
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Policy 7.3
Up to one-half acre of "debit" wetland (filled or drained) will be allowed to be
replaced through wetland "credit" in a bank which is located outside of Prior Lake's city
limits, but State and County governments are exempt from this policy (M.S. 103G.222
(e)).
Policy 7.4
Restrict clearing and grading within close proximity of the wetland boundary to
provide for a protective buffer strip of natural vegetation to promote infiltration of
sediment and nutrients. In the event that grading occurs close to the wetland boundary
native plant materials shall be reestablished as a buffer strip.
Policy 7.5
Establish for City use a wetland bank account to allow for wetland debits and
credits for city projects.
Policy 7.6
Require that a wetland assessment be prepared for any project that includes a
wetland.
The Wetland Management Plan, section 4 of this report, incorporates assessments
of some of the larger wetlands within the 2020 growth area. These assessments will
serve in lieu of this requirement for these specific locations until the spring of201O. For
projects that involve wetlands not assessed or for projects involving assessed wetlands
after 2010, a wetland assessment using MnRAM methodology must be prepared and
submitted with other review materials as spelled out in the PWDM.
3.3.8 Erosion and Sediment Control
Goal 8:
Prevent erosion of soil into surface water systems through enforcement of City
SWPPPP.
Policy 8.1:
Erosion control plans shall be required for all land disturbance activities. The
erosion control plans shall be consistent with the criteria as outlined in MPCA's
"Protecting Water Quality In Urban Areas". Items to be checked are: silt fence and
strawbale locations, maximum slopes, grading limits, etc.
Policy 8.2:
Temporary sediment basins shall be constructed in areas of new development to
prevent sediment from leaving the construction area, as required by the NPDES Permit.
Policy 8.3:
Streets and property adjacent to construction areas shall be kept free from
sediment carried by construction traffic.
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Policy 8.4:
The City may prohibit work in areas having steep slopes and high erosion
potential. The provisions of the shore land ordinance should be followed to prevent
impact to these erosion sensitive areas.
Policy 8.5:
The City shall maintain a street sweeping program to minimize sediment entering
the drainage system. Streets will be swept twice yearly, once in the spring and once in
the fall, unless overridden by the City SWPPP.
Policy 8.6:
Establishment of temporary and permanent vegetation shall be required to
minimize the time that a graded area remains in an exposed condition.
Policy 8.7:
All eXIstmg storm drain inlets and conveyance systems shall be adequately
protected from sedimentation.
3.3.9 Prior Lake's NPDES Permit
Goal 9:
Operate and manage the City's surface water system consistent with best current
practices and the City's NPDES Permit
Policy 9.1:
Projects to correct existing deficiencies, to the extent they are identified, will be
prioritized as follows:
1. Projects intended to reduce or eliminate flooding of structures in known problem
areas
2. Projects intended to improve water quality in the City's lakes
3. Projects intended to retrofit water quality treatment into downtown redevelopment
activities
4. Projects intended to reduce maintenance costs
5. Projects intended to restore wetlands and habitat
Policy 9.2:
The City will actively inspect, and properly operate, maintain and repair its storm
water system. The City will follow a regular inspection, cleaning, and repair schedule.
Frequency of maintenance will be event-based and informed by experience and
inspection history.
Policy 9.3:
The City will follow best management practices on its own lands and for its own
projects including street reconstruction projects - in accordance with the NPDES
construction site permit and the City's NPDES MS4 Permit.
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3.3.10 Financial Management
Goal 10:
Ensure that the costs of the surface water system are equitably distributed.
Policy 10.1:
The City will continue to update and apply area based charges so that the surface
water related costs of development can be fairly borne by the development.
Policy 10.2:
The City will periodically update its storm water utility rate structure to
accomplish the following:
1. Meet the requirements of its NPDES permit
2. Provide for the maintenance of ponds and outfall structures
3. Conduct repairs to the system
4. Update its system planning efforts
5. Implement rainwater gardens or other water quality retrofits with its downtown or
other untreated redevelopment
3.4 County, State and Federal Agency Requirements
This section of the LSWMP presents a synopsis of the current agency requirements while
acknowledging the existence of other requirements that may be applicable. The City is
committed to the preservation and enhancement of its wetlands and water resources
through full compliance with local, state, and federal wetland regulations.
3.4.1 Minnesota Department of Natural Resources
At the state level, Types 3, 4, and 5 wetlands are protected by the DNR by statute. These
are areas typically recognized as wetlands and are generally characterized by open water
and emergent vegetation throughout most of the year. The state has jurisdiction over
only those wetlands appearing on the state's inventory of protected waters. Further,
wetlands in the inventory were generally those in excess of 10 acres in rural areas or in
excess of 2.5 acres in municipalities and incorporated areas. Map 1 shows some of the
protected waters within the Prior Lake LSWMP study area.
If an area meets the jurisdictional criteria but is not on the state's inventory, it is not
regulated by the DNR. If it does not meet the statutory criteria but is listed on the
inventory, it still is subject to MNDNR regulation. There is no mechanism presently for
adding or deleting wetlands. The inventory was begun in the late 1970s and all state
inventories were completed during the early 1980s.
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The MNDNR rules specify that permits may not be issued for any project except those
that provide for public health, safety, and welfare. Any private development projects are
effectively excluded from permit consideration by this requirement.
The other powers and duties of this Minnesota state agency and its commissioner are
wide-ranging. As they affect surface water management within the City they include:
· Regulation of all public waters inventory waterbodies within the City - to the
extent of their ordinary high water level.
· Regulation of certified floodplains around rivers, creeks, lakes and wetlands.
· Management of the Flood Hazard Mitigation program
3.4.2 U.S. Army Corps of Engineers (USACE)
Under Section 404 of the Clean Water Act, including subsequent modifications, the U.S.
Environmental Protection Agency (EPA) and the U.S. Army Corps of Engineers
(USACE) regulate the placement offill into all wetlands of the U.S. In 1993, there was
a modification of the definition of" discharge of dredged material" to include incidental
discharges associated with excavation. This modification of the "discharge of dredged
material" definition meant that any excavation done within a wetland required the
applicant to go through Section 404 permitting procedures. In 1998, however, this
decision was modified so that excavation in wetlands is now regulated by the USACE
only when it is associated with a fill action.
3.4.3 Board of Water and Soil Resources (BWSR)
The local and regional wetland rules are governed by the Wetland Conservation Act
(WCA). The WCA, passed in 1991, extends protection to all wetlands unless they fall
under one of the exemptions of the WCA. The WCA follows a "no net loss" policy. The
wetlands covered under the WCA must not be drained or filled, wholly or partially,
unless replaced by restoring or creating wetland of at least equal public value under an
approved replacement plan. Replacement ratio is typically 2: 1 (2 acres created for every
1 acre filled) for wetland impacts.
A designated Local Government Unit (LGU) is responsible for making exemption and
no-loss determinations and approving replacement plans. Currently, Prior Lake acts as
the LGU for WCA within the City's subdivision authority.
The powers and duties of this Minnesota state agency also include:
· Coordination of water and soil resources planning among counties, watersheds,
and local units of government.
· Facilitation of communication among state agencies in cooperation with the
Environmental Quality Board.
· Approval of watershed management plans.
3.4.4 Minnesota Pollution Control Agency
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The USACE implements provisions of Section 404 of the Clean Water Act with guidance
from the EP A through a permitting process. The Section 404 permit also requires a
Section 401 water quality certification before it is valid. The EPA has given Section 401
certification authority to the MPCA.
The powers and duties of this Minnesota state agency and its commissioner include:
· Fulfilling mandates from the EP A, particularly in regard to the Clean Water Act.
· Administration of Prior Lake's NPDES Phase II MS4 permit.
· Administration of the NPDES construction site permit program.
· Administration of the NPDES industrial site discharge permit program.
· Development ofTMDLs for waterbodies and watercourses in Minnesota (often in
conjunction with other agencies or joint powers organizations such as watersheds).
3.4.5 Environmental Protection Agency
As it relates to surface water management, this agency is charged with interpreting and
applying aspects of the Clean Water Act. This has led to the City's need for its NPDES
MS4 permit. Total maximum daily load limits, a new initiative mandated by the EP A,
also stem from the EPA's role as steward of the Clean Water Act.
3.4.6 Prior Lake Spring Lake Watershed District and Scott County Watershed
Management Organization
The powers and duties of these Minnesota statutory authorities include:
· Approval authority over local water management plans.
· Ability to develop rules regarding management of the surface water system
· Ability to determine a budget and raise revenue for the purpose of covering
administrative and capital improvement costs.
· Regulation of land use and development when one or more of the following apply:
o The City does not have an approved local plan in place
o The City is in violation of their approved local plan
o The City authorizes the watershed toward such regulation
3.4.7 State and Federal Jurisdictional Boundaries for Public Wetlands and Waters
Wetlands are delineated in accordance with the Federal Manual for Identifying and
Delineating Jurisdictional Wetlands (1987). Wetlands must have a predominance of
hydric soils. Hydric soils, by definition, are inundated or saturated by surface water or
groundwater at a frequency and duration sufficient to support, under normal
circumstances, a prevalence ofhydrophytic (water tolerant) vegetation typically adapted
for life in saturated soil conditions. The USACE and the BWSR regulate wetlands as
defined by a jurisdictional delineation.
For wetlands that fall under the MNDNRjurisdiction, the Ordinary High Water Level
(OHW) determines the boundary ofMNDNRjurisdiction. The OHW is established by
the DNR. A summary of agency jurisdiction is presented in figures 4 and 5.
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MINNESOTA STATE AND FEDERAL JURISDICTION OVER "WATERS"
LAW
EPA / CORPS - CWA. SECTION 404
I~
I
I
I
I
FLOODPLAIN
I . CORPS - RHA, SECTION 10
I
I
I MPCA - CWA, SECTION 401
I
I
LGU/BWSR- ,I
WCA
I
I
I
I
NON
WETLANDS \ '
WATERS OF
THE U.S.
(WETLANDS)
CWA "" CLEAN WATER ACT
RHA == RIVERS AND HARBORS ACT OF 1899
WCA == MINNESOTA WE-lLAND CONSERVATION ACT OF 1991
AGENCY
EPA == u.s. ENVIRONMENTAL PROTECTION AGENCY
CORPS == U.S. ARMY CORPS OF ENGINEERS
BWSR == MINt..jESOTA BOARD OF WATER AND SOIL RESOURCES
MnDNR '" MINNESOTA DEPARTMENT OF NATURAL RESOURCES
LGU == LOCAL GOVERNMENTAL UNIT
MPCA = MINi'lESOTA POLLUTION CONTROL AGENCY
PUBLIC WATERS: WATER COURSE
I
I
I
I
I
\.0
I
FEDERAL
OHW
DNR
OHWL
I
I
I
I I
I I
I I
I I
r I I
I I
I I
I I
FEDERAL DNR
OHW OHWL
WATERS OF THE U.S.
PUBLIC WATERS
WATER COURSE
* THIS LINE COULD CHANGE ELEVATION
AT ANY GIVEN X-SECTION. CONSEQUENTLY
THE JURISDICTION CHANGES
CllY OF PRIOR LAKE
SURFACE WATER MANAGEMENT PLAN
56603113f4.dwg
3.30.05
FIGURE 4
J[lj Bonestroo
-=- Rosene
~ Anderlik &
. \J. Associates
MINNESOTA STATE AND FEDERAL JURISDICTION OVER "WATERS"
I I I 1 I
, CORPS/MP~ I · C9RPS/~CA I
,SEC. 404/4011 SE1. 404 1401 I.
CWA DNR CWA
I I' .1 LGU/ 1 I
I LGU /BWSR I BWSR-I 1
WCA I IWCA', 1
I
I
I
I PUBLIC WATERS WATER BASIN
I .
1
I WATERS OF THE U.S. (WETLANDS)
I~~:L~I
(BASED ON ELEVATION
DETERMINED BY DNR)
DNR PROTECTED WATER BODY
I NON-WETLAND
I
I
I
1
I
I
I
JURISDICTIONAL
WETLAND
DELINEATION
I
I
I
I
I
I
I
I
JURISDICTIONAL
WETLAND
DELINEATION
I
I
. I
I
LGU / BWSR- WCA 1
I
1
I
I
I
I
I
I
WATERS OF THE U.S. I
(WETLANDS) I
JURISDICTIONAL
WETLAND
DELINEATION
· CORPS/MPCA
SEC. 404/401
CWA
I
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I
1
1
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1
I
JURISDICTIONAL
WETlAND
DELINEATION
NON-DNR PROTECTED WATER BODY
LAW
CWA = CLEAN W,A.TER ACT
weA == MINNESOTA WETLAND CONSERVATION ACI
· U.S. ARMY CORPS OF ENGINEERS.
THE CORPS HAS JURISDICTION ON
WETUINDS THAT ARE PART OF, OR
CONNECTED BY TRIBUTARY, TO A
NAVIGABLE WATER.
AGENCY
CORPS = U.S. ARMY COI'<PS OF ENGINELHS
BWSR = MINNESOT,t, BOt\RD OF WATER AND SOIL RESOURCES
DNR = MINNESOTA DEPARTMENT OF NATURAL RESOURCES
!_GU 0= LOCAl. GOVERNMENTAL UNIT
MPCA = MINNESOTA POLLUTION CONTROL ,t\GU~CY
CITY OF PRIOR LAKE
SURFACE WATER MANAGEMENT PLAN
FIGURE 5
J{]j Bonestroo
-=- Rosene
~ Anderlik &
. \j. Associates
PUBLIC WATERS: WATER BASIN
56603113f5.dwg
3.30.05
MINNESOTA STATE AND FEDERAL JURISDICTION OVER "WATERS"
LAW
I.
I
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I
FLOODPLAIN
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I
NON
WETLANDS: .
WATERS OF
THE U.S.
(WETLANDS)
CWA = CLEAN WATER ACT
RHA == RIVERS AND HARBORS ACT OF 1899
WCA "" MINNESOTA WETLAND CONSERVATION ACT OF 199 i
AGENCY
EPA == u.s. ENVIRONMENTAL PROTECTION AGENCY
CORPS == U.S. ARMY CORPS OF ENGINEERS
BWSR == MINNESOTA BOARD OF WATER fiND SOIL RESOURCES
MnDNR == MINNESOTA DEPARTMENT OF NATURAL RESOURCES
LGU "" LOCAL GOVERNMENTAL UNIT
MPCA = MINNESOTA POLLUTION CONTROL AGENCY
PUBLIC WATERS: WATER COURSE
EPA / CORPS - CWA, SECTION 404
I _ CORPS - RHA, SECTION 10
I
I
I MPCA - CWA, SECTION 401
I
I
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I
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DNR
OHWL
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I-
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FEDERAL
OHW
DNR
OHWL
I
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I
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rl
I
I
I
FEDERAL
OHW
WATERS OF THE U.S.
PUBLIC WATERS
WATER COURSE
* THIS LINE COULD CHANGE ELEVATiON
Al ANY GiVEN X-SECTION. CONSEQUENTLY
THE JURISDiCTION CH/',NGES
CITY OF PRIOR LAKE
SURFACE WATER MANAGEMENT PLAN
56603113f4.dwg
3.30.05
FIGURE 4
J[]J Bonestroo
-=- Rosene
~ Anderlik &
. ~. Associates
MINNESOTA STATE AND FEDERAL JURISDICTION OVER "WATERS"
I I I I I
, .CORPS/MPC~ I · c9~PS/~CA I · CORPS/MPCA
I SEC. 404/4011 SE9. 404 1401 I' SEC. 404/401
CWA DNR CWA
I I ILGU/I 1 CWA
I LGU/BWSR I BWSR-1 I LGU / BWSR- WCA
WCA 1 I'WCA'I 1
1
I
I
1
I
1
I
I
I
JURISDICTIONAL
WETLAND
DEUNEATION
I NON-WETLAND
1
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JURISDICTIONAL
WETLAND
DEUNEATION
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I WATERS OF THE U.S.
I (WETLANDS)
JURISDICTIONAL
WETLAND
DEUNEATION
I
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1
I PUBLIC WATERS WATER BASIN
I'
I
I WATERS OF THE U.S. (WETLANDS)
1~8II:c~1
(BASED ON ELEVA~ON
OETERMINED BY DNR)
DNR PROTECTED WATERBODY
JURISDIC~ONAL
WETlAND
DEUNEA~ON
NON-DNR PROTECTED WATERBODY
LAW
CWA == CLEAN WATER ACT
WCA == MINNESOTA WETl.AND CONSERVATION ACT
. U.S. ARMY CORPS OF ENGINEERS.
THE CORPS HAS JURISDICTiON ON
WETLN1DS THAT ARE PART OF, OR
CONNECTED BY TRIBUTARY, TO A
NAVIGABLE WATER.
AGENCY
CORPS = U.S. ARMY COF~PS OF ENGINEmS
BWSR = MINNESOTA BOf\RD OF WATER AND SOIL RESOURCES
DNR = MINNESOTA DEPARTMENT OF NATURAL RESOURCES
LGU ::: LOCAL GOVERNMENTAL UNIT
MPCA = MINNESOTA POLLUTION CONTROL AGH~CY
FIGURE 5
J[]J Bonestroo
-=- Rosene
~ Anderlik &
. \j. Associates
PUBLIC WATERS: WATER BASIN
CITY OF PRIOR LAKE
SURFACE WATER MANAGEMENT PLAN
56603113f5.dwg
3.30.05
3.5 Agency Contacts
The primary contacts for local regulating agencies described above are presented below.
These contacts are accurate as of December, 2004.
City of Prior Lake
City Engineer
City of Prior Lake
16200 Eagle Creek Ave. S.E.
Prior Lake, MN 55372
(952) 447-9830
Director of Public Works
City of Prior Lake
17073 Adelmann St. S.E.
Prior Lake, MN 55372
(952) 440-9890
Scott County WMO
c/o Scott County Natural Resources Manager
Scott County
200 Fourth Avenue West
Shakopee, MN 55379
(952) 496-8054
Prior Lake Spring Lake Watershed District
District Administrator
15815 Franklin Trail, Suite 100
Prior Lake, MN 55372
(952) 447-4166
Scott County Soil and Water Conservation District
District Manager
Scott County Soil and Water Conservation District
7151 West 190th Street, Suite 125
Jordan, MN 55352
(952) 492-5425
Minnesota Department of Natural Resources
Area Hydologist
Minnesota Department of Natural Resources
n City of Prior Lake
-=-
lit Local Surface Water Management Plan
3-16
1200 Warner Road
St. Paul, MN 55106
(651) 772-7910
Board of Water and Soil Resources
Board Conservationist
Board of Water and Soil Resources
One West Water Street, Suite 200
St. Paul, MN 55107
(651) 296-3767
3.6 Water Resource Management-related Agreements
The City of Prior Lake is party to a 1981 joint powers agreement with the City of
Shakopee and the Prior Lake Spring Lake Watershed District regarding the outlet channel
for Prior Lake.
3.7 Impacts of the Prior Lake LSWMP on Other Units of Government
Upon approval of this LSWMP by the two watersheds with jurisdiction over the City, it is
the City's intent to assume all permitting powers within it jurisdiction. Currently, the
Scott County WMO does not issue permits, so no impact to this organization would
occur. The Prior Lake Spring Lake Watershed District does issue permits for any
planned activity that disturbs more than 10,000 square feet of land area. This threshold
rises to one acre if the activity is not near a lake, wetland, or the Prior Lake outlet
channel.
Since the watershed would still permit activities outside the City's jurisdiction its permit
process would remain in place. Within its jurisdiction, the City will use the permit
submittal requirements outlined in the watershed rules and updates. This will ensure
consistency of approach for all projects.
The PLSL WD would continue in its role as a project review agency though it may defer
to the City review process for projects that don't have a direct impact on Prior Lake or
the Prior Lake outlet channel. The Prior Lake-Spring Lake Watershed District will also
continue to have responsibility for water quality monitoring.
The Prior Lake LSWMP envisions the City and its two watersheds as partners in
implementing this plan. In the PLSL WD lands, the City envisions the watershed taking
the lead on water quality and lake water quality issues. The City and watershed would be
equally responsible for implementation of the volume management targets discussed in
Section 5 of this Plan with the City taking the lead in the 2020 expansion areas and the
watershed taking the lead in areas outside the 2020 boundary,
n City of Prior Lake
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lit Local Surface Water Management Plan
3-17
The Scott County WMO portion of the City's current and 2020 boundary drains toward
the City of Shakopee and Credit River. The flows and routes that discharge from Prior
Lake into Shakopee were developed in consultation with the City of Shakopee and in
conjunction with their surface water planning efforts. Further coordination will be
needed to address the concerns of the WMO when areas within the upper reaches of the
WMO watershed are proposed for development.
3.8 Watershed Goals and Strategies that Affect the City of Prior Lake
The City of Prior Lake goals and policies, outlined above, are a close reflection of those
of the watersheds, only presented through the municipal filter. The PLSL WD has, over
the past two years, developed goals related to volume management that will have a
profound affect on the City as it implements its surface water system. Specifically, the
PLSLWD is looking for 1,500 to 3,000 acre-feet of retention storage so that the impact of
future development on Prior Lake and its outlet channel can be mitigated.
The PLSLWD has determined that increasing the outlet channel's permitted capacity
above the current 65 cubic feet per second is not a viable option due to high cost,
permitting problems, and downstream environmental impacts. What is needed is
retention storage, which is storage without discharge. This retention volume, once filled,
is emptied through evaporation, infiltration, or transpiration. One method of increasing
the capacity of volume storage is through the restoration and creation of new wetland
areas within the agricultural areas of the district. Through partnership with the District,
these potential wetland creation sites can be planned for and set aside for the benefit of
water quality and volume storage. District identified wetlands and low areas can be
pursued through the City process as a result of this partnership
The 1,500 to 3,000 acre-feet of storage needed presents a challenge to both the City and
the watershed, as does obtaining this storage while managing the City's wetlands
according to the Wetland Management Plan. A large part of the volume storage will
occur in existing natural wetlands. Each wetland will be rated for its susceptibilities to
retaining the necessary volumes, forming a balance needed for wetland preservation with
the system's volume storage needs. This approach, as well as the creation of new
wetland areas will allow both wetland preservation and volume management to be
accomplished.
n City of Prior Lake
-=-
lit Local Surface Water Management Plan
3-18
4. WETLAND MANAGEMENT PLAN
4.1 Wetland Inventory Goals
The goal of this wetland inventory is the management of wetlands based on the functions
they perform and to determine appropriate protection strategies for stormwater discharge
to the wetlands if a land use change occurs that triggers a NPDES permit. Since smaller
wetlands are not typically used as major components in a stormwater storage system, we
focused our inventory on wetlands shown on the National Wetland Inventory (NWI) Map
that were over 0.5 acres in size.
The inventory and assessment of wetlands allows the city to set up priorities. This plan
includes a wetland inventory and ranking system that will assist the city in establishing
priorities and focusing available resources for wetland protection, enhancement and
restoration. Because all wetlands have value, all are protected, to some degree, in this
plan.
The plan is designed to provide the following benefits:
· Provide wetland inventory, assessment, and management information:
· Aid in administration of the Wetland Conservation Act (WCA) by providing
information regarding the wetlands functions:
. Enhance wildlife values of wetlands:
. Provide and enhance recreational values:
. Designate wetland restoration/enhancement opportunities:
· Protect wetlands and adjacent resources that provide valuable ecological support:
. Provide stormwater protection for wetlands.
It should be noted that this wetland inventory has been created for planning purposes
only. Regulation of activities potentially impacting individual wetlands will be based on
a site-specific delineation of the wetland boundary as part of a proposed project.
4.2 Wetland Identification
Wetland identification numbers used in the wetland inventory are based on the township,
range and section in which the wetlands exist.
n City of Prior Lake
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lit Local Suiface Water Management Plan
4-1
Each wetland is identified by the following numbers: county code (CC), township (T),
range (R), section (S) and then an individual number for the wetland within the section.
The following is an example of the wetland ID.
19 115 22 23 003
CC T R S Wetland No.
The wetland designation can be found on the Wetland Inventory Maps located in the back
of the report.
4.3 Wetland Mapping
An ARC/INFO Geographic Information System (GIS) was used to aid in the inventory
and final mapping of wetlands within the study area. The GIS database provides the city
with a map that can be easily updated and integrated with other mapped data. The
wetland Map includes the wetland location, estimate of the wetland boundary and
Rankings that describe overall quality of the basin. The Stormwater Drainage Map
includes the wetland location, estimated wetland boundary and Stormwater Rankings that
describe susceptibility of the wetland to stormwater impacts. Preliminary layouts for
future development should consider the wetland boundaries on the map as a guide. The
wetland boundaries should be delineated early in the platting process to avoid
development within the wetlands and buffer zones.
Since smaller wetlands are not typically used as major components in a stormwater
storage system, the scope included collecting field information on all wetlands that are
over 0.5 acres in size based on the NWI. The wetlands evaluated are shown on the
Functional Ranking Map. It is important to note that several of the "not inventoried"
wetlands shown on the map were less than the 0.5 acres based on the NWI, but based on
Minnesota Land Cover Classification System (MLCCS) and field observations the
boundaries have been enlarged to encompass the approximate entire wetland boundary.
4.4 Wetland Evaluation Methodology
4.4.1 Minnesota Routine Assessment Method
Wetlands are valued for a wide range of functions they perform, such as improving water
quality, flood water attenuation, and providing fish and wildlife habitat. Recently,
wetland scientists have developed methods to assess the functions of individual wetlands.
The assessment evaluates characteristics such as plant community diversity and structure,
connectivity to other habitat types, location in the watershed, and a wide range of other
factors. The assessment is like a "report card" which evaluates the wetland's functions
and quality.
A combination of the Minnesota Routine Assessment Method Version 2.0 and 3.0
(MnRAM) was used to assess the functions of all the wetlands inventoried for this plan.
This method was developed by the Minnesota Interagency Wetland Group as a field
n City of Prior Lake
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lit Local Surface Water Management Plan
4-2
evaluation tool to assess wetland functions on a qualitative basis. It is intended to
document the field observations and interpretations of professionals who have had
training and experience in wetland science. This method is not intended to be a rigid
procedure but rather an aid to complement trained observation and interpretive skills with
additional qualitative evaluation.
Wetlands were visited by trained personnel using MnRAM to assess wetland functions
for Hydrologic Regime, Aesthetics, Restoration Potential, Wildlife Habitat, and Floral
Diversity/Integrity. A ranking for Exceptional, High, Medium, and Low were provided
for each function assessed and the results are provided in Appendix E.
4.4.2 Database
All the MnRAM data sheets were entered into a database available for use by the City.
The database allows for quick retrieval of information for each wetland and allows
queries to be performed to complete special searches within the database. For example, a
search can be done to list all the wetlands that have high floral diversity.
4.5 Required Submittals at the Time of Development
In addition to a wetland delineation a Minnesota Routine Assessment Method 3.0
(MnRAM) should be provided to the City for each wetland located on the property at the
time of development. MnRAM can be applied by a wetland professional hired by the
applicant or it can be completed by the City with the time billed back to the applicant.
The City or County will utilize the completed MnRAM and compare the condition of the
wetland at the time of this plan's inventory with the existing condition and determine if
the wetland ranking for storm water and buffer protections should be modified. The
Wetland Ranking will be determined based on the following section "Wetland Ranking
Methodology" .
4.6 Wetland Ranking Methodology
Following the assessments of wetland functions, the next step in developing this plan was
the ranking of each wetland for future management. Management recommendations are
closely related to the functions each wetland performs in comparison to other wetlands in
the study area.
It is important to note that the comparison domain for the wetlands is the study area. It is
possible that a wetland found within the study area may not be considered to be of high
quality if compared to a wetland in northern Minnesota, but in comparison with wetlands
in the area, the wetland may be valuable for the functions it performs.
4.6.1 Habitat ProtectionlWetlaod Ranking
An overall functional ranking for the wetlands within the study area categorized the
wetlands into Unique, High, Moderate, and Low. These rankings are based on the
wetland floral diversity/integrity combined with the wildlife habitat ranking determined
n City of Prior Lake
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lit Local Surface Water Management Plan
4-3
from MnRAM. The process that was used to determine the overall functional ranking is
presented in detail in the Overall Functional Ranking Flow Chart (Figure 6). The ranking
for the wetlands is shown on the Functional Ranking Map.
4.6.2 Stormwater Protection Ranking
One of the purposes of this Wetland Inventory was to determine stormwater protection
standards for wetlands. There are many types of wetlands, each determined by its
hydrology and vegetative composition.
The wetland's sensitivity to stormwater input is dependent on the wetland's community
type and the quality of its plant community. Some wetlands (e.g., sedge meadows with
carex species) are sensitive to disturbance and will show signs of degradation unless
water quality, bounce and duration are maintained at existing conditions after
construction. On the other hand, there are other wetlands (e.g., floodplain forests) which
are better adapted to handle the fluctuating water levels and influx of sediment often
associated with stormwater.
Site visits to the wetlands included a determination of the wetland plant community (-
ities) and Floral Diversity using the key provided in MnRAM Version 2.0 and 3.0. The
Guidance for Evaluating Urban Storm Water and Snowmelt Runoff Impacts to Wetlands
completed by the State of Minnesota Storm Water Advisory Group was used as a guide
in the determination of wetland sensitivity to stormwater.
This document divides wetlands into rankings that include: highly susceptible,
moderately susceptible, slightly susceptible, and least susceptible. These rankings are
provided on the Stormwater Susceptibility Map. The following are the procedures that
were used to determine the wetland susceptibility ranking.
Highlv Susceptible: A wetland is considered highly susceptible if:
. Forty percent or more of the wetland complex has a highly susceptible wetland
community (-ities) as shown in Table 4.1 and;
. Highly susceptible wetland community (-ities) have medium to exceptional floral
diversity/integrity.
Moderately Susceptible: A wetland is considered moderately susceptible if:
. Forty percent or more of the wetland complex has a moderately susceptible
wetland community (-ities) as shown in Table 4.1 and;
. Moderately susceptible wetland community (-ities) have medium to exceptional
floral diversity/integrity.
Slightly and Least Susceptible: Wetlands with low floral diversity, as determined by
n City of Prior Lake
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U Local Surface Water Management Plan
4-4
MnRAM, were considered to be least susceptible wetlands. Wetlands that do no fall
under the high, moderate, or least susceptible categories are considered slightly
susceptible. (Note: This category also includes wetlands or wetland complexes that
contain 40 percent floodplain forest, which is a slightly susceptible wetland
community, with medium to exceptional floral diversity.)
n City of Prior Lake
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lit Local Surface Water Management Plan
4-5
Wetland is a National Heritage Site or
has rare, threatened and/or endangered .....
plant and animal species as mapped by
the National Heritage Program
associated with wetlands present within
9 km
I
No
Exceptional or high floral
diversity/integrity
No
Medium floral
diversity/integrity
No
Low floral diversity/integrity
Yes
Yes
Overall Functional Ranking Flow Chart
- Yes
Unique
Yes
Yes
Exceptional or high wildlife y,,~
High
No
Medium high or
medium wildlife
~~~ Moderate
Exceptional to high
wildlife
No
~I Low
Figure 6
Surface Water Management Plan
Prior Lake, Minnesota
\566\S6603114\cad\graphics\FuncRank_FIoWChrt.ppt
~ =roo
-=- Andertik &
1\11 Associates
Engineers & Architects
Table 4.1
Wetland Community Susceptibility to Stormwater Impacts
Highly Susceptible Wetland Communities* Moderately Susceptible
Wetland Communities*
Low Prairies Shrub-Carrs
Coniferous Swam s Alder Thickets
Hardwood Swam s Fresh wet Meadows
Seasonall Flooded Basins Shallow Marsh
Calcareous Fens Dee Marsh
* Wetland community (-ities) determined using key provided in MnRAM Version 2.0.
4.7 Wetland Management Standards and Recommendations
All of the inventoried wetlands within the study area were classified for Stormwater and
Habitat Protection. Stormwater Protection standards are listed in Table 4.2 and 4.3 and
Habitat Protection Recommendations are listed in Table 4.4. The Stormwater Protection
Standards include Water Quality and Quantity Protection. The Habitat Protection
Recommendations include Buffer Zones and No Grading Recommendations.
The following sections provide details of each protection strategy developed for wetlands
within the City and County.
4.7.1 Water Quality
Water quality plays a significant role in the overall quality ofa wetland. When the
quality of the incoming water declines, the wetland's plant community may change to
fewer numbers of species and retain only those species that are tolerant of high nutrient
and sediment loads. Once a wetland's plant community is changed, the wetland's
character and ecosystem will change, often to a less valuable system in terms of
biodiversity, habitat for wildlife, and aesthetic enjoyment. Pretreatment
recommendations have been developed to maintain the character of the wetland. BMPs
can be used to accomplish the pretreatment requirements given in Table 4.2.
Table 4.2
Stormwater Protection Standards
Management Category Stormwater Phosphorus Pretreatment
Recommendations
150 b
200 b
200 b
Least Susce tible 250 b
1) Includes lakes, creeks, streams, and rivers (as defined by the USGS).
2) A multi-cell configuration with lower cell being a constructed wetland or infiltration basin is
recommended to achieve these levels of removal.
n City of Prior Lake
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lit Local Surface Water Management Plan
4-7
4.7.2 Water Quantity
In the recent past, surface water management plans have protected wetlands from
nutrients but not water fluctuations or duration. In fact, it was common to use wetlands
to reduce flooding potential through sizing storm sewer pipes to maximize bounce and
detention time in wetlands.
This plan addresses stormwater quantity impacts to wetlands by providing protection
strategies to maintain the existing integrity of the wetland through special protection
strategies for highly, moderately, and slightly susceptible rankings and are described in
Table 4.3 below.
Table 4.3
Wetland Quantity Standards
Hydroperiod Highly Moderately Slightly Least
Standard Susceptible Susceptible Susceptible Susceptible
Storm Bounce Existing Existing plus Existing plus 1 No limit
lOO-year 0.5 feet foot
Discharge Rate Existing Existing Existing or less Existing or less
Inundation Existing Existing plus I Existing plus 2 Existing plus 7
Period for 1 & day days days
2 yr
precipitation
event
Inundation Existing Existing plus 7 Existing plus Existing plus
Period for 10 yr days 14 days 21 days
precipitation
event
Outlet control Existing Existing (0-2 ft above (0-4 feet above
elevation existing existing
overflow) overflow)
"Existing" in this chart means the existing hydrologic conditions. If there have been
recent significant changes in conditions, it means the conditions that established the
current wetland.
4.7.3 Wetland Buffer Strip and Setback Protection
A wetland buffer is a vegetated area that surrounds a wetland and reduces negative
impacts to wetlands from adjacent development. The needs identified for the
establishment of wetland buffers are related to the functions that wetlands perform.
Wetlands perform a variety of functions such as groundwater recharge, stormwater
retention to improve water quality and reduce flooding, and wildlife habitat. Wetlands
n City of Prior Lake
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lit Local Surface Water Management Plan
4-8
are often neighborhood amenities because they can provide screening from adjacent
neighbors and wildlife viewing opportunities.
Wetland buffers can help mitigate potential development impacts to wetlands by reducing
erosion by stormwater; filtering suspended solids, nutrients, and harmful substances; and
moderating water level fluctuations during storms. Buffers also provide essential wildlife
habitat for feeding, roosting, breeding, and rearing of young, and cover for safety,
movement, and thermal protection for many species of birds and animals.
Buffer Width Effectiveness for Wetland Protection
Buffer strips help mitigate the impacts of development adjacent to wetlands. Catch
basins and storm sewers typically collect street and front yard drainage and direct the
drainage to an appropriately sized pond for pretreatment prior to discharge to a wetland
or waterbody. Backyard drainage typically reaches wetlands or waterbodies without
pretreatment, thereby allowing lawn and garden chemicals, sediments, pet wastes,
fertilizer and other types of contaminants to directly impact the receiving waterbody.
Buffer strips can provide needed treatment of stormwater drainage to protect wetlands
from human impacts as areas develop. A secondary benefit is valuable habitat protection,
especially near aquatic areas. Habitats adjacent to aquatic areas generally have a higher
density of bird species than other habitats (Johnson, 1992). The reasons for this include:
the proximity of habitat requirements (i.e., food, cover, and water), the increased number
of niches (because of wider diversity of plant species and structure), and the high edge-
to-area ratio that results from the linear shape of most riparian zones (MPCA, 1997).
As the buffer width increases, the effectiveness of removing sediments, nutrients, and
other pollutants from surface water increases. In additions, as buffer width increases,
direct human impacts, such as dumped debris (i,e., garbage, lawn and garden cuttings, or
fill) and trampled vegetation will decrease. A field study of wetland buffers in Seattle
showed that 95% of buffers less than 50 feet wide suffered a direct human impact within
the buffer, while only 35% of buffers wider then 50 feet suffered direct human impact
(Schueler, 1995). An overview of scientific literature on wetland buffers suggests the
following minimum buffer widths for protection of these buffer functions (MPCA, 1997):
Water Quality Protection: 25 feet or more
(Depends on vegetation, slope, density and type of adjacent land use and quality of
receiving water)
Protection from human encroachment:
50 - 150 feet or more
Bird Habitat preservation:
50 feet or more
Protection of threatened, rare or endangered species:
100 feet or more
n City of Prior Lake
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lit Local Surface Water Management Plan
4-9
Although these buffer widths are suggested by the MPCA, the Wetland Conservation Act
may require a different minimum buffer width to obtain wetland credits. The most recent
Wetland Conservation Act Rules should be reviewed to determine the minimum buffer
widths for credits.
Setbacks of 10 feet between structures and the edge of the buffer are the minimum
recommended by the Minnesota Pollution Control Agency (MPCA, 1997). A setback of
20 feet from the wetland is recommended as part of this plan to insure there is usable
space between structures and buffers and to prevent encroachment of lawns into buffer
areas. For purposes of this plan a structure is anything which is built or constructed, an
edifice or building of any kind, or any piece of work artificially build up or composed of
parts jointed together in some definite manner.
Buffer strip features outlined in Table 4.4 below, are recommended standards based on
the wetland management classifications that are shown on the Functional Classification
Map. The purpose of these features it to mitigate the impacts (e.g., stormwater, human
encroachment, etc.) of development. The PWDM will spell out specific buffer
requirements required for development.
Wetland T e
Buffer Strip
Average
Width
Buffer Strip
Minimum
Width
Structural
Setback
Distance
Native
Vegetation in
Buffer Stri
Table 4.4
Recommended Wetland Buffer Strip Features
Uni ue
40 feet from
delineated
wetland ed e
30 feet from
delineated
wetland ed e
20 feet - from
upslope buffer
edge to
building or
other structure
* Requirements
below
Hi h
30 feet from
delineated
wetland ed e
20 feet from
delineated
wetland ed e
20 feet - from
upslope buffer
edge to
building or
other structure
* Requirements
below
Moderate
20 feet from
delineated
wetland ed e
15 feet from
delineated
wetland ed e
20 feet - from
upslope buffer
edge to
building or
other structure
** Optional
Low
15 feet from
delineated
wetland ed e
10 feet from
delineated
wetland ed e
20 feet - from
upslope buffer
edge to
building or
other structure
** Optional
* Buffer area vegetation shall be considered adequate when the buffer has a continuous
dense layer of perennial grasses, flowers, trees and/or shrubs. Vegetation shall be
considered unacceptable if:
I) It is composed of noxious weeds (70% or more); or
2) Topography or sparse vegetation tends to channelize the flow of surface water; or
3) For some other reason the vegetation is unlikely to retain nutrients and sediment.
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** While native vegetation is not required as part of this plan, a buffer may not be
acceptable for Public Value Credit under the Wetland Conservation Act ifit does not
contain native vegetation.
4.8 Wetland RestorationlEnhancement Opportunities
Wetland restoration/enhancement sites were identified during the field inventory and will
be further investigated at the time of development. The wetland restoration portion of the
filled out MnRAM will be reviewed at the time of development to determine the potential
for restoration of wetlands on the property. The potential for wetland restoration will be
determined based on the ease with which the wetland could be restored, the number of
landowners within the historic wetland basin, the size of the potential restoration area, the
potential for establishing buffer areas or water quality ponding, and the extent and type of
hydrologic alteration.
Wetlands and low areas identified by the PLSLWD for volume management and wetland
restoration should be considered. Wetlands that have hydrologic restoration proposed
would likely qualify as wetland banking or mitigation sites if restored. Wetland banking
is a type of mitigation, or replacement for wetland losses, allowed under State and
Federal rules. Wetland banking allows the appropriate amount and type of wetland
acreage to be purchased from an account holder who has a "bank" of functioning
wetlands. These wetlands may have been restored from previously drained or filled
wetlands, or created where wetlands did no previously exist. Wetland banking is
contrasted with project-specific replacement where the project sponsor creates or restores
a wetland specifically to replace a wetland that is being drained or filled. Project specific
replacement is usually done on-site, while wetland banks are typically located in another
place in the community or watershed. Site-specific replacement should be encouraged
when a wetland restoration or creation is possible on-site. When site-specific
replacements are not ecologically appropriate, then wetland banks located within the City
and County should be the next priority. The funding for the wetland restoration sites can
come from a variety of sources, which include:
· BWSR Banking Money for Road Construction Projects
· Department of Natural Resources, Conservation Partners and Community
Environmental Partnerships grants
· Department of Natural Resources Greenway grants
· Soil and Water Conservation District grants
· Prior Lake Spring Lake Watershed District grants
4.9 Wetland Stewardship
There are a number ofthings that residents, cities, or counties can do voluntarily to
enhance wetlands and buffer strips that surround wetlands. This section describes some
of these practices.
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4.9.1 Enhancement
Native wildflowers, grasses, shrubs and trees can be planted in the wetland or the
adjacent buffer areas to enhance habitat and stormwater filtering. Habitat can be
enhanced by creating more vertical layers (such as adding trees or shrubs where these are
absent), and by adding plants that provide food and cover, such as fruiting shrubs.
Increasing the structural and plant species diversity in the landscape provides additional
habitat niches, and can increase the numbers and species of animals using the area. Many
of these plants also make the landscape more attractive for human inhabitants.
Species that are native to the area will probably require the least maintenance, survive
harsh Minnesota weather more easily, and provide the greatest habitat benefits. The book
Landscaping for Wildlife by Carroll Henderson and other references that are available in
most bookstores or from Minnesota Extension Services, can help landowners to add
plants that enhance the wetland and increase the variety of attractive plants and wildlife.
4.9.2 Control of Invasive Exotic Species
Several non-native species (sometimes called exotics) have become problems in
Minnesota wetlands and adjacent uplands. These include purple loosestrife, European
buckthorn, black locust, reed canary grass, and leafy spurge. These plants invade native
plant communities and can take over rapidly, eliminating native plants that provide
important food and habitat benefits.
Invasion by exotic species can be controlled by minimizing disturbance to wetlands and
buffer areas as much as possible to avoid the creation of openings for exotics to invade.
Small populations of many exotic species can be controlled by hand removal or direct
application of appropriate herbicides that are licensed for use near water. The Minnesota
DNR provides information about identifying or controlling exotic species around
wetlands.
4.9.3 Habitat Structures
Wetlands provide important habitat for many species of birds and other animals. Adding
wood duck nest boxes and other types of nesting structures for ducks and other birds can
augment nesting habitat, help birds to avoid predators, and enhance opportunities to view
and enjoy wildlife. The Minnesota DNR, Minnesota Waterfowl Association, and other
habitat enhancement organizations can provide information about the types and sources
of structures available. Retaining or adding stones, logs, and dead trees near wetlands
and within buffers provides habitat for turtles, other reptiles and amphibians, and resting
areas for birds and animals.
Habitat areas may also become refuges for large populations of deer, geese, and wildlife
that may become a nuisance in urban areas. When needed, population control measures
should be included in management plans for these areas. Minnesota DNR staff can
provide assistance in the development and implementation of these plans.
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4.9.4 Learning Opportunities
Schools and other organizations can adopt wetlands and adjacent areas for use as outdoor
classrooms. Students, parents, and teachers can add native wetlands and upland plants,
habitat structures, and other enhancements to increase learning opportunities and
encourage other wetland owners in the area to make similar enhancements.
II City of Prior Lake
U Local Surface Water Management Plan
4-13
5. SYSTEM ASSESSMENT AND DESIGN
5.1 General
This section of the Prior Lake Local Surface Water Management Plan (LSWMP) serves
two functions. The system assessment portion catalogues the various assessments of
problems that the Plan must address whether they relate to water quality, wetland
protection, flooding, volume management, or lakes management. The intent is to identify
the source of problems and, more importantly, specific actions the City will take to
address these problems either independently or in collaboration with some other
organization - most commonly one of the watershed management organizations.
The system design portion of this section describes the 2030 growth area surface water
management system. This system is shown in maps I through 5. The discussion of the
proposed system revolves around answering the following questions:
· What are the general drainage patterns of the 2030 and existing system?
· What does the 2030 system entail in terms of storage, conveyance, volumes, and
discharge rates?
· Where does the proposed system discharge and what constraints in the existing
system limit discharge of the 2030 system?
· What is the impact of agricultural drainage, outside the 2030 growth area, on the
proposed and existing urban system?
· How have proposed wetland bounce, and duration of HWL, been determined by
management guidelines of the Wetland Management Plan, section 4 of the
LSWMP?
· What opportunities exist for obtaining the retention storage identified by the
PLSL WD both in the 2030 growth area and outside it?
· What is the impact of the City of Prior Lake's 2030 urban system on agricultural
areas and other municipalities?
· Are there any existing ponds where calculated HWL is a concern?
Maps I through 5 show the major drainage divides, storage areas, conveyance (including
pipe and channels), wetlands and lakes that have been incorporated into the Prior Lake
LSWMP.
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The purpose of Maps I through 5, and the system design portion of this section, is to
identify and quantify the infrastructure needed to allow continued development in Prior
Lake while avoiding the negative impacts, such as flooding and water resource
degradation, often associated with development.
5.2 System Assessment
5.2.1 Water Quality Assessments
5.2.1.1 Clean Water Act Assessments
A number of water bodies within the existing City and its 2030 growth boundary are
listed in the state impaired waters list. Known as the 303(d) list from the applicable
section of the federal Clean Water Act, these waters are ones that do not currently meet
their designated use due to the impact of a particular pollutant or stressor. If monitoring
and assessment indicate that a water body is impaired by one or more pollutants, it is
placed on the list. At some point a strategy would be developed that would lead to
attainment ofthe applicable water quality standard. The process of developing this
strategy is commonly known as the Total Maximum Daily Load (TMDL) process and
involves the following phases:
I. Assessment and listing
2. TMDL study
3. Implementation plan development and implementation
4. Monitoring of the effectiveness of implementation efforts
Responsibility for implementing the requirements of the federal Clean Water Act falls to
the U.S. Environmental Protection Agency (USEPA). In Minnesota the USEPA
delegates much of the program responsibility to the state Pollution Control Agency
(MPCA). Information on the MPCA program can be obtained at the following web
address: htto://www.oca.state.mn.us/water/tmdVindex.html. The following is an excerpt
from the MPCA website describing the program and its need:
The Clean Water Act requires states to publish, every two years, an updated list of streams and
lakes that are not meeting their designated uses because of excess pollutants. The list, known as
the 303(d) list, is based on violations of water quality standards and is organized by river basin.
Environmental organizations and citizen groups have sued the EP A because states have not made
adequate progress to meet Section 303( d) requirements. The EP A has been sued for various
reasons. Over the past 10 years, lawsuits have been filed in 42 states and the District of Columbia.
Of those, 22 have been successful. There is currently no such lawsuit in Minnesota. However,
beyond the federal requirements, there are many reasons for us to move forward with the
development ofTMDLs. Foremost is the need to clean up our rivers, streams and lakes to
maximize their contributions to the state's economy and quality of life and to protect them as a
resource for future generations.
For each pollutant that causes a water body to fail to meet state water quality standards, the federal
Clean Water Act requires the MPCA to conduct a TMDL study. A TMDL study identifies both
point and nonpoint sources of each pollutant that fails to meet water quality standards. Water
quality sampling and computer modeling determine how much each pollutant source must reduce
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its contribution to assure the water quality standard is met. Rivers and streams may have several
TMDLs, each one determining the limit for a different pollutant.
Table 5.1 lists the 303(d) impaired waters within the existing boundary and 2030 growth
boundary.
Table 5.1
303(d) 2004 Final List ofImpaired Waters
Within the City of Prior Lake and its 2030 Growth Area
TMDL
Water Body Year First DNR# Affected Pollutant or start!
Listed Use Stressor TMDL
complete
Spring 2002 70-0054 Aquatic Excess 2004/2008
recreation nutrients
Spring 1998 70-0054 Aquatic Mercury, 1999/2011
consumotion FCA
Upper Prior 2002 70-0072 Aquatic Excess 2004/2008
recreation nutrients
Upper Prior 2002 70-0072 Aquatic Mercury , 2002/2015
consumption FCA
Lower Prior 2002 70-0026 Aquatic Mercury, 2002/2015
consumption FCA
Pike 2002 70-0076 Aquatic Excess 2007/20 II
recreation nutrients
Notes: FCA stands for fish consumption advisory and is thus not an independent pollutant or stressor.
Source: MPCA
The Minnesota River, downstream of the Prior Lake outlet channel, is also listed. This
listing will potentially affect management of drainage that directly discharges to the
outlet channel. The river's affected uses are aquatic consumption, aquatic recreation, and
aquatic life and the pollutants or stressors that have been identified as causing these
impairments are the following:
. Fecal coliform
. Low oxygen
. Mercury
. PCB
. Turbidity
The absence of a waterbody from the 303d List does not necessarily mean the reach is
meeting its designated uses. It may be that the reach has either not been sampled or there
are not enough data to make an impairment determination. Additionally, where mercury
is identified as a stressor, the TMDL approach will be regional in nature as mercury is
most commonly an air-borne pollutant.
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Most likely the PLSLWD will be the lead agency charged with developing TMDL's for
the basins identified above. The City must be involved in developing the implementation
plan. It is likely that once a TMDL plan is in place this LSWMP will have to be amended
to incorporate the requirements of the TMDL.
Only a handful ofTMDLs have been completed or are in process - none for the water
bodies identified above. As shown in table 4.1 the first TMDL implementation plan is
due in 2008 for excess nutrients in Spring Lake and Upper Prior Lake.
City of Prior Lake Actions: The City of Prior Lake will, through its development review
and permitting process, quantify the change in nutrient loading due to implementation of
water quality treatment in developments. This quantification will be linked to the city's
GIS mapping so that a database can be maintained of how nutrient loading has been
changed. The City will use its hydrologic model to determine the extent to which volume
management has reduced nutrient loadings to the lakes and outlet channel.
5.2.1.2 Prior Lake Spring Lake Watershed District Assessments
When discussing nutrient impacts to lakes the nutrient most commonly identified is
phosphorus. Through its own monitoring efforts and those of the Citizen Assisted
Monitoring Program (CAMP) run by Metropolitan Council, the PLSLWD has been
collecting data on nutrient loading into the impaired waters, and others, identified above.
The PLSL WD water quality data collection and monitoring efforts consist of
tributary/outlet monitoring and in-lake monitoring. Tributary/Outlet monitoring
and other sampling occurs at the following locations:
· County Ditch 13 at the second/upstream crossing of Hwy 13 (PLSL WD site CD I)
· Outlet ofthe Hwy 13 Treatment Wetland (PLSLWD site CD2)
· Outlet of the desiltation basin on County Ditch 13 tributary just upstream of Spring
Lake (PLSL WD site CD3)
· Outlet from Lower Prior Lake (PLSLWD site PLO)
· Outlet from Spring Lake (PLSL WD site SLO)
A total of six lakes within the PLSL WD were monitored in 2003 as part of the
Metropolitan Council's CAMP. These were:
· Spring Lake
· Upper Prior Lake
· Lower Prior Lake
. Pike Lake
· Fish Lake
. Cates Lake
Each ofthe lakes was monitored by volunteers in one location, usually the deepest area of
the lake. Samples were collected approximately every two weeks between April and
October and were sent to the Metropolitan Council's laboratory for analysis of Total
Phosphorus, Total Kjeldahl Nitrogen, and Chlorophyll-a. Volunteers also measured
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surface water temperature and Secchi disk transparency, and rated the physical condition
and recreational suitability of the lake during each visit.
Summaries of the CAMP monitoring program results are provided the Metropolitan
Council's Environmental Services (MCES) 2003 report. Table 5.2 summarizes some of
this data and is reprinted from the PLSL WD 2003 Annual Report
Table 5.2
Growing Season (May -Sept.) Average Lake Monitoring Results, 2000-2003
From PLSL WD 2003 Annual Report
Lake Total Phos horus, 1l1!!L Secchi disk. meters Chlorophyll-It, "2 L
2000 2001 2002 2003 2000 2001 2002 2003 2000 2001 2002 2003
Sprin~ 170 93 148.8 103.9 1.54 l.l 0.6 1.6 50 58.3 116.6 44.2
Upper 85 88* 102 64.5 1.36 0.8* 0.7 1.4 63 80* 62.3 54.9
Prior I
Upper 96.0 0.8 67.0
Prior 2
Lower 24 21 26.5 40.4 2.89 2.4 2.5 3.3 12 14 13.0 8.0
Prior I
Lower 28 29 37.3 1.99 1.8 1.8 17 22 27.5
Prior 2
Fish 46 66 76.4 53.5 2.59 2.8 1.0 2.4 18 19 37.5 25
Pike I 136 139 198 225.6 0.31 1.3 0.5 0.8 13 102 57.0 120.3
Pike 2 97.0 0.5 74.0
Cates 22.2 29.1 1.7 1.8 7.7 4.8
Notes: *Samples unintentionally weighted toward poorer-quality late summer months, which may have biased
results.
+Three TP data points are missing from the database.
Lake water quality is often described by the "trophic" or nutrient status. In oligotrophic
lakes low concentrations of nutrients lead to a reduced ability to support aquatic life,
including algal blooms. Oligotrophic lakes are considered clean. Further along the
spectrum of nutrient concentration are mesotrophic, eutrophic and hypereutrophic lakes.
Mesotrophic lakes are still considered suitable for recreational purposes while eutrophic,
and, particularly, hypereuthrophic lakes frequently see algal blooms thereby reducing
their suitability for recreational purposes.
Scientists use a tool called the Carlson Trophic State Index (TSI) to determine where a
lake lies on the spectrum from oligotrophic to hypereutrophic. TSIs are calculated based
on water quality indicators such as total phosphorus concentration (TP), chlorophyll-a
concentration (Chi-a), and See chi disk transparency. Phosphorus is often the nutrient that
limits plant growth is lake systems. Additions of phosphorus (e.g., external P inputs) will
therefore enhance plant growth, including algae. Chl-~ is a green pigment in algae. Chl-
a concentration provides an indication of how much algae are in the water body. Secchi
depth, the third trophic state indicator, is a measure oflake transparency or clarity.
Murky and cloudy lakes have low Secchi disk readings, which usually correspond to
higher TP and Chl-a concentrations.
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TSIs are calculated based on relationships between these indicators and trophic status.
Higher TSIs correspond to high nutrient status. Table 5.3 comes from the MPCA's lake
data website and explains the relationship between the TSI value and lake nutrient status.
Table 5.3
Carlson's Trophic State Index (TSI) Explanation
TSI <30 Classic Oligotrophy; Clear water, oxygen through the year in the hypolimnion,
salmonid fisheries in deep lakes.
TSI30-40 Deeper lakes still exhibit classical oligotrophy, but some shallower lakes will
become anoxic in the hypolimnion during the summer.
TSI40-50 Water moderately clear, but increasing probability of anoxia in hypolimnion during
summer.
TSI50-60 Lower boundary of classical eutrophy: Decreased transparency, anoxic hypolimnion
during the summer, macrophyte problems evident, warm-water fisheries only.
TSI60-70 Dominance of blue-green algae, algal scums probable, extensive macrophyte
problems.
TSI 70-80 Heavy algal blooms possible throughout the summer, dense macrophyte beds, but
extent limited by light penetration. Often would be classified as hypereutrophic.
TSI > 80 Algal scums, summer fish kills, few macrophytes, dominance of rough fish.
From: The Minnesota Pollution Control Agency (MPCA) lake data web site.
MCES, in the context of its CAMP program, develops lake grades for its monitored
basins. Table 5.4, from the PLSL WD 2003 Annual Report, illustrates the relationship
between trophic status and this lake grade.
Table 5.4
Relationship of MCES Lake Grade to Trophic Status
MCES Lake Grade
B
Tro hie Status
Table 5.5 presents the TSI values with the MCES lake grades.
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Table 5.5
Trophic Status of District Lakes, 2003
2003 Previous MCES Grades
Lake TSI TSI TSI TSI MCES Trophic Status 2002 2001 2000
lTP) (Chi-a) (So) (Ave) Grade
Spring 71 68 53 64 C Eutrophic F D D
Upper Prior 1 64 70 55 63 C Eutrophic D D D
Lower Prior B Mesotrophic B B B
I 57 51 43 50
Fish 62 62 47 57 C Eutrophic D B C
Pike I 82 78 63 74 F Hypereutrophic F D D
Cates 53 46 52 50 B Mesotrophic B N/A N/A
According to the PLSL WD 2003 Annual Report:
All of the lakes in the District are either eutrophic or hypereutrophic except for Cates Lake and
Lower Prior Lake, which are on the upper boundary of meso trophy. Review of Table 4.5 and
comparison with the TSI descriptions in Table 4.3 shows that both Cates Lake and Lower Prior
Lake are very close to the boundary for a eutrophic lake, and this boundary is where problems
really start to become evident. The western end of Lower Prior Lake is mesotrophic/eutrophic
largely because of water flowing through this end from Upper Prior Lake to the outlet. The rest of
Lower Prior Lake has a limited watershed and is isolated from a majority of the inflowing water
from Upper Prior Lake.
The continued assessment of these lakes has led the PLSL WD to emphasize reduction in
phosphorus loading to the lakes. This will also be the focus of a watershed-based TMDL,
when developed, for the impaired waters listed in table 5.1. Since the mercury TMDL
will be regional in nature, the City of Prior Lake and PLSL WD will focus their efforts on
reducing nutrient loading. According to the PLSL WD:
For noticeable improvements to occur in lake water quality, TSI values need to be reduced to 55 or
less. On the reverse, if these lakes are allowed to decline further, algae blooms will become worse
and fish kills are probable.
In addition to collecting and reporting on the above data, the PLSL WD has created a
model to quantify the internal and external phosphorus load for Spring and Upper Prior
Lakes. This modeling effort is summarized in the 2003 Annual Report:
In summary, sediment phosphorus release and recycling accounts for approximately 43 to 78% of
the total phosphorus load for Spring Lake and 49% of the total phosphorus load for Upper Prior
Lake. As a result, significant water quality improvements in each lake will require
implementation of lake improvement options that would greatly minimize the potential for
sediment phosphorus release. In addition, significant reductions in phosphorus from County Ditch
13 and Spring Lake should result in significant water quality improvements in Spring Lake and
Upper Prior Lake, respectively. To a lesser degree, senescing macrophytes and bottom-feeding
fish also affect the water quality of Spring and Upper Prior Lakes, since each of them contribute
approximately 5 to 15% of the total phosphorus load to each lake.
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City of Prior Lake Actions: The City of Prior Lake and Prior Lake Spring Lake
Watershed District should consider whether moving forward jointly on a nutrient TMDL
is warranted, Near term development of a TMDL and the subsequent implementation
plan makes sense for several reasons:
· The TMDL will target nutrients. Nutrient reduction, and improvement in lake
water quality, is a primary goal of both the City and watershed district.
· The tributary area to all the impaired lakes is completely under the jurisdiction of
either the City (existing boundary and 2030 growth boundary) or watershed, which
makes for clearer lines of authority in implementation.
· Certain efforts toward volume retention - necessary for managing Prior Lake and
the outlet - will tend to reduce nutrient loading. It is not known if efforts
completed before creation of a TMDL will be credited toward the TMDL
implementation. Both the City and the watershed would stand to benefit if their
volume management strategies, as they are implemented, were also considered
steps to TMDL implementation.
· The more of the City that develops prior to a TMDL, the more of the City that
might be subject to retrofits to meet the requirements ofthe TMDL
implementation plan. If the TMDL precedes development then the cost of
implementation can be borne by development rather than directly through the
City's storm water utility and general fund or the watershed levy.
5.2.1.3 Scott County Watershed Management Organization Assessments
The Scott County WMO is in the early stages of its existence and thus has not had the
time to organize around an assessment of water quality within its jurisdiction. Its 2004
Comprehensive Water Resources Management Plan includes limited assessments of a
couple lakes including Markley within the City of Prior Lake. To date no long term
water quality trend is obtainable from the Markley Lake monitoring, which has been
occurring for seven years. In 2003 the lake was given a grade of "C", which indicates
that it lies between mesotrophic and eutrophic. Refer to table 4.3 for a description of
these terms.
Campbell Lake and Howard Lake have not been assessed as to their water quality. It is
generally understood that these lakes are subject to frequent late summer algal blooms
indicating that they likely tend toward the eutrophic to hypereutrophic end of the water
quality spectrum.
5.2.2 Water Quantity Assessments
5.2.2.1 City Identified Problem Areas
Cates channel should be monitored for future erosion.
5.2.2.2 Pike Lake and Jeffers Pond Districts
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In 2001 the City of Prior Lake prepared its Trunk Storm Sewer Fee Determination Study.
The purpose of this study was to take the land slated for development at that time
(generally land within the City limits) and develop a plan for providing a stormwater
management system for that land. The study's emphasis was toward developing a
defensible area charge. In order to estimate costs for the future stormwater management
system some modeling was conducted. Within the Jeffers Pond and Pike Lake Districts
this modeling became the basis for an agreement between the Prior Lake Spring Lake
Watershed District and the City of Prior Lake regarding allowable discharge rates from
the Pike Lake and Jeffers Pond districts.
The agreement that was arrived at between the Watershed and City is summarized in a
memorandum to the City from the Watershed's engineer. This memorandum is dated
July 21, 2003 and is included in appendix E. The essence of the agreement is that
allowable 100-year rates of 35 cfs and 300 cfs were set for the Jeffers Pond and Pike
Lake Districts, respectively. The allowable 2-year rates were set at 23.3 cfs and 206 cfs
from the Jeffers Pond and Pike Lake Districts, respectively. Tables 5.6 and 5.7 are
reprinted from that memorandum and distribute the allowable flow to specific
subdistricts. The subdistrict nomenclature is from the 2001 Study. Similar tables appear
in section 6 of this Plan detailing the revised targets for these areas in the context of more
recent modeling efforts. The allowable rates will remain those in tables 5.6 and 5.7. The
more recent modeling results in section 6 are not intended to revise that already agreed
upon rates and are presented merely as illustration that the allowable rates have been
considered in the modeling and system planning efforts. This rate scheduled will be
replaced when a Joint Powers Agreement (JPA) for the operation and maintenance ofthe
Prior Lake Outlet channel is approved by the City.
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Table 5.6
PLSLWD memo Table #1: 2-YR Peak Flow Rates
Discharge Subwatersheds Total Subwatershed Subwatershed Discharge/
Sub watershed Upstream Subwatershed Regulated Regulated Acre
Area (ac) Peak FlowT Peak Flow + (cfs)
(cfs) 25% (cfs)
JP-2 JP-l 105.2 18.6 23.3 0.22
Jeffers District Total 18.6 23.3 0.22
PL-5 PL-l, 2, 3, 4, 350.1 51.5* 51.5* 0.20
17
PL-7 PL-6, 8, 9, 10 232.7 45.8 57.3 0.25
PL-ll 144.2 29.7 37.1 0.26
PL-12 PL-13 173.5 7.6 7.7* 0.04
PL-16 34.0 7.6 9.5 0.28
PL-18 7.7 1.8 2.3 0.30
PL-21 4.9 1.6 2 0.41
Pike Lake District Total 164.7 206.0 0.22
Table 5.7
PLSL WD memo Table #2: 100- YR Peak Flow Rates
Discharge Subwatersheds Total Subwatershed Discharge/Acre
Subwatershed Upstream Subwatershed Regulated Peak (cfs)
Area (ac) Flow" (cfs)
JP-2 JP-l 105.2 35 0.33
Jeffers District Total 3S 0.33
PL-5 PL-l, 2, 3, 4,17 350.1 51.5 0.20
PL-7 PL-6, 8, 9, 10 232.7 93.5 0.40
PL-ll 144.2 114.0 0.79
PL-12 PL-13 173.5 7.7 0.04
PL-16 34.0 26.4 0.78
PL-18 7.7 3.2 0.42
PL-21 4.9 2.7 0.55
Pike Lake District Total 299 0.42
T Peak flow values taken from Prior Lake - Spring Lake Watershed District Outlet Channel XP-SWMM model.
* Maximum runoff value assumed as I OO-yr peak flow rate from City of Prior Lake Trunk Storm Sewer Study if runoff
value in Prior Lake - Spring Lake Watershed District Outlet Channel XP-SWMM model was greater.
** Peak flow values taken from City of Prior Lake Trunk Storm Sewer Study.
5.2.2.3 PLSLWD Volume Management
The Water Resources Management Plan for the PLSLWD, completed in 1999, identified
several planning efforts, which would occur subsequent to the Plan, to address issues
with the Prior Lake water levels and outlet operation. These included:
· Calibrating an hydrologic model for the watershed
· Designing improvements to the outlet channel for full-development conditions
· Addressing flood prone structures on Prior Lake
· Addressing increases in runoff volume as development occurs
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The PLSL WD report Prior Lake Outlet Channel and Lake Volume Management Study
(May, 2003) addresses these issues in detail. The 100-year floodplain elevation for Prior
Lake established by FEMA is 908.9 MSL. There are 79 homes around the lake with low
openings lower than this floodplain elevation. Fifty-one of these have low openings
below 907.6 and ten have low openings below or within one foot of the lakes 904.0
OHW. According to PLSLWD information this 904.0 elevation has been exceeded a
total of259 days since 1983.
Since development tends to improve drainage pathways and increase runoff volume, the
impact of future development on Prior Lake could, without mitigation, increase the
frequency of water levels above the 904.0 OHW.
To assess the impact development might have on water levels in Prior Lake, the
PLSL WD created a calibrated model of the watershed. The calibration of this model
started with standard curve numbers for the subwatersheds tributary to the lake and,
through the calibration process, modified these until modeled results matched monitored
lake levels for the 1998 to 200 I period. The hydrologic modeling for the LSWMP is
based upon this calibrated watershed model. The difference between the two, is that the
LSWMP model looks at the conditions that will exist when build out occurs in the 2030
growth area. Additionally, the LSWMP model includes more detail on the storage and
conveyance system necessary to serve the 2030 growth area.
Table 5.8 summarizes the volumetric increase in runoff volume as the study area converts
from current uses to future, with future consisting primarily of residential. The reader
should refer to the study itself for the finer distinctions between existing and future land
use assumptions. It should also be noted that the volume calculations assumed no
application of any runoff management techniques.
Table 5.8
PLSL WD Volume Study
Model Results Summary
Runoff Volume (ac-ft)
Year Existing; Future
1998 19,700 23,900
1999 19,400 23,000
2000 10,800 12,900
2001 17,300 20,600
Based on the representative years and the assumptions regarding land use change, annual
runoff volume increases anywhere from 2,100 ac-ft from the year 2000 rainfall season to
4,600 ac-ft in the 1999 rainfall season. This increase runoff volume has a direct
correlation to levels in Prior Lake and the frequency of exceedence ofthe 904.0 level.
The conclusions are that if no water resource management actions are taken to reduce
these developed runoff volumes, then Prior Lake will experience more frequent flooding
and this flooding will occur for longer durations than currently seen.
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The PLSL WD volume assessment identified five strategies for addressing future lake
levels. These included:
· Outlet structure modifications
· Rule revisions
· A Land Management Program for acquiring and preserving upstream storage
· Low home flood-proofing or buyout
· Outlet channel improvements
· Implementing volume management rules for new and re-development
The district eliminated the option of increasing the overall capacity of the outlet (by
adding a second pipe) due to the high cost, difficulty in permitting, and downstream
environmental impacts. Instead, the district intends to improve the efficiency of the
current structure and optimizing outlet operation, which will provide a small calculated
reduction in the future HWLs in Prior Lake.
Flood proofing and buyout have high potential but require substantial funding. Whether
this strategy can be used or not depends upon this funding as well as the willing
participation of landowners around the lake.
The most effective mitigation strategies, other than removing the homes, are retention
storage in the watershed and volume control measures. The District's goal is to acquire
between 1,500 and 3,000 acre-feet of retention storage. By District estimates this would
be 38 to 75 acre-feet per year.
5.3 System Design
5.3.1 Hydrologic Modeling Discussion
Stormwater runoff is defined as that portion of precipitation which flows over the ground
surface during, and for a short time after, a storm. The quantity of runoff is dependent on
the intensity of the storm, the amount of antecedent rainfall, the length of the storm, the
type of surface upon which the rain falls, and the slope of the ground surface.
The intensity of a storm is described by the amount of rainfall that occurs over a given
time interval. Storms are typically characterized by their return frequency. A return
frequency designates the average time span during which a single storm of a specific
magnitude is expected to recur. Thus, the degree of protection afforded by storm sewer
facilities is determined by selecting a return frequency for analysis.
For the Prior Lake SWMP the following return frequencies were used:
· 10-year Rational Method for storm sewer design
· 100-year, 24-hour (Type II distribution) event for overland drainage and pond
storage design
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A 100-year, 24-hour frequency event (6.0 inches in 24 hours for Scott County) has a 1 %
chance of occurring or being exceeded in any given year. This design rainfall return
period is commonly used for flood control throughout Minnesota.
As development occurs in Prior Lake, actual storm sewer design should be a 10-year
minimum recurrence for lateral, or local, systems in residential and commercial areas.
This implies that no street, parking lot, or backyard ponding would occur for the 10-year
design event. Trunk facilities should be analyzed and designed to accommodate the 100-
year ponded discharges plus 10-year rational flows from areas that enter the trunk to be
carried to the next storage area downstream.
In general, complete protection against large, infrequent storms with return intervals
greater than 100 years is only justified for important flood control projects. For most
developing areas like Prior Lake, the cost of constructing a large capacity storm drainage
system (for events greater than the 100-year) is much greater than the amount of property
damage that would result from flooding caused by a larger than 100-year event occurring
in a system designed for the 100-year event.
The excess runoff caused by storms greater than the 10-year will be accommodated by
transient street ponding and overland drainage routes. Providing areas for this short-term
flooding and overland drainage reduce flood damage due to larger than design events.
Provisions should be made to provide or preserve overland drainage routes for emergency
overflows.
A number of methods have been developed to determine the expected maximum rate of
runoff from a known area for a specific design storm, given land use and soil moisture
conditions. The preliminary trunk storm sewer design presented in this plan is based on
the Rational Method and the pond design on the XP-SWMM computer program.
The modeling involves the selection or computation of a time of concentration and a
runoff coefficient. The time of concentration is the time required for the runoff from a
storm to become established and for the flow from the most remote point (in time, not
distance) of the drainage area to reach the design point. The time of concentration will
vary with the type of surface receiving rain and the slope of the surface.
A minimum time of concentration of 15 minutes was selected for the design of the trunk
storm sewer system. Shorter times may be utilized in lateral system design. As the
stormwater runoff enters the system, the flow time in the storm sewer is then added to the
time of concentration, resulting in a longer time of concentration and thus lower average
rainfall intensity as the flow moves downstream from the initial design point.
The percentage of rainfall falling on an area that must be collected by a storm sewer
facility is dependent on watershed variables such as:
· Soil perviousness
. Ground slope
. Vegetation
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. Surface depressions
· Development type
· Antecedent rainfall
These factors are taken into account when selecting a runoff coefficient (C) in the
Rational Method or a runoff curve number (CN) for use in XP-SWMM.
In the Rational Method, the runoff coefficient for urban areas varies from 0.2 for parks to
0.95 for asphalt and concrete surfaces, while in XP-SWMM (or more correctly, the SCS
methodology which XP-SWMM incorporates), the CN varies from 58 for parks to 98 for
asphalt and concrete surfaces. CN values depend on the type of soil, cover type and
hydrologic condition. Under fully developed conditions, the values of CN will rise with
increases in impervious area caused by street surfacing, building construction, and
grading.
Table 5.9 provides CN values and runoff coefficients used in the SWMP modeling. To
ensure consistency with this Plan future analyses, whether they be for development
proposals or other city projects, should use the values contained within Table 5.7. For
other types of land use not identified in the table, SCS Technical Release 55 (TR-55)
curve numbers should be used.
As noted earlier, the predominant hydrologic soil group (HSG) within the study area is
HSG B to HSG D. Table 4.7 CN values reflect HSG B. To the extent that soils fall into
the C or D categories they should be modified accordingly. The CN values also reflect
Antecedent Moisture Condition II (AMC II), which is a typical assumption in hydrologic
analyses. AMC II simply implies that average soil moisture conditions apply prior to
simulation of the design event.
Table 5.9
Runoff Coefficients
Land Use Type Average Runoff Coefficient C for Rational Method CN Value
5 year 10 year 100 year
Park/Open Space 0.16 0.25 0.30 60
Low Density
Residential (30% 0.33 0.45 0.50 72
impervious)
Medium Density
Residential (65% 0.59 0.63 0.72 85
impervious)
High Density
Residential (72% 0.66 0.70 0.77 88
imoervious)
CommerciallIndustrial 0.76 0.79 0.85 92
(85% imoervious)
Ponds 1 1 1 99
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As mentioned, the computer modeling of stormwater quantities for pond design and trunk
pipes was carried out using the computer software HydroCAD, HydroCAD stormwater
runoffhydrographs are calculated in accordance with SCS TR-20 methodology.
Hydrograph routing through channels and detention basins is performed using the
Storage-Indication method. Storm distributions of SCS 24-hour Type I, lA, II, and III
storm distributions are allowed in the model. All analyses performed within the context
of this report have been conducted using Type II storm distributions.
5.3.2 System Design Recommendations and Discussion
The City of Prior Lake has a Public Works Design Manual (PWDM) first prepared in
2002, is currently being revised to include rules governing development hydrology and
water resource management issues. The following discussion is meant give background
information and provide the technical basis of some of the PWDMs requirements. This
discussion and background information should not be considered rule and it does not
substitute for or supersede the specific requirements of the City's Ordinances and
PWDM, or the Rules of the Prior Lake-Spring Lake Watershed District and the Scott
Watershed Management Organization.
5.3.2.1 Conveyance and Storage System Concepts
Storm Sewer and Channels
In the Prior Lake SWMP, a combination of storm sewer and channels has been used to
transport simulated stormwater runoff. Only major storm sewer trunks, 24 inches and
larger and related facilities have been considered in this study. A complete system
consists of a complex web of trunks, manholes, lateral lines, overland drainage ways,
catch basin leads, catch basins, pond inlets and outlets and all other items.
Proper design of a storm sewer system requires that all sewer lines be provided with
access through manholes for maintenance and repair operations. Generally, spacing of
manholes should be no greater than 400 feet. Intervals on larger diameter lines can be
increased when the pipes are sufficiently large for a person to physically enter the storm
sewer pipe for maintenance operations. Regardless of sewer size, manholes should
normally be provided at all junction points and at points of abrupt alignment or grade
changes.
Although lateral systems are designed for the 10-year storm event, their performance
must be analyzed for storms exceeding the design storm. Lateral and trunk pipes will
surcharge when the design storm is exceeded. During surcharging, the pipes operate as
closed conduits and become pressurized with different pressure heads throughout the
system. Low areas that are commonly provided with catch basins become small
detention ponds often performing like pressure relief valves with water gushing out of
some locations. For this reason, it is extremely important to ensure that these low areas
have an acceptable overland drainage route with proper transfer capacity.
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At a minimum, ponding on streets must meet all of the requirements ofthe 100-year
design criteria. For safety reasons, the maximum depth should not exceed two feet at the
deepest point and the lowest exposed building elevation should be at least one foot above
the elevation to which water could rise before overflowing through adjacent overland
routes,
All storm sewer facilities, especially those conveying large quantities of water at high
velocities, should be designed with efficient hydraulic characteristics. Manholes and
other structures at points of transition should be designed and constructed to provide
gradual changes in alignment and grade. Pond outlet control structures should be
designed to allow water movement in natural flow line patterns, to minimize turbulence,
to provide good self-cleaning characteristics, and to prevent damage from erosion.
Intake structures should be liberally provided at all low points where stormwater collects
and at points where overland flow is to be intercepted. Inlet structures are of special
importance, since it is a poor investment to have an expensive storm sewer line flowing
partially full while property is being flooded due to inadequate inlet capacity. Inlets
should be placed and located to eliminate overland flow in excess of 1,000 feet on minor
streets, or a combination of minor streets and swales, and 600 feet on collector streets and
arterials. Additionally, inlets should be located such that 3 cfs is the maximum flow at
the inlet for the 10-year design storm. Intake grates and opening should be self-cleaning
and designed to minimize capacity reduction when clogged with twigs, leaves and other
debris.
Effective energy dissipation devices or stilling basins to prevent stream bank or channel
erosion at all stormwater outfalls should be provided. The following recommendations
should be kept in mind when designing an outlet:
· Inlet and outlet pipes of stormwater ponds should be extended to the pond
normal water level whenever possible.
· Outfalls with velocities of less than 4 fps that project flows downstream into
the channel in a direction 30 degrees or less from the normal channel axis
generally do not require energy dissipators or stilling basins, but do require rip
rap protection.
· Where an energy dissipator is used, it should be sized to provide an average
outlet velocity of less than 6 fps, unless rip rap is also used. In the latter case,
the average outlet velocity should not exceed 8 fps.
· Where outlet velocities exceed 8 fps, the design should be based on the unique
site conditions present. Submergence of the outlet or installation of a stilling
basin approved by the City is required when excessive outlet velocities are
experienced.
· In the case of discharge to channels, rip rap should be provided on all outlets
to an adequate depth below the channel grade and to a height above the outfall
or channel bottom. It should be placed over a suitably graded filter material
and filter fabric to ensure that soil particles do not migrate through the rip rap
and reduce its stability. Rip rap should be placed to a thickness at least 2.5
times the mean rock diameter so as to ensure that it will not be undermined or
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rendered ineffective by displacement. If rip rap is used as protection for
overland drainage routes, grouting may be recommended.
· Overland drainage routes where velocities exceed 8 fps should be reviewed by
the City Engineer and approved only when suitable stabilization measures are
proposed.
Open channels and swales are recommended where flows and small grade differences
prohibit the economical construction of an underground conduit and in areas where open
channel type drainage will enhance the aesthetic qualities of a development. Whenever
possible, a minimum slope of 2% should be maintained in unlined open channels and
overland drainage routes. Slopes less than 2% and greater than 1 % are difficult to
construct and maintain and may require an underdrain system. Slopes less than 1%
should not be allowed. Side slopes should be a maximum of 4: I (horizontal to vertical)
with gentler slopes being desirable. Where space permits, slopes should be cut back to
match existing grade.
In general the flatter the channel side slopes and the more meandering the channel
alignment the more natural the channel will appear. Natural looking channels use
significantly more space than common ditches. One method of providing this space is to
incorporate greenway corridors over the channel area.
Rock rip rap should be provided at all points of juncture between two open channels and
where storm sewer pipes discharge into a channel. The design velocity of an open
channel should be sufficiently low to prevent erosion of the bottom. Rip rap or concrete
liners should be provided in areas where high velocities cannot be avoided. Periodic
cleaning of an open channel is required to ensure that the design capacity is maintained.
Therefore, all channels should be designed to allow easy access for equipment.
Sanitary sewer manholes that could be subject to temporary inundation, due to their
proximity to ponds, channels, or roadway low points, should be equipped with watertight
castings. Precautions should be taken during construction to prevent the entrance of
stormwater into the sanitary sewer. When access is required at all times, sanitary
manholes located near ponding areas should be raised above the 100-year high water
level. Future storm drainage construction should include provisions for improving the
water tightness of nearby sanitary sewer manholes. All newly constructed sanitary
manholes in the vicinity of ponding areas and open channels described in this report
should be waterproof.
Ponds
Stormwater ponding areas are an essential part of any storm drainage system. These
areas provide locations where stormwater flows can be reduced to provide flood
protection for downstream areas. The numerous natural depressions found throughout
Prior Lake have been incorporated into the Plan as ponding areas. The effective use of
ponding areas enables the installation of outflow storm sewers and channels with reduced
capacities, since the duration of the design storm is effectively increased over the total
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time required to fill and empty ponds. Smaller capacity trunk storm sewer and channels
provide a cost savings to the City.
The use of ponds to control stormwater runoff rates is a recent phenomenon.
Historically, older cities (including the older portions of Prior Lake) have piped
stormwater directly to the nearest large receiving water or river. Continued use of this
practice has both cost and regulatory implications. In terms of cost, few cities have the
funds necessary to build pipes that provide 100-year protection to properties. In fact, the
older cities that have historically piped all their stormwater find that the systems they
constructed provide nowhere near the 100-year protection found in newer cities that have
used ponds. In terms of the regulatory control, many direct discharges (without ponding)
to waters of the state are precluded. At present, even direct discharges to wetlands that
are not considered waters of the state are regulated through the NPDES construction
permit.
Cost and regulatory considerations aside, well designed ponds:
1. Improve water quality
2. Recharge the groundwater table
3. Provide aesthetic, recreational and wildlife benefits
Ponds improve stormwater quality by allowing nutrients and sediments carried by runoff
to settle before discharge to important receiving waters. Groundwater recharge is
increased by restricting the outflow rate from a pond, thus allowing more water to
infiltrate into the soil. Careful planning of ponds can enhance a development's appeal
and still provide efficient stormwater management. In fact, lots with pond frontage
command a higher price than lots without.
Most of the ponding areas proposed in this plan collect water from large regional
drainage areas. To provide proper protection for adjacent property, the design storm for
ponding areas is the maximum flood from a Type II, 24-hour, IOO-year rainfall event (6.0
inches of rain in 24 hours). To provide an additional safety factor, the lowest exposed
structure elevation in a development should be at least two feet above the IOO-year high
water level. The lowest exposed elevations of structures that are adjacent to ponds
should be certified by the builder during basement construction to ensure adequate
freeboard.
Runoff determinations for pond design vary from those for storm sewer calculations. The
critical storm for storm sewer design is the short, high intensity storm, whereas the
critical storm for pond design is of longer duration, since water is being stored for longer
periods of time and released at a slower rate.
The use of computer modeling in the analysis of the ponding system has allowed the
efficient review of several complicated routing patterns, each comprised of several ponds.
The pond storage and outflow rates, adjusted by lag time, were determined by the
program for all the ponds identified in this Plan. The lag time is significant as it
represents the attenuation of peak flows at each pond and generally shows that the peaks
are not occurring at the same time, This implies that the direct runoff to a pond has
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generally passed through to the downstream trunk system before the inflow of large
volumes of runoff from upstream ponds.
5.3.2.2 Water Quality System Concepts
Establishing the highest water quality goals that are both reasonable and sustainable is
one of the objectives of the Prior Lake Local Surface Water Management Plan, The only
effective way to maintain high quality water bodies is to prevent sediment, nutrients and
other materials from entering the storm drainage system. Complete interception of
stormwater for treatment at the point of discharge is not currently feasible, though the
City encourages the implementation of techniques such as rainwater gardens, infiltration
areas, and filtration swales etc. that capture a portion of runoff at the point of generation.
Application of these small-scale techniques should be on a site specific basis.
Pollutant Control
The three main sources for degradation of water quality are:
I. Solids and associated chemicals (including calcium chloride and salt) from
erosion and street sanding,
2. Composted decay around ponds, and
3. Fertilizers and other chemicals from farming practices, impervious surfaces, or
lawn care.
Identification of the source and implementation of reasonable control measures can
minimize the degradation of Prior Lake's water bodies.
In areas where extensive development is taking place, stormwater runoff frequently
contains substantial quantities of solids. Most commonly, these sediments are carried by
runoff into the storm sewer from large grading sites though fully developed areas also
generate sediment loads particularly from winter sanding operations and in areas of
structurally failing pipes. For developing areas, strict on-site erosion control practices are
required to prevent sediments from entering downstream water bodies. Inspections
should be conducted by the City to verify that the erosion control practices have been
installed and maintained properly. Even with extensive erosion control practices,
sediment and airborne particulates will continue to enter surface waters of the City.
The importance of erosion control measures during construction cannot be
overemphasized. The Best Management Practices (BMPs) recommended in the
Minnesota Pollution Control Agency's (MPCA) Protecting Water Quality in Urban Areas
should be followed for all development. The Minnesota general National Pollutant
Discharge Elimination System (NPDES) stormwater permit for construction activity
requires a permit for construction activities that disturb one or more acres.
When disturbing 10 or more acres, developers are required to provide temporary settling
ponds to treat the runoff from their grading sites. These ponds are intended to prevent the
introduction of sediment and its associated pollution into the storm sewer system and are
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required to function, in their various forms, until grading has ceased and adequate cover
has been established. At a minimum, these temporary sedimentation basins should meet
the requirements set forth in the NPDES general permit for construction activities.
When the outlet for a siltation basin, either permanent or temporary, is located below the
normal water surface, the basin can also serve to confme floating solids that may
otherwise enter a downstream pond or lake. This practice is typically referred to as
skimming. If a hazardous material such as fuel oil were to spill, a skimmer structure
would retain it within the basin and thus isolate it for easy access and prompt cleanup.
Skimmer structures should be used for all constructed ponds upstream of wetlands, lakes,
rivers and streams. For constructed ponds that discharge into other constructed ponds,
skimmer structures are not as important.
Ideally, some sort of solids removal system should be installed wherever a storm sewer
outlets into a pond. In certain cases, settling chamber (sump) type catch basins or
manholes can be provided for storm sewers that discharge into ponds. The Prior Lake
Public Works Design Manual requires 3-foot sumps in the last manhole prior to discharge
into a water body. These can provide effective removal of sand and gravel, which may
be flushed into the storm sewer from streets and highways, but are ineffective in the
removal of finer particles such as silts and clays. Use of this type of catch basin or
manhole should be limited to those areas where regular maintenance is practical and to
where the sump can be realistically expected to intercept sand from winter sanding
operations and gravel from driveways and construction sites.
Of late a concern regarding West Nile virus and mosquito breeding habitat has called into
question the use of sump manholes. The latest data suggests that many different breeding
environments exist for the mosquitoes that carry the virus including ponds, wetlands,
catch basins, and manholes. Obviously, eliminating these elements of the system is not
feasible. Though they should be used sparingly, sump manholes should not be prohibited
due to a concern over West Nile virus.
It bears repetition that a solids removal structure must be regularly maintained if it is to
remain effective. Since maintenance is the controlling factor in the long term
performance of sediment control measures, ponds are recommended over sump
manholes. Sump manholes, if numerous, often go without maintenance. An individual
pond requires more maintenance time than a sump, but system maintenance time goes
down when ponds are the preferred method of sediment removal as long as pond slopes
and benching allow access by maintenance equipment (see Chapter 5 for pond grading
requirements). For this reason sump manholes should be limited to storm sewer lines
discharging directly to wetlands, lakes, rivers, streams, ravines, and constructed channels
and should be avoided upstream of constructed ponds. In all cases, the location, type,
and number of sediment control structures must be established at the time of final design
of that portion of the storm sewer system. Maintenance of the system is discussed further
in Section 6.
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Even with the best and most expensive solids removal system, contamination of ponds
and lakes will occur unless particular attention is paid to those activities that occur after
development of a site. Developers must utilize the BMPs to minimize erosion during the
mass grading phase of construction. But property owners must also use care in the
development and maintenance of their lawns and open areas. Debris is frequently raked
from lawns into gutters; from there, if it is not removed, it washes into the storm sewer
system.
Generally speaking, water quality ponding within a development has to treat storm water
to the level required by the downstream receiving water body and its attendant
management strategy. The Prior Lake Public Works Design Manual calls for detention
pond design according to the design program developed by William Walker. At a
minimum, though, detention ponds should contain wet volume equivalent to the runoff
from a 2.5 inch rainfall over their tributary area.
Occasionally, with small plats (of 5 acres of less), water quality ponding cannot be
constructed to the extent required by the Plan without severely hampering the site
development or destroying other habitat such as upland grasslands and forests. In such
cases, it is within the City's discretion to reduce the required water quality ponding
and/or require other methods such as filtration swales or filter beds.
Water Quality Modeling
When necessary for modeling a series of water quality ponds, PondNet water quality
management model or P-8 model is recommended. PondNet is an empirical model
developed from data collected by the Environmental Protection Agency (EP A)
Nationwide Urban Runoff Program (NURP). The model predicts the phosphorus
removal efficiency of a large number of hydrologically connected ponds. Phosphorus is
the primary nutrient modeled because it has been found to be the nutrient most likely to
promote the growth of algae in lakes.
A limitation of the PondNet model is its inability to predict phosphorus concentrations in
large, deep water bodies. In general, water bodies larger than 20 acres or with mean
depths greater than 10 feet should be modeled with in-lake models, many of which are
now available.
Values for average runoff phosphorus concentrations, average annual summer runoff
coefficients and the resulting phosphorus export coefficients were determined for use in
the model. The values are shown in Table 5.10 along with the range of published
literature values.
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Table 5.10
Phosphorus Concentrations and Export Coefficients
Model Parameters Published
Land Use Values
P concentration Runoff P export P export
(I!g/l) Coefficient* Coefficients Coefficients
(%) Obs/ac) (lbs/ac)
Park/Open Space 200 0.07 0.14 0.09 - 0.3
Low Density
Residential (30% 450 0.21 0.97 0.45 - 2.7
impervious)
Medium Density
Residential (65% 500 0.47 1.64 0.45 - 2.7
impervious)
High Density
Residential (72% 500 0.55 2.31 0.45 - 2.7
impervious)
CommerciallIndustrial 600 0.68 3.08 0.70 - 3.0
(85% impervious)
* 2-year storm frequency (2.5" of precipitation in 24 hours)
The export coefficients reflect a large increase in nutrient loading as land use changes
from open space and agricultural to urban. The main reason for this increase is the large
increase in runoff rate and volume, caused by the amount of impervious area, which
washes off the pollutant buildup from those surfaces.
Computer models that predict concentrations and removal efficiencies for heavy metals
are currently available. These models predict removal efficiency in terms of inflow
particle distribution and the pond's ability to remove suspended solids. Based on a
number of studies recently performed by various agencies, it can be assumed that wet
detention ponds which remove 60 percent of phosphorus also remove high percentages of
heavy metals. Table 5.11 shows the benefits of wet detention ponds as estimated by the
DNR in Wisconsin.
Table 5.11
Benefits of Wet Detention Ponds
Pollutant Average Reduction (%)
Lead 70
Zinc 70
Bacteria 70
Diazinon (pesticide) 17
Phthalate 80
Sediment 90
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Based on these findings, it can be assumed that water quality ponds which reduce
phosphorus loadings by 60% under standard runoff concentrations will also reduce heavy
metal concentrations by 70% and sediments by 90%. For this reason it is sufficient to
model for phosphorus and from those results infer removals of other pollutants according
to the percentages in table 4.3.
Actual modeling of water quality basins and their treatment capacities can be
cumbersome for developers and their engineers. A simple criterion is that every water
quality basin should provide wet volume (volume below the normal water level)
equivalent to the post development site runoff for the 2.5-inch rainfall event. Ponds
designed in this manner will meet a 60% removal efficiency while providing excess
volume for sediment storage.
Local vs. Regional Water Quality
The ponds shown in maps I through 5 are flood control basins and are not generally
considered areas for water quality treatment. Water quality treatment is not considered a
regional element but rather something to be installed with individual developments.
Regional water quality treatment is considered less effective than local treatment and
some analyses suggest that regional water quality basins can become pollutant sources
rather than sumps. Additionally, by dispersing water quality to the local level, a wider
range of techniques can be used such as:
· filtration swales
· infiltration swales
· infiltration basins
. structural units like swirl separators
. sand filters
· reducing and disconnecting impervious surface
5.3.2.3 The Use of Wetlands in the Surface Water System
This LSWMP seeks to use the abundant wetlands within the City as a part of the natural
storm drainage system, This involves maintaining water quality, reducing flooding and
erosion, and stabilizing or restoring water levels. Wetlands are important physical,
educational, ecological, aesthetic, recreational and economic assets to the City.
Historically, most of the wetlands in the City have been affected by agriculture or
urbanization. In urbanizing areas, wetland degradation can be an ongoing process.
However, some degraded wetlands can be improved by stabilizing water levels and
reducing sediment loads.
Water quality plays a significant role in the overall quality ofa wetland, When the
quality ofthe incoming stormwater declines, the wetland's plant community may become
less diverse, retaining only those species that are tolerant of high nutrient and sediment
loads. Once a wetland's plant community is changed, the wetland's character and
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ecosystem will change, often to a less valuable system in terms of diversity, wildlife
habitat and aesthetic qualities.
Aerial photos show that numerous wetlands within the 2030 growth area have been
drained, via tile or ditching, and are now vegetated primarily with reed canary grass.
These areas are potential restoration sites since the wetland's natural storage will be
needed when the area develops. Storage can be restored by breaking tile lines and
berming across ditches,
In the recent past, LSWMPs have addressed wetland protection from nutrient loadings,
but not from water fluctuations or sustained water levels. Wetlands were commonly used
for flood storage. The Minnesota Stormwater Advisory Group published a guidance
document that looked at the implications of this practice. It was found that stormwater
bounce and duration affected some types of wetlands, but not others. The guidance
document presented specific guidelines on what wetlands were most affected by
stormwater bounce and sustained water levels. A wetland's sensitivity is affected by
vegetation type, hydrology, soils, topography and chemistry. Section 4, Wetland
Management Plan, applies the concept of susceptibility to the Prior Lake surface water
system.
In all instances, future developments (including road projects and redevelopment
projects) should incorporate some ponding upstream of wetlands. It is the method of the
Plan to show storage in wetland locations with the understanding that minimum control
measures upstream of the wetland will be installed. These minimum control measures
include:
· Water quality ponding for runoff generated by the 2.5-inch rainfall event
· Rate control to predevelopment conditions for the 10-year, 24-hour rainfall event
Where wetlands are identified as high quality and susceptible to negative impacts from
urban runoff quality and volume then additional control measures should be implemented
including:
· Infiltration in addition to 2.5-inch water quality criterion
· Filtration (via swales or rainwater gardens) in addition to 2.5-inch water quality
criterion
· Rate control to predevelopment conditions for the 100-year, 24-hour event
In order to determine what level of upstream control is necessary, assessments of
wetlands using standard assessment methodology (Minnesota Routine Assessment
Methodology, for instance) should be provided in submittals for developments and
projects that involve discharges to wetlands, unless these wetlands were included in the
assessment summary in Section 4. In such a case, only after 2010 would these also have
to be assessed by project proposers.
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Depending on the quality of the wetland in question, rate control upstream of the wetland
could be to the 100-year predevelopment rate. Generally, the rate control proposed in the
Plan is to a lower rate, which approaches a preagricultural rate. The additional rate
reduction would occur in the wetlands themselves. In many cases this will involve
constructing multi-stage control structures for wetland outlets. Construction ofthese
wetland outlets should be a part of any project that uses the wetland for a portion of its
flood storage. In some cases developments and other projects will rely solely on
constructed ponds that discharge directly to waterways or lakes. In some cases these
wetlands, along with other basins, would also incorporate a retention volume - as
discussed above regarding volume impacts to Prior Lake.
Table 5.12 lists differing wetland types and their susceptibility to impacts from
stormwater. Table 5.12 also appears as table 4.1 in section 4. It is reprinted here for ease
of reference.
Table 5.12
Wetland Community Susceptibility to Stormwater Impacts
Highly Susceptible Wetland Communities* Moderately Susceptible
Wetland Communities*
Low Prairies Shrub-Carrs
Coniferous Swam s Alder Thickets
Hardwood Swam s Fresh wet Meadows
Seasonall Flooded Basins Shallow Marsh
Calcareous Fens Dee Marsh
* Wetland community (-ities) determined using key provided in MnRAM Version 2.0.
Subsection 5.4 describes the surface water system proposed for development within the
2030 growth area. In general, storage is located in existing topographic low areas and
these areas are often wetlands. Appendix C provides calculated flood storage for the
proposed basins.
In many cases, the numbered ponds shown on maps I through 5 will be wetlands and
these wetlands will have differing susceptibilities to stormwater impacts as indicated in
the table above and, more substantively, in section 4. To the extent that some of the
proposed ponds in maps I through 5 are wetlands, then a portion of the flood storage
indicated in appendix C will fall outside and not within the wetland area.
The following general criteria should be used to determine what type of rate control and
water quality treatment would likely occur upstream of a wetland.
· Case 1: regional pond is a "least susceptible" wetland or not a wetland
· Water quality volume can be built into the regional pond ifit is not a
wetland, otherwise water quality volume per the requirements of the
NPDES construction permit is required.
· All flood storage can occur within the regional pond
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· Case 2: regional pond is a "slightly susceptible" or "moderately susceptible"
wetland
· Water quality volume is built upstream of the regional pond/wetland
· Flood storage is provide upstream of regional pond/wetland such that
the predevelopment 10-year rate is maintained to the wetland
· Case 3: regional pond is a "highly susceptible" wetland
· Water quality volume is built upstream of the regional pond/wetland
· Flood storage is provided upstream of the regional pond/wetland such
that the predevelopment I DO-year rate is maintained to the wetland
Tables 4.2 and 4.3 provide numeric targets that augment the concepts described above.
5.4 System Description
This subsection provides information on the surface water management system for Prior
Lake's 2030 growth area. The model was built based on the Prior Lake Spring Lake
Watershed District (PLSL WD) model and served the purpose of determining overall
infrastructure capital requirements.
The Study area is broke into 21 major drainage districts, which are further divided into
sub-districts. Total acreage for major drainage districts should roughly match the
existing work completed by the Watershed District.
The model included outlets for many ofthe landlocked basins within the City, to
determine the potential future infrastructure needs. The inclusion of a landlocked basin
outlet in the model is not a guarantee that the outlet will be approved or built. Proposals
to add outlets to landlocked basins will be evaluated on an individual basis at the time of
development, and must meet the requirements of the PWDM and the Rules of the
PLSL WD or Scott WMO, and as stated in chapter 3 it is the policy to encourage that
landlocked basins remain disconnected whenever possible.
Some changes are bound to occur in the growth assumptions on which this model is
based. Further study, or a more detail added to the District's model may result in
recommendations for changes to the stormwater management rules implemented by the
City of Prior Lake. The City should work under the leadership of the Watershed District
on issues of floodplain management by assisting in periodically refining the active
surface water management model and changing stormwater management policy when
warranted.
The following table lists the major drainage districts and abbreviation used for mapping.
The following sections describe each drainage district in detail.
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ramal2:e IStricts
Drainae:e District Abbreviation Acres
Buck Lake BLK 4336.7**
Camnbell Lake CALK 422.0
County Ditch 13 CD 13 5638.2*
Credit River CR 976.4
Crystal Bay CRBA 529.0
Crystal Lake CRLK 164.6
East Rice Lake ERLK 435.1
Howard Lake HLK 558.8
Jeffers Pond JP 812.8
Louisville Swamn LSW 1987.3
Lower Prior Lake LPPL 2859.0
Marklev Lake ML 528.6
Mystic Lake MLK 227.8
Pike Lake PL 1507.8
Rice Lake RLK 727.0
Sioux Community SC 771.2
Snring Central SPC 416.2
Snring Lake SPLK 1857.4
Snring West SPW 384.5
Unner Prior Lake UPPL 1516.2
Table 5.13
D D.
*Includes Swamp and Sutton Lake drainage area
**Includes Fish Lake drainage area
Appendix A details the drainage areas for the subdistricts within each drainage district.
Appendices Band C detail the pond and trunk storm sewer data. Appendix D lists the
proposed pond and storm sewer costs for each district. Refer to the sytem maps at the
end of the report for detailed topography, storm sewer, pond locations and drainage
districts. The discussion of specific major drainage districts is generally organized in an
upstream to downstream manner.
The system maps that show the 2030 system include the "V c" notation on several ponds.
This designator refers to the PLSL WD having identified that basin as a potential
component in its volume management strategy - the general parameters of which are
discussed earlier in this section. The "V" refers to volume and the subscript "c" refers to
the Watershed's desire that the City take the lead in implementing volume management
in areas that are, or soon will be, within the City limits. In other locations a "Vw" is
noted. These are also potential components in a volume management system with the
subscript "w" indicating that the Watershed would take the implementation lead in these
locations since they lie outside the 2030 growth boundary and thus will not fall within
City jurisdiction.
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EAST RICE LAKE (ERLK)
The East Rice Lake drainage district is the farthest upstream area tributary to Crystal
Lake. Under existing conditions it consists of a system of wetlands connected by
agricultural ditches. Runoff flows from south to north. ERLK-PI provides the primary
rate control before discharging to Rice Lake under Panama Avenue. This major district's
tributary area is primarily agricultural and outside the 2030 Urban Boundary. ERLK is
shown on map 3.
RICE LAKE (RLK)
Rice Lake is the central feature of this district. Rice Lake is located west of Panama
Avenue and south of County Road 13, as shown on map 4. The majority ofland that
constitutes this district lies outside the 2030 growth area but will drain into the growth
area with no additional rate control over what occurs today. Modeling for the Rice Lake
district assumes existing land cover and discharge patterns.
Under existing conditions Rice Lake (RLK-Pl) and Crystal Lake are connected via a
channel. For proposed conditions the outlet has been revised to a 48" outlet to restrict
flows upstream into fully developed Prior Lake. Three subdistricts, beyond RLK-PI,
provide additional rate control and flood storage. There are 170.5 acres of agricultural
drainage tributary to RLK-PI from the south via an existing ditch. The 100-year
discharge from the agricultural land is 33.6 cfs and the location where this enters the
2030 system is indicated on map 4.
Some grading and excavation is necessary at RLK-P2 to provide additional flood storage.
RLK-P4 is designated a highly susceptible wetland (MNRAM number 191142210013).
Since it is the farthest upstream in the system, meeting the water quality and quantity
standards for wetlands of this type should be feasible. Even with the small drainage area,
local ponding is required to maintain the integrity ofRLK-P4.
CRYSTAL LAKE (CRLK)
The Crystal Lake drainage district consists only of Crystal Lake and a small existing
NURP pond, CRLK-P2 northwest of County Road 13. The total drainage area is 176.4
acres, and is shown on map 3 and 4. Proposed development in CRLK is medium density
residential and preliminary plans for Heritage Landing are currently being developed for
the area tributary to CRLK-P2. Crystal Lake is tributary to UPPL-P13 via a 24" outlet.
CRLK-Pl receives runoff from RLK-Pl. To protect the downstream fully developed
storm sewer system, significant rate control was provided in Rice Lake and Crystal Lake
due to the tributary off-site agricultural drainage from the south.
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BUCK LAKE (BLK)
The Buck Lake drainage district consists of 4,336.7 acres. It is located on the south side
of Spring Lake. It lies fully outside the study area, but is tributary to SPLK-P8 and
therefore was modeled to determine the impact from the agricultural drainage on the
urban system.
The extensive Buck Lake drainage system begins as far south as Fish Lake and consists
of channels and wetlands. Overall the land tributary to this system is fairly well drained.
It should be noted that the channel that connects Fish Lake to Buck Lake and Buck Lake
to DNR wetland 206W (BLK-Pl) is a DNR protected waterway. Buck Lake provides the
majority of the flood storage and rate control before discharging to SPLK-P8. The
drainage area is well connected by channels, although several large wetlands provide
storage on the east side of Fairlawn Avenue.
As stated, Buck Lake and its drainage is not within the 2030 growth boundary.
Consequently, modeling of the 2030 system assumes the entirety ofthis drainage remains
under its current land cover, which is primarily agricultural. Based on existing conditions
modeling, the 100-year discharge from the Buck Lake drainage district into the urban
system is 497.6 cfs. The 100-year HWL in the farthest downstream wetland is 922.0,
which overtops Trunk Highway 13 (Langford Boulevard). The simulated HWL for
BLK-PI backs into Buck Lake to a limited extent. Ultimately, though, not a lot of
storage in Buck Lake can be used before the water tops Trunk Highway 13.
It should be noted that the PLSL WD has not targeted Buck Lake as a location for volume
management. Considering that the basin is quite large and apparently under utilized for
storage, it would make sense for the City and Watershed to study the feasibility of
controlling discharge from Buck Lake with the intent of creating more storage in this
basin.
Refer to system map 4 for detailed topography, subdistricts, ponding and storm sewer.
The off-site drainage area to Buck Lake includes Concord, Spring Southeast and Spring
East from the Prior Lake Spring Lake Watershed District (PLSLWD) model.
SPRING CENTRAL (SPC)
The Spring Central drainage lies between the Buck Lake and County Ditch 13 drainage
districts and, like these districts, discharges into Spring Lake's southwest lobe. The
Spring Central drainage currently consists of ditches that pass through wetland areas
designated as least susceptible in the Wetland Management Plan (section 4). A total of
416 acres drain to Spring Lake via the Spring Central system - approximately 186 acres
of which lie outside the City's 2030 growth boundary,
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Two basins are proposed upon development ofthis drainage. These basins, SC-Pl and
P2, occur in locations identified by the PLSL WD as strategic locations for considering
retention storage - with the City taking the lead on implementation. The proposed
normal water levels for these basins were developed with the idea of providing some
retention in the basins. Thorough study of retention implementation is not within the
scope ofthis Plan and no specific direction is provided by the Watershed as to the
amounts of retention desired in anyone location.
COUNTY DITCH 13 (CD13)
County Ditch 13 drainage district lies on the southwest side of Spring Lake, shown on
system map 4 and 5. As with Buck Lake, a significant drainage area outside the 2030
growth boundary is tributary to the small portion of this drainage within the boundary.
This drainage spans 3763.5 acres and reaches as far south as Sutton Lake. These areas,
though within PLSL WD jurisdiction, would lie outside City jurisdiction well beyond the
2030 timeframe. County Ditch 13 is the primary conveyance from Sutton Lake to CD-PI
- the only 2030 proposed basin within this drainage. The high modeled flow under
existing agricultural conditions (assumed conditions for 2030 as well) result from the lake
of storage evident is this district due to decades of agricultural drainage practices.
CD13-PI is a very important wetland because it is strategically located to intercept and
control the extensive upstream drainage, and, as a wetland determined to be "least
susceptible" to impacts from urban stormwater, it is appropriate to use the basin as a
major flood storage component without running into excessive concerns for the impacts
of bounce. This Plan proposes a substantial bounce on this wetland, from 916.0' to
930.0' - a HWL that would require raising Langford Blvd. If the PLSLWD were to
pursue storage upstream ofCD13-PI, the storage proposed in this basin could be
reduced. Note that CD13-PI is designated as a strategic location for considering
retention storage - with the City as the lead in implementation.
SPRING WEST (SPW)
Spring West is located southwest of Spring Lake. It consists of385 acres, shown on
system map 5. Under existing conditions it consists of a system of agricultural ditches,
ponding behind Langford Avenue before discharging to SPLK-PI. Drainage flows from
the south to north.
Grading revisions are necessary at SPW-PI to create a large pond, outside the existing
channel. The modeled outlet is a 24" rather than the existing 36" to provide additional
rate control. The existing channel was primarily used for ponding at SPW -P2. The
proposed outlet under Langford Avenue, from SPW-P2, is also smaller.
The locations for ponds SPW-PI and P2 do not occur on inventoried wetland sites but on
otherwise low areas adjacent to the ditches that drain these districts. Note that both
subdistricts SPW-I and SPW-2 have locations where potential retention areas are
identified.
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SPRING LAKE (SPLK)
Spring Lake is located directly upstream of Upper Prior Lake. The primary waterbody is
Spring Lake, SPLK-PI in modeling nomenclature. There are 1,858 acres within the
Spring Lake district, plus 11,629 acres of other drainage much of which, like the Buck
Lake and CD 13 drainage, lies outside the 2030 growth area. Subdistrict I is by far the
largest within this system and includes Spring Lake itself, wetland areas adjacent to the
lake and slopes that drain directly to the lake.
Other than the large central subdistrict and Spring Lake itself, the remainder of this
district is organized into a system of drainage areas and ponds around the periphery of
Spring Lake. The predominant 2030 land use is urban low density residential. SPLK-2
consists primarily of land that forms Spring Lake Regional Park and as such is one area
where no change to existing drainage patterns is anticipated. Otherwise notable features
of the 2030 system include regional ponds for SPLK-2 and SPLK-5 subdistricts. SPLK-
P5 is a potential wetland/storage restoration to provide rate control and sedimentation
capacity to an existing drainage notable for the amount of sediment it currently delivers
to Spring Lake.
Proposed pond SPLK-P7 would be formed by excavation and berming. Otherwise the
storage in other proposed ponds would be provided by existing topography. There are
several wetlands identified as least susceptible by the Wetland Plan. Among these,
SPLK-P2, P3, P4, and P5 have been proposed with more restrictive outlets to maximize
rate control under proposed conditions. Appendix B provides detail on the outlet sizes
proposed for these basins while appendix C provides the basin discharge rates.
SPLK-P5, P6, P7 and P9 have all been designated as potential retention areas. The table
at the end of this section provides prospective retention volumes based on the modeling
assumptions that guide this Plan. As with any retention basin, if a valved outlet were
provided then flood storage and retention volume could overlap. Without the ability to
draw a basin down, any increase in retention volume comes at the expense of guaranteed
flood storage.
Spring Lake discharges into Upper Prior Lake via an existing channel. No modifications
to this route or its capacity are proposed in this Plan.
CRYSTAL BAY (CRBA)
The Crystal Bay district lies west of Upper Prior Lake, bounded by County Road 82 to
the north and Howard Lake Road to the west. It is shown on system map 1. A large
portion of Spring Lake Regional Park lies within CRBA-2. Of the 529 acre drainage
area, only the eastern edge ofCRBA-2 has been developed (as urban low density
residential). The entire CRBA drainage district is tributary to Upper Prior Lake via the
18" culvert under Fremont Avenue. Only a small portion west of Arctic Lake, CRBA-
P2, is left to be developed medium density residential.
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The topography on the east side ofCRBA-P2 is characterized by steep slopes. Drainage
travels west to east. No agricultural drainage from off-site is tributary to the CRBA
system. CRBA-Pl, P2, and P3 were all designated as least susceptible wetlands in the
Wetland Management Plan - incorporated as section 4 of this report. Therefore, flood
storage and rate control have been maximized in these basins.
UPPER PRIOR LAKE (UPPL)
The Upper Prior Lake drainage district consists of 1,516 acres and is shown on system
maps I, 3 and 4. The major waterbody is UPPL-P4, Upper Prior Lake, It is directly
tributary to Lower Prior Lake (LPPL-PI) via an existing channel under County Road 21.
No changes are proposed for this channel connection between the two portions of Prior
Lake.
The majority of the area in UPPL is fully developed. Therefore no major changes are
proposed to the existing trunk storm sewer routing. Several least susceptible wetlands are
utilized for regional ponding. UPPL-P5 is a moderately susceptible wetland (MNRAM
number 19114220300 I). Refer to tables 4.2 and 4.3 for specific requirements regarding
treatment for these two wetland types.
UPPL-PI3 receives discharge from CRLK and its upstream tributaries via an existing 24"
outlet. UPPL-P13 discharges via a 48" culvert under County Road 13 to an existing
backyard ravine. This is a potential problem area due to the high discharge and should be
evaluated for erosion.
UPPL-PI4 overflows the road during 100-year events, assuming the existing outlet.
The ravine that UPPL-P7 and P9 discharge into adjacent to UPPL-P4 has been utilized
for proposed conditions. To protect the ravine a 36" outlet from P7 and a 12" outlet from
P6 was modeled. The revised HWLs provide adequate freeboard to adjacent structures.
LOWER PRIOR LAKE (LPPL)
The Lower Prior Lake drainage district is the downstream portion of the Prior Lake
chain. Much of its drainage area is developed and includes the Prior Lake downtown.
No significant changes are proposed for the existing drainage patterns and routing. The
only additions are outlets to currently landlocked ponds LPPL-P6, P2, PlO, P8 and P3.
With ponds P2 and P3, small lift stations and force main were modeled. It is also
possible to raise the NWL of the ponds and use gravity pipe as outlets (i.e. provide a
piped emergency overflow and allow infiltration to operate below this).
There are 2,859 acres within the LPPL district, 1,921 acres of which are tributary to
LPPL-Pl. The district is shown on system maps 1,2 and 3. LPPL-PI outlets to JP-P5
via the Prior Lake outlet channel.
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JEFFERS POND (JP)
Jeffers Pond drainage district is located in the northwest comer of Prior Lake. The
topography is characterized by steep wooded bluffs and several interconnected wetlands.
The district is bounded to the north by County Road 42 and to the east by County Road
21, as shown on system map I. Drainage is routed from southwest to northeast.
The majority of the Jeffers Pond district is undeveloped woods and grasslands. The
southwest portion is developed as low density residential. Preliminary design is
underway for the undeveloped portions of Jeffers Pond district in JP-5 and JP-6. There
are 732 acres of tributary area, with 19,298 acres of ponded tributary area from LPPL-Pl.
Lower Prior Lake outlets via the Prior Lake Outlet Channel into the Jeffers Pond district
at JP-P5. The channel is noted on map I by dashed lines. Proposed conditions modeling
assumes the upgrades noted in the Environmental Assessment Worksheet (EA W) for the
Prior Lake Outlet and Channel Improvement Project, prepared by PLSL WD in 2004,
have been implemented. These improvements include, but aren't limited to: a new outlet
for LPPL-PI, a weir waterfall overflow from JP-P5, upgrades to the channel crossing
from JP-P7 under County Road 42. The improvements are designed to improve wildlife
habitat, aesthetics and stabilize the outlet channel. The channel stabilization will also
improve the water quality in Jeffers Pond and downstream Pike Lake. It is important to
provide water quality ponds in new developments before discharging to the Jeffers Pond
district regional ponds. The cost estimates in Appendix D do not include any upgrades
that are a part of the EA W because these costs will be borne by the PLSL WD.
Berming is necessary on the east side of JP-PI, P2, and P3 to provide additional storage.
Construction costs have been included for these ponds since they lie outside the EA W
study area. The final downstream pond in the system is JP-P7. JP-P7 acts primarily as a
channel during low flow events; during high flow 100-year events, ponding occurs
behind County Road 42.
Section 5 of this report summarizes an agreement regarding discharge rate from a small
portion of the Jeffers Pond district into the outlet channel. Table 5.12, which follows the
discussion of Pike Lake, summarizes the modeling conducted for this plan and compares
the new rates obtained from this modeling to those previously agreed to.
PIKE LAKE (PL)
Pike Lake drainage district is in the far northern portion of the study area, adjacent to
Shakopee. There are a total of 1,508 acres within the district and 19,405 acres of ponded
area tributary to PL-PI6 (Pike Lake). The region is shown on system maps 1 and 2.
Drainage travels from east to west via a system of wetlands, proposed ponds, and trunk
storm sewer.
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Pike Lake is the farthest downstream portion of the Prior Lake watershed within the study
area - it receives discharge from JP-P7 through the Prior Lake outlet channel under
County Road 42. Pike Lake discharges via a 36" outlet north to Shakopee. For fully
developed conditions, 100-year event, PL-PI6 overflows the existing road, elevation 824,
roughly 0.5-feet.
PL-P9 is a highly susceptible wetland (MNRAM number 191152225002). Refer to
tables 4.2 and 4.3 for quantity and quality standards when discharging to a highly
susceptible wetland. It is important to construct local ponds to protect the regional PL-
P9.
Berming and some excavation are necessary for the ponds north of County Road 42 to
provide additional storage. Significant excavation is required for PL-P7.
Table 5.14 compares the proposed discharges from the Prior Lake storm water model to
the agreements for Pike Lake and Jeffers Pond discharge that were enacted in 2003. The
Prior Lake storm water model was created for the Surface Water Management Plan from
XP-SWMM software using a model provided by the PLSL WD as a base. The original
agreement on subwatershed discharge rates resulted in part from HydroCAD modeling
done for Prior Lake in the preparation of the 2001 Trunk Storm Sewer Fee Determination
Study. The new software was chosen to promote consistency between City and
Watershed modeling efforts. The new software uses different algorithms for calculating
peak water levels and discharges and leads to slightly different results for these over what
the older HydroCAD model provided - even given the same input parameters. The
strategy in modeling Pike Lake and Jeffers Pond in XP-SWMM has been to keep the
modeled flows below those memorialized in the agreement with the intent that the
agreement remains the final word on rate control for the Jeffers Pond and Pike Lake
Districts,
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Table 5.14
Comparison of SWMP Modeled Flows
to 2003 City of Prior Lake/PLSL WD Agreement
Rainfall 2003 AlITeement 2005 SWMP
Event District Area Subwatershed Discharge District Area Discharge Discharge
Regulated Per Acre Rate Per Acre
Peak Flow +
25%
(ac) (efs) (cfs/ac) (ae) (efs) (cfs/ae)
2-YR lP-2 105.2 23.3 0.22 JP-2 105.3 15.9 0.15
Table 5.6 105.2 23.3 0.22 105.3 15,9 0.15
Jeffers Pond
PL-5 350.1 51.5 0.20 PL-5 437.0 32.3 0.07
PL-7 232.7 45.8 0.25 PL-7 289.2 22.7 0.08
PL-II 144.2 29.7 0.26 PL-12 172.2 7.0 0.04
PL-12 173.5 7.6 0.04 PL-14 130.4 13.0 0.10
PL-16 34.0 7.6 0.28 PL-15 31.3 0.8 0.03
PL-18 7.7 1.8 0.30 PL-18 16.1 1.2 0.07
PL-21 4.9 1.6 0.41 PL-20 38.1 2.6 om
Table 5.7 947.1 206 0.22 1114.3 79.6 0.07
Pike Lake
100- YR lP-2 105.3 28.6 0.18
Table 5.6 105.2 35 0.33 JP-2 105.3 28.6 0.18
Jeffers Pond
PL-5 350.1 51.5 0.20 PL-5 437.0 48.0 0.11
PL-7 232.7 93.5 0.40 PL-7 289.2 71.5 0.25
PL-II 144.2 114.0 0.79 PL-12 172.2 7.4 0.04
PL-12 173.5 7.7 0.04 PL-14 130.4 17.4 0.13
PL-16 34.0 26.4 0.78 PL-15 31.3 4.3 0.14
PL-18 7.7 3.2 0.42 PL-18 16.1 5.9 0.37
PL-21 4.9 2.7 0.55 PL-20 38.1 4.3 0.11
Table 5.7 947.1 299 0.42 1114.3* 158.8 0.14
Pike Lake
* PL-P 19 not part of comparison since it is outside of the previous study area. Pond added due to level of detail of SWMP.
HOWARD LAKE (HLK)
Information collected for the Wetland Inventory indicates that Howard Lake is highly
susceptible to impacts associated with urban stormwater. Consequently, the use of this
basin in the 2030 urban system must be restricted compared to similarly sized basins
elsewhere in the City. Fortunately, Howard Lake's smaller natural tributary drainage
area also means that the future urban drainage area to the lake will also be small - so
meeting the management strategy for Howard Lake should not be an issue. In part, the
high quality of the lake is a function of its limited drainage area.
Howard Lakes drainage totals 559 acres and includes small wetlands separated from
Howard Lake by steep slopes and hills. In order to protect Howard Lake, existing
discharge and bounce is essentially maintained in the 2030 proposed system. This holds
true for the I, 10, and 100- YR rainfall events. Currently the lake discharges through a
culvert under Marschall Road into a complex of wetlands within the Campbell Lake
subdistrict CALK-2. The 2030 drainage plan anticipates that the future Howard Lake
drainage continue in this pattern to proposed basin CALK-P2, which will discharge into
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Campbell Lake. Alternately, if development in CALK-2 benefits from routing discharge
around this subdistrict and its wetland complex, then Howard Lake's discharge could be
sent directly to Campbell Lake via a pipe under Marschall Road. From a stormwater
management perspective, either route would be acceptable since the choice of route has
no bearing on the calculated discharge from Campbell Lake.
CAMPBELL LAKE (CALK)
Campbell Lake drainage district is located northwest of Spring Lake, bounded by 170th
Street and Marschall Road. It consists of roughly 422 acres and is shown on system map
5. The district is characterized by steep slopes surrounding Campbell Lake: CALK-PI.
Campbell Lake is designated as a least susceptible wetland in the Wetland Inventory
(section 4).
Given Campbell Lake's status as "least susceptible" to impacts from urban stormwater
and its large size, it follows that maximum rate control should be obtained within
Campbell Lake. This said, the proposed bounce of926.0' to 927.8' would be considered
moderate and more in line with a higher susceptibility ranking.
Though not identified as such by the PLSL WD in their initial look at retention potential
because it lies outside their jurisdiction, Campbell Lake holds some promise as a
retention basin and implementing some retention here would promote reduction in
volumetric discharge to Louisville Swamp and, ultimately, the Minnesota River.
LOUISVILLE SWAMP (LSW)
The Louisville Swamp drainage district is located on the far western portion of the 2030
growth area and is shown on system map 5. It is bounded on the western edge by
Baseline Road. It covers roughly 1,987 acres of currently agricultural land. Proposed
development is urban low density residential in this districts northern portion and a mix
of urban medium and high density residential and planned industrial in its southern
portions.
Under existing conditions, the LSW drainage is characterized by agricultural ditches.
Two specific ditch alignments occur in the 2030 growth area portion of the drainage:
1. A northerly ditch beginning at Campbell Lake through DNR 57W (LSW-P9)
under Baseline Road to Marystown Road (County Road 15).
2. A southerly ditch beginning at pond LSW -P2 and meeting the northerly ditch east
of Marystown Road. These ditches are more defined and other tributary ditches
more prevalent in areas east of Baseline Road - areas that are outside the 2030
growth boundary.
East of Marystown Road the combined ditches become a DNR protected waterway that
crosses under V.S, Highway 169. This unnamed protected waterway enters Louisville
Swamp, which lies adjacent to the Minnesota River near the confluence of Sand Creek
with the River.
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For the 2030 system this Plan envisions eliminating the current ditch and drained wetland
system and restoring storage to the landscape. At LSW-P2, for instance, the future
storage could be significantly increased and infiltration promoted if the large wetland
south of the proposed storage basin were restored and used for retention storage and
infiltration, This implementation concept would be particularly appropriate here as the
surrounding landscape is envisioned as planned industrial in the 2030 growth plan -
implying that more runoff will be generated in these areas than elsewhere in the
Louisville Swamp drainage.
Similarly, within proposed basin LSW-P9 a significant amount of storage exists such that
retention volume and infiltration, in addition to the rate control set by this Plan, could
occur - significantly lowering both peak rates and annual runoff volumes discharged
downstream into Louisville Swamp and the Minnesota River. LSW-P9 holds a strategic
location for implementation of the retention concept since it receives runoff from
approximately 756 acres of agricultural land in Shakopee from an area designated as their
Sand Creek Drainage by Shakopee's 1999 Comprehensive Stormwater Management
Plan. According to Shakopee's Plan developed discharge rates of 1/3 cfs/acre will be
allowed from this area upon development. This is approximately the existing rate off the
currently agricultural land. It would make sense for Prior Lake and Shakopee to work
cooperatively toward lowering this discharge rate particularly since the ditch system that
currently delivers runoff south across the border drains a particularly large wetland within
Shakopee - a wetland that seemingly has significant potential for decreasing the target
discharge rate and implementing some retention.
There are 756.3 acres of off-site agricultural land tributary from the north in Shakopee.
This runoff enters at West 160th Street East, roughly 500-feet west of Baseline Avenue.
CREDIT RIVER (CR)
The Credit River district lies on the southeast comer of the study area, shown on system
map 3. It consists of976 acres of primarily developed area. Subdistricts 1 through 4 are
tributary to CR-P2 which is currently landlocked. Drainage districts 5-7 travels west to
east, toward the Credit River. Since Credit River district is tributary to existing
agricultural area outside the City limits, this is a priority region for volume control since
increased volume associated with urban stormwater systems has been shown to
negatively impact agricultural lands due to frequent inundation of crop during late season
times when crop drying is important.
The bounce and inundation period requirements are being met for CR-P5, the only
moderately susceptible wetland within the system without an existing piped outlet.
CR-P5 requires berming on the east side to close contours around the pond and protect
developed lots to the east.
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MARKLEY LAKE (ML)
The Markley Lake drainage district consists of 529 acres in the southeast comer of Prior
Lake, as shown on system map 3. The drainage moves southwest to northeast to Markley
Lake (ML-P2), ML-P2 will discharge via a 12" forcemain to the Credit River - as
proposed in the 2001 Trunk Storm Sewer Fee Determination Study. Much of Markley
Lake is developed in a mix of commercial/industrial and residential. Portion of the ML-
1,2, and 3 subdistricts remain to be developed.
There are several moderately and least susceptible wetlands in Markley Lake district. To
maintain the integrity of these wetlands, especially as commercial/industrial development
progresses, local water quality ponding is necessary.
ML-PI requires excavation to create a pond adjacent to Markley Lake. Most of the other
ponds and wetlands utilized for regional ponding in Markley Lake don't require any
grading. Since ML discharge east outside the Prior Lake City Limits to existing
agricultural land, this area has been identified as a priority for volume control.
MYSTIC LAKE (MLK)
Mystic Lake drainage district lies just south of County Road 42. It consists of 228 acres,
shown on system map I. Much of the Mystic Lake drainage area is Shakopee
Mdewaketan Sioux Community Trust Land and Mystic Lake Casino property.
MLK-PI is tributary to SC-PI (Haas Lake) which is a moderately susceptible wetland.
Therefore rate control from MLK-PI has been greatly restricted by the 12" outlet on the
north side of the lake.
SIOUX COMMUNITY (SC)
The Sioux Community drainage district drains south to north into Shakopee. It is so
named due to the large amount of SMSC Trust land within the district. It consists of 771
acres, shown on system map I. County Road 42 splits the drainage area.
Much of the drainage area within the Sioux Community remains to be developed. A
portion of SC-3 and the majority of SC-7 are SMSC Trust Land with low development
densities. The remaining land is slated for medium density residential, except a small
portion of commercial in SC-2 along County Road 42.
Haas Lake (SC-PI) is the largest regional pond in the system. It receives discharge from
MLK-Pl. SC-PI is characterized by steep slopes adjacent to the lake and is designated a
moderately susceptible wetland (MNRAM number 19115227006). Currently discharge
is restricted from MLK-Pl to sustain existing inundation and bounce in SC-Pl. Future
development around the lake should required local ponding to protect the lake and
ravines that surround it. Shoreline protection is required by the City around the lake.
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There are a series of ravines north of County Road 42 that SC drains to at the Shakopee
city limits. To prevent erosion in these ravines, ponds are located upstream with
significant rate control. Additional excavation and grading is required at SC-P7 in order
to provide adequate flood storage before discharging north into Shakopee due to the large
ponded and direct tributary area.
POTENTIAL RETENTION
As stated earlier, this Plan identifies certain areas identified by the Prior Lake Spring
Lake Watershed District as having high potential for volume retention. In the discussion
of specific drainage systems other basins are identified that also have some potential for
application of this concept. The system maps identify only those prospective locations
for retention identified by the Watershed, and do not show those that may be discussed in
the preceding text. It should be emphasized that any large basin, regardless of which
Watershed it lies in, has potential for application of retention and infiltration concepts.
This is particularly true for basins termed "least susceptible" where the fluctuations in
water level that occur due to retention storage will have less impact on current wetland
functions and values. Table 5.15 summarizes retention volumes that might be available
between this Plan's proposed outlet elevations and the lowest elevation evident in a
particular basin. Table 5.15 is not intended to stipulate that a particular amount retention
is required, it is only intended a first step in defining what the current surface water
modeling provides by way of potential retention volume.
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Table 5.15
Potential Retention Volume
Pond Number Retention Volume (ac-ft)
SPLK-P5 5.6
SPLK-P6 7.7
SPLK-P7 6.1
SPLK-P9 3.5
SPC-PI 9.2
SPC-P2 9.0
CRLK-PI 41.5
RLK-PI 262.8
SPW-PI 7.7
SPW-P2 3.2
CD13-PI 42.9
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6. IMPLEMENTATION PLAN
6.1 General
The Implementation Plan section of the Prior Lake LSWMP describes a range of
activities and programs that support improvement of the City's surface water
management program. Capital outlay for the surface water system (pipes, channels, and
ponds) shown on the system maps will be large. For this reason a financing mechanism,
called an area charge, is developed in this section. Based on the Capital Improvement
Plan and the developable acreage, an area charge is developed and application of this
charge is discussed.
The concept of an area charge to [mance expansion of the trunk stormwater management
system is not a new concept for the City. Since its report titled Trunk Storm Sewer Fee
Determination Study (February, 2001) the City has quantified future trunk and ponding
needs and developed an area charge based on actual costs of these needs spread across
the potential developable acreage. With the analysis contained within the LSWMP the
City will update the fees for the 2030 growth area,
Section 6 also includes:
. An overview of the City's NPDES permit
. A discussion of operation and maintenance procedures and strategies
. An outline of an education program
. Financial considerations for the storm water utility
. A section referencing applicable design standards for stormwater management
. A section on Watershed implementation priorities
. Implementation priorities for the City
. A discussion of the process for amending this plan and an annual report to council
6.2 Cost Analysis and Capital Improvement Plan
One of the basic objectives of this study was to determine the cost of completing the City
of Prior Lake's trunk stormwater system and at the same time to determine new trunk
area charges that will insure availability of sufficient funds for future trunk and pond
construction.
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The cost estimates presented in this report are based on 2004 construction costs and can
be related to the Engineering News Record (ENR) cost index of7017 (April, 2004).
Future changes in this index are expected to fairly accurately reflect changes in
construction costs for the trunk stormwater system. This cost analysis is completed for
estimation purposed and should not be interpreted as policy.
6.2.1 Cost Estimation Methods
To minimize excavation ponds have generally been proposed for existing low areas.
Where natural topography does not lend itself to ponds either excavation or berming is
proposed to create the requisite pond areas.
Ponds serve to reduce peak flows. In that capacity they are desirable in and of
themselves. Ponds also have an added benefit of reducing downstream pipe sizes and
thus trunk pipe costs. Since ponds themselves involve cost it is desirable to reach a
balance point between ponds and larger pipes so that the least expensive system is
proposed. Generally, when pipes larger than 48-inches are prevalent, overall system
costs might be reduced by additional ponding areas.
Trunk pipes are generally located in existing drainage ways so that excessive pipe depth
can be avoided. This keeps pipe costs down and is the specific reason why it is best to
install trunk pipes in existing drainage ways.
Appendix B provides detail on the pipe system and channel reaches while appendix D
provides cost estimates for building these reaches used in the analysis. Appendix D also
includes construction costs for ponds, which follow from the pond data provided in
appendix C.
6.2.1.1 Pipe Costs
Pipe costs are based upon:
· Pipe construction
· Easement Acquisition
· Indirect Costs
Pipe Construction:
The appendix D pipe costs are based upon a pipe cost matrix that relates pipe cost to
diameter and pipe depth. This matrix is based on an analysis of bid tabulations and
discussions with large utility contractors. The matrix is updated annually based on the
ENR index and more recent bid tabulations as they become available. The per linear foot
pipe costs given in appendix D include pipe material costs, installation, manholes, and
bedding, as well as restoration and are thus comprehensive in terms of the various costs
associated with installing pipe. In many cases, existing channels are used in lieu of trunk
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pipe. The costs associated with channels are for the excavation and shaping that is
usually necessary to make channels function properly.
Easement Acquisition Assumption:
For each pipe reach a cost is included for permanent and temporary easement. Permanent
easement is calculated at 100% the fee title value of upland areas - estimated at $100,000
per acre in the year 2004. Appendix D includes no costs for temporary easement.
Temporary easement is usually not necessary since construction of facilities occurs on
development sites. As pipe depths increase the permanent easement width around the
pipe also increases. This is reflected in the cost estimates. For the purposes of estimating
costs, it is assumed that 75% of future trunk pipe will fall in dedicated easement or right-
of-way, so easement costs are applied to only 25% of trunk footage.
Indirect Costs:
A 30% factor for indirect costs is included in the cost estimate presented in Appendix D -
applicable to pipe, channel and pond construction, Indirect costs include engineering,
administration, contingencies, and fiscal costs. For easement acquisition a 10% indirect
cost has been applied. This accounts for the appraisal and administrative costs associated
with easement acquisition.
6.2.1.2 Pond Costs
Pond costs involve the following:
· Pond construction (excavation and berming)
· Easement acquisition
Pond Construction:
The primary element of pond construction is excavation. To some extent berming will
also be necessary to create the ponds shown in the system maps at the end of this report.
Pond construction costs vary considerably depending on whether excavation is necessary.
At one extreme are ponds that obtain their requisite flood storage solely by excavation.
At the other extreme are ponds that are existing depressions with the required storage
provided or ponds that can be created by berming. To account for the variability in pond
construction costs, three different pond construction costs are used. Each of the three is
based on a unit cost per acre of pond at high water level (HWL). The three costs and the
rationale behind their use are as follows:
I. Minimal excavation or berming: $4,500/acre of pond at HWL
2. Combination of excavation and berming: $9,000/acre of pond at HWL
3. Full excavation of flood storage volume: $13,OOO/acre of pond at HWL
Appendix D details the costs of pond construction. The construction cost is for providing
flood storage only and does not include costs associated with providing water quality
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treatment. As stated previously, water quality treatment is considered a development cost
and not a trunk cost. Creation of water quality volume could occur in the ponds proposed
within the developable area, but this cost would be strictly a developer cost with no
reimbursement or participation by the City.
Pond Easement Acquisition:
For each pond, a cost is included for permanent easement. Permanent easement is
calculated at 50% the fee title value of upland areas (50% x $100,000/acre =
$50,000/acre). 50% fee title, instead of 100%, is used to account for the fact that many of
these low areas are otherwise undevelopable since many are jurisdictional wetlands or
would be used for the required water quality ponding.
No easement cost is associated with existing NWI wetlands or wetlands inventoried as
part of the Wetland Management Plan (section 4 of this Plan), although there may be a
construction cost to account for required berming or expansion. The easement costs for
pond are applied only to those areas that appear to be non-wetland by the methods
employed in preparing this plan.
6.2.2 System Costs and Capital Improvement Plan
Appendix D summarizes the analyzed system costs by element, by major watershed, and
for the system as a whole. Appendix D serves as the City's stormwater CIP for future
development and for calculating area charges.
The analyzed system, as shown in system maps, has an estimated cost of $10,992,289.
This cost includes indirect costs of 30% on trunk and pond construction and indirect costs
of 10% on easement acquisition.
Table 6.1 presents the stormwater management CIP for the City of Prior Lake. The cost
elements come directly from the 2030 stormwater system design as described in the
system maps and the appendices to this report. The various trunk elements are organized
by prospective year of implementation as well as whether they constitute a pond cost or
trunk pipe cost. Total costs for the 2030 system are $10,836,957.
Table 6.1
Capital Improvement Plan
Trunk Storm Sewer Element
YearlPhase Pipe Pond Cost ($) Trunk Cost ($)
From Point To Point
2004 CRLK-P2 CRLK-Pl 32,163 147,987
RLK-P2 RLK-Pl 543,201 147,987
RLK-P3 RLK-Pl 145,174 30,003
RLK-Pl CRLK-Pl 157,536
SPLK-P3 SPLK-Pl 18,909
SPLK-PlO 16,283
Total 736.822 502.422
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2006 CR-P3 CR-P2 18,609 315,277
CR-P5 CR-P6 7,564
CR-P4 8,723
CR-P6 Credit River 90,763
CRBA-P3 CRBA-P2 215,563
HLK-P2 HLK-P1 68,073
JP-P1 JP-P2 23,261 38,605
JP-P2 JP-P3 8,141 19,303
JP-P3 JP-P4 23,843 38,605
JP-P4 JP-P5 90,763
LPPL-P2 LPPL-P1 41,647
LPPL-P4 LPPL-P1 43,615 30,254
LPPL-PlO LPPL-P1 75,636
LPPL-P12 LPPL-P1 115,816
LPPL-P5 1,163
LPPL-P9 7,560
LPPL-P11 LPPL-P1 45,360 115,816
ML-P5 ML-P7 19,191
ML-P6 ML-P1 166,400
ML-P1 ML-P2 414,548 15,001
ML-P2 Credit River 113,128
ML-P11 Credit River 7,564
SPLK-P6 SPLK-P7 44,270
SPLK-P7 SPLK-P1 40,707 45,382
SPLK-P5 SPLK-P1 29,077 18,909
SPLK-P4 SPLK-P1 18,909
UPPL-P15 UPPL-P8 397,828 52,945
UPPL-P8 UPPL-P9 26,473
UPPL-P6 UPPL-P4 71,854
UPPL-P7 UPPL-P4 25,055
Total 1,081,626 1,862,013
2008 CALK-P2 CALK-PI 29,658 13,615
CALK-P3 CALK-PI 17,446 75,636
CALK-PI LSW-P9 200,436
ERLK-P1 RLK-P1 114,562 13,281
Total 161.666 302.967
2010 LSW -P2 55,827
LSW -P9 LSW-P7 218,075 12,868
LSW-PlO LSW-P7 59,316 7,564
LSW-P11 Shakopee 15,120 7,564
SPW -P2 SPLK-P1 387,132 200,020
Total 735.470 228.015
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2012 CD13-Pl SPLK-Pl 350,664 94,521
PL-Pl PL-P2 214,422 98,327
PL-P2 PL-P4 19,191 310030
PL-P3 PL-P4 34,892 40,004
PL-P4 PL-P5 41,870 154,422
PL-P5 PL-P16 26,751 115,816
PL-P6 PL-P7 25,006 166,400
PL-P9 PL-P8 109,672
PL-PI0 PL-P8 45,382
PL-P8 PL-P7 6,397 25,737
PL-P7 PL-PI6 106,087 87,694
PL-Pll PL-PI6 75,636
PL-P13 PL-PI2 125,581
PL-P12 PL-PI6 20,935 30,254
PL-P14 PL-P16 55,827 30,003
PL-PI5 PL-PI6 45,382
PL-P16 Shakooee 50, III
PL-P17 Shakooee 165,762 3,782
PL-PI8 PL-PI6 33,152 7,564
PL-PI9 PL-P16 66,305 37,818
PL-P20 PL-P16 79,566 41,600
SC-Pl SC-P2 75,636
SC-P2 SC-P7 45,941 450,396
SC-P7 Shakooee 106,087 200,442
SC-P3 SC-P4 81,415 136,145
SC-P4 Shakooee 23,261 35,003
SC-P5 ShakoDee 18,028 52,945
SC-P6 Shakooee 20,354 15,127
SC-P8 ShakoDee 55,827 7,564
SC-P9 Shakooee 152,501 7,564
SPC-P2 SPC-Pl 171,358 20,002
SPW-Pl SPW-P2 51,756 238,067
Total 1.973.354 2.937.627
2014 LSW-P3 75,599
Total 75.599
2024 SPLK-P9 SPLK-Pl 26,169 22,691
SPLK-P8 SPLK-Pl 15,217
SPC-Pl SPLK-Pl 175,387
Total 26.169 213.205
Grand Total 10.836.967
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6.3 Financing and Cost Recovery
6.3.1 Area Charges and Cost Recovery Calculations
The City of Prior Lake currently recovers the cost of its trunk stormwater system through
an area charge. This Plan is updating those charges for the 2030 growth area.
At present, the City reduces gross acreage by wetland area and park area to determine a
developable acreage. This developable acreage is reduced further to account for area that
would not be subject to area charges: storm pond easements, major road right-of-way,
greenway corridors, park dedication, and wetland buffers.
The total acreage then available for application of area charges was set at 3,524 acres out
of the developable acreage within the 2030 growth area. The acreage to which area
charges apply is termed the financing area and includes only those areas developing
within the financing analysis period.
For financing purposes, the net developable acreage within the 2030 growth area is 3,524
acres. In order to determine an equitable financing schedule it is also necessary to
convert acres into equivalent acres and base the financing analysis on a reasonable break
even period.
Typically stormwater area charges are applied to equivalent area with and equivalent acre
adjusted to reflect the greater burden placed upon the stormwater system by more
impervious land uses like commercial, industrial, and high density residential. Table 6.2
provides a summary of equivalency factors used by the City of Prior Lake. Equivalency
factors are based upon runoff coefficients for the different land uses with urban low and
medium density residential considered as a base case.
Table 6.2
Land Use Based Equivalency Factors
Land Use Factor
Urban Low and Medium 1.0
Density
Urban High Density 1.65
Residential
Commercial and Industrial 2.07
Table 6.3 presents the cost recovery analysis based upon application of these equivalency
factors and a 20-year break even assumption.
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Table 6.3
Stormwater Area Charge
50% Land Cost for Constructed Pond Easement
A B-1 B-2 B-3 C D E
Net
Net Developable Net Developable Developable REVENUE
(Assessable) (Assessable) (Assessable) Column C x Area
Acreage Added - Acreage Added - Acreage Added Equivalent Charge of EXPENDITURE City
Phase Residential Non-Residential -TOTAL Area Added $2790/EqAc Trunk Costs
(yr) (ac) (ac) (ac) (EaAc) ($) ($)
2004-2009 1312 133 1445 1587 4,428,595 4,647,517
2010-2014 1467 224 1691 1931 5,386,597 5,950,066
2015-2019 243 10 253 264 735,723 0
2020-2024 135 0 135 135 376,650 239,374
Totals
3,157
367
3,524
3,917
10,927,565
10,836,967
Note: Developable Area within municipal boundary phased in between 2004-2014.
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In some instances developers will dedicate or construct at their own cost the trunk
infrastructure necessary to serve future upstream development. This Plan serves as a
guide as to what is necessary for upstream development. In these cases developers may
seek a credit toward area charges - a credit that can be quantified based upon the analysis
of the preceding sections.
6.3.2 Area Charge Summary
Sections 6.2 and 6.3 develop an area charge for the City of Prior Lake that can be applied
to future development within the City. The area charge has been constructed
methodically as follows:
I. Pond and trunk costs for near term development have been estimated. A
stormwater CIP has been created as shown in appendix D and table 6.1.
2. Net assessable acreage has been determined.
3. The base area charge has been modified into a land use based area charge through
the use of equivalent acres.
6.4 NPDES Permit
Refer to City MS4 permit and Storm Water Pollution Prevention Plan for additional
operations, maintenance, and reporting requirements.
6.5 Operation and Maintenance
6.5.1 Activities
A storm water system is a major investment for the City of Prior Lake - both in terms of
initial capital cost and in terms of ongoing maintenance costs. The capital improvement
program outlines the costs for new trunk system construction which will be funded by
area charges. System maintenance is funded by the city's storm water utility.
The city's storm water system maintenance responsibilities include the following:
· Street sweeping
· Cleaning of sump manholes and catch basins
· Repair of catch basins and manholes
· Assessing pipe condition (typically by televising)
· Inspection of storm sewer inlet and outlet structures
· Pond mowing and other vegetation maintenance
· Excavation of accumulated sediments from ponds
The city has maintained its pipe system for decades and staff has a strong grasp on the
costs associated with this. As new development brings more ponds into the system, city
staffwill find that pond maintenance becomes an increasingly large portion of both staff
time and maintenance budget. It is important to quantify the extent of this future
commitment so that the funds necessary for pond maintenance activities can be collected
via the city's storm water utility.
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Table 6.4 provides a typical maintenance schedule for wet ponds.
Table 6.4
Wet Pond Maintenance Schedule
Activity Schedule
Inspect regional pond outlets for clogging. After significant
rainfalls
Inspect for damage. Annual inspection
Note signs of hydrocarbon build up.
Monitor for sediment accumulation in the facility and forebay.
Examine to ensure that inlet and outlet devices are free of debris
and operational.
Repair undercut or eroded areas. As needed
Mow slopes Twice annually
Remove sediments from forebay 5 to 7 year cycle
Remove sediment accumulated in main pool 20 to 30 year cycle
Adapted from Watershed Management Institute. 1997. Operation, Maintenance, and Management of Storm water
Management Systems.
As the city obtains more ponds, the management of these might be facilitated by creation
of a GIS database for all storm water system infrastructure. Via this database the city
could reference, via interactive mapping, maintenance records, videotapes, and
maintenance costs for portions of their system.
6.5.2 Storm water Basins
Stormwater basins represent a sizable investment in the City's drainage system. General
maintenance of these facilities helps ensure proper performance and reduces the need for
major repairs. Periodic inspections are performed to identify possible problems in and
around the basin. Inspection and maintenance cover the following:
. Basin outlets
. Basin inlets
. Side slopes
. Illicit dumping and discharges
. Sediment buildup
Basin Outlets
A key issue with stormwater basins is ensuring that the outlets perform at design
capacity. Inspection and maintenance of basin outlets address the following:
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· The area around outlets is kept free and clear of debris, litter, and heavy vegetation.
· Trash guards are installed and maintained over all outlets to prevent clogging of
the downstream storm sewer.
· Trash guards are inspected at least once a year, typically in the spring, to remove
debris that may clog the outlet. Problem areas are addressed more frequently, as
required.
· Emergency overflow outlets are provided for all ponds when possible. These are
kept clear of debris, equipment, and other materials and properly protected against
erOSIOn.
Basin Inlets
Inspection and maintenance of basin inlets address the following:
· Inlets are inspected for erosion.
· Where erosion occurs near an inlet, energy dissipaters or riprap are installed.
· Inlets are inspected for sediment deposits, which can form at the inlets due to poor
erosion practices upstream.
· Where sediment deposits occur, these are removed to ensure design capacities of
storm sewers entering the basin are maintained.
Side Slopes
Inspection and maintenance of basin side slopes address the following:
· Side slopes are kept well-vegetated to prevent erosion and sediment deposition into
the basin. Severe erosion along side slopes can reduce the quality of water
discharging from the basin and require dredging of sediments from the basin.
· Noxious weeds are periodically removed from around basins.
· Some basins in highly developed areas require mowing. If mowing is performed, a
buffer strip of 20 feet or more adjacent to the normal water level is typically
maintained. This provides filtration of runoff and protects wildlife habitat.
Illicit Dumping and Discharges
Inspection and maintenance of illicit dumping and discharges into basins address the
following:
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lit Local Surface Water Management Plan
6-11
. Basins are periodically inspected for evidence of illicit dumping or discharges.
The most common of these is dumping of yard waste into the basin.
. Where found, illicit material is removed, and signs are posted as needed
prohibiting the dumping of yard waste.
. Water surfaces are inspected for oil sheens. These can be present where waste
motor oil is dumped into upstream storm sewers.
· Skimmer structures are installed as needed at outlet structures to prevent oil spills
and other floatable material from being carried downstream.
· Skimmer structures are periodically inspected for damage, particularly from
freeze-thaw cycles.
Sediment Buildup
Inspection and maintenance of sediment buildup in basins address the following:
. Basins are inspected to determine if sediment buildup is causing significant loss
of storage capacity from design levels. Excessive sediment buildup significantly
reduces the stormwater treatment efficiency of water quality ponds.
. Sediment removal is performed where excessive sediment buildup has occurred.
As a general guideline, ponds require dredging every 15 to 20 years.
6.5.3 Sump Manholes and Sump Catch Basins
Sump manholes and sump catch basins are included in storm sewer systems to collect
sediments before they are transported to downstream water bodies. These structures keep
sediments from degrading downstream water bodies. Once sediments are transported to a
lake or pond, they become much more expensive to remove.
Sediments originate primarily from road sanding operations, although construction
activity and erosion can also contribute. Since these structures are designed to collect
these sediments, they are routinely inspected and cleaned to provide capacity for future
sedimentation. Suction vacuum equipment is typically used.
6.5.4 Storm Sewer Inlet Structures
To fully utilize storm sewer capacity, inlet structures are kept operational in order to get
runoff into the system. All efforts are made to keep catch basins and inlet flared ends
free of debris and sediments so as not to restrict inflow and cause flood damage. Leaf
and lawn litter are the most frequent cause of inlet obstructions. On a routine basis, City
staff visually inspects inlet structures to ensure they are operational.
~ City of Prior Lake
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lit Local Surface Water Management Plan
6-12
6.5.5 Open Channels
Overland flow routes constitute an important part of the surface water drainage system.
Open channels are typically vegetated and occasionally lined with more substantial
materials. The lined channels typically require little or no maintenance. Vegetated
channels are periodically inspected and maintained, as high flows may create erosion
within the channel.
Eroded channels can contribute to water quality problems in downstream water bodies as
the soil is continually swept away. If not maintained, the erosion of open channels would
accelerate and the repair would become increasingly more costly. The use of
bioengineering and natural stream technology, which mimics the characteristics of
natural streams to promote channel stability, can reduce the potential for erosion.
6.5.6 Piping System
The storm sewer piping system constitutes a multimillion-dollar investment for the City.
The City performs a comprehensive maintenance program to maximize the life of the
facilities and optimize capital expenditures. The following periodic inspection and
maintenance procedures are followed:
. Catch basin and manhole castings are inspected and are cleaned and replaced as
necessary .
. Catch basin and manhole rings are inspected and are replaced and/or regrouted as
necessary .
. Catch basin and manhole structures are inspected and are repaired or replaced as
needed. Pipe inverts, benches, steps (verifying integrity for safety), and walls are
checked. Cracked, deteriorated, and spalled areas are grouted, patched, or
replaced.
. Storm sewer piping is inspected either manually or by television to assess pipe
condition. Items looked for include root damage, deteriorated joints, leaky joints,
excessive spalling, and sediment buildup. The piping system is programmed for
cleaning, repair, or replacement as needed to ensure the integrity of the system.
6.5.7 De-Icing Practices
Minnesota receives approximately 54 inches of snow during a typical year. This requires
a large amount of de-icing chemicals (primarily salt) to be applied to roads and sidewalks
each winter.
Estimates indicate that 80 percent ofthe environmental damage caused from de-icing
chemicals is a result of inadequate storage of the material (MPCA 1989). Improper
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lit Local Surface Water Management Plan
6-13
storage as well as overuse of salt increases the risk of high chloride concentrations in
runoff and groundwater. High chloride concentrations can be toxic to fish, wildlife, and
vegetation.
The following procedures are used for storing de-icing chemicals in the City.
I. De-icing material is stored in waterproof sheds. Where this is not possible,
stockpiles are covered with polyethylene and placed on impervious surfaces.
2. Road de-icing stockpiles are not located near municipal well areas or in other
sensitive groundwater areas.
3. Runoff from stockpiles is not allowed to flow directly into streams or
wetlands where environmental damage can occur.
Prior Lake has established a detailed "snow and ice removal policy" to address winter
maintenance needs. Street conditions are assessed for each individual event and ice
control material application is adjusted accordingly. Equipment is maintained in good
working order to place ice control material on roadways and is properly calibrated to
prevent excessive application. The City is in the process of building its own sand/salt
storage facility.
6.5.8 Street Sweeping
Street sweeping is an integral part of the City's effective surface water management
system. It greatly reduces the volume of sediments that have to be cleaned out of sump
structures and downstream water bodies. The City has a "street sweeping policy" that
includes three sweeping operations in a year, or more often as dictated by the City
SWPPP. Spring sweeping begins either late March or early April after the risk oflater
snowfall has passed and targets sand left from winter sanding operations. Fall sweeping
occurs after leaf fall. The downtown area is swept every other week.
Prior Lake does not allow residents to rake leaves into the street for pick up, but does
provide a compost site where residents can bring their leaves. This greatly reduces the
incidence of inlet blockages and protects the water quality of downstream water bodies.
The objective of both programs is to minimize impacts from leaf litter, sand, salt and
other debris on the surface waters of the City.
6.5.9 Detection of Illicit Connections
As presented in the goals section Prior Lake will modify its ordinance to prohibit the
dumping of hazardous material into the stormwater system, As staffing allows the City
will also inspect storm sewer outfalls during dry periods to determine if any illicit
sanitary sewer connections are evident.
n City of Prior Lake
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lit Local Surface Water Management Plan
6-14
The City will also begin the process, as staffing allows, of mapping its storm sewer
outfalls and integrating this mapping with inspection data,
6.6 Education
6.6.1 General
Education can play an important role in any effort to implement a stormwater
management program like the one outlined in this LSWMP. The objectives of an
education effort are different, depending on the target audience. In general, the target
audience for this education program is City staff, City residents, and the development
community. The following sections describe why education of each of these groups is
important and presents educational methods that may be used for each audience.
6.6.2 City Staff
City Staff have a wide range of responsibilities for implementing this plan. These
include:
. Implementing street sweeping and spill response programs.
. Maintaining detention basinlstormwater management pond performance and
system operability.
. Planning for, and management of projects to enhance pollutant removal
performance, wetland quality, etc.
. Carrying out grounds maintenance of City-owned lands/facilities in a way that
sets a good example for residents.
. Utilizing BMPs in application of ice control material.
. Application of Best Management Practice policies and regulations to new and
redevelopment projects.
. Planning and delivering education programs.
. Working out cooperative arrangements with regulatory and non-regulatory
organizations to achieve LSWMP objectives.
. Assisting the City Council in the application of the LSWMP policies.
Because these responsibilities involve many different levels of City staff, City staff
members are trained to have a basic understanding of the LSWMP, including:
. A description of the major stormwater management issues (including known
stormwater management problem areas, stormwater management expectations for
a City of Prior Lake
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lit Local Suiface Water Management Plan
6-15
new and re-development projects, and incorporation of stormwater mitigation into
capital improvement projects, and regulatory jurisdictions).
. The objectives ofthe LSWMP and the general approach outlined in the LSWMP
for resolution of these issues.
. The responsibilities of the different work units in implementing the LSWMP.
. The information the LSWMP provides.
. Identification of in-house experts.
This information is disseminated in presentations at staff meetings, coverage in internal
newsletters, and issuance of internal memos.
6.6.3 City Residents
In order to obtain the necessary political and economic support for successful LSWMP
implementation, it is vital to inform City residents about basic stormwater management
and water quality concepts, policies and recommendations in the LSWMP, and the
progress of stormwater management efforts.
For example, the City has incorporated stormwater management practices into a number
of utility reconstruction projects that benefit stormwater quality in the watersheds of
some of the City's most visible lakes. It is important that residents know about these
projects (including how they were funded) so that they develop an awareness that the
City is being responsive to the public interest in protecting these high priority resources
and that dedicated financial resources such as revenue from the stormwater utility are
being put to work.
This information is presented to the public through the City newsletter: The Wavelength,
press releases on the City website or to local papers, through the Mayor's and City
Manager's columns, and at public meetings as appropriate. Periodic updates on the
progress of LSWMP implementation and information on specific improvement projects is
also provided to the public. Again, the City newsletter and press releases to local papers
are good methods by which this information is disseminated.
The City's Lake Advisory Committee provides educational brochures on a periodic basis.
The City also contributes, through its stormwater utility, to the Citizens Assisted
Monitoring Program (CAMP). This Metropolitan Council sponsored program has as one
of its primary goals the development of a lakes water quality database to facilitate
understanding of the processes involved in urban lakes. The City also conducts bacteria
testing at beaches and makes these results available to the public.
6.6.4 Development Community
The LSWMP is designed to provide the official policy direction that City staff and the
City Council desire to guide stormwater mitigation for new and redevelopment projects.
n City of Prior Lake
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lit Local Surface Water Management Plan
6-16
The information contained within this plan is disseminated to developers and their
consulting engineers as early as possible in the development review process. In this way,
developers know what is expected of them and can consider the requirements in their
initial assessments of the site as well as incorporate the necessary BMPs in any
subsequent designs. Much of the necessary information is disseminated to the developers
in an information packet in the development submittal information they receive from the
City.
While dissemination of information is valuable, there is no substitute for a meeting
between key City staff and the developer as early as possible in the review process. This
helps define expectations for submittals, clarify regulatory compliance issues, and
provide additional detailed guidance. Developers are encouraged to do this as soon as
possible after they have reviewed the LSWMP information and thought about how it
applies to their site.
6.7 Financing and the Stormwater Utility
6.7.1 Current Status - Summary
The City of Prior Lake implemented a stormwater utility in 1993. The current quarterly
residential charge is $6.00 per residential unit. Annual revenue from the stormwater
utility has grown as shown in table 6.5.
Table 6.5
Storm Water Utility Revenue
Year Annual Revenue ($)
1997 137,000
1999 177,000
2001 257,000
2002 283,000
Generally, revenue has grown not because of increases in the charge (the charge has gone
from $5.63 in 1997 to $6.00 in 2005, an increase of 6.6%) but due to development
bringing in more properties over which to collect the charge. With this increased
revenue, though, has come an increase in the City's maintenance responsibilities.
In the past the stormwater utility has funded a staff position, programs, and capital
expenditures. The 2002 capital projects totaled $140,000 and included a dredging
project, a lake bank stabilization project, and some storm drainage improvements.
6.7.2 The Stormwater Utility into the Future
In order that storm water utility (SWU) funding keeps pace with increase in municipal
maintenance responsibilities, the city should plan for the costs to conduct periodic pond
maintenance. Limited data on maintenance activities has been developed by watershed
management organizations. A review of this data suggests an annual maintenance budget
n City of Prior Lake
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lit Local Suiface Water Management Plan
6-17
of$I,250 per acre-foot of wet volume or $4,350 per acre of surface at NWL. Either
parameter is relatively easy to track. This $1,250 per acre-foot maintenance item can be
translated into a per household cost by virtue ofthe fact that one acre-foot is sufficient
pond wet volume for 20 acres of residential development. Assuming 2.5 units per gross
acre, then $1,250 per year is spread among 50 units - $25 per unit per year.
The current residential rate is $24 per unit per year. The current charges provide
approximately $300,000 per year in revenue of which only about $20,000 to $40,000 has
been used for pond maintenance. As the city's maintenance responsibilities grow the
storm water utility funding also needs to grow to keep pace.
Prior Lake is a regulated MS4 under the Phase II NPDES Permit. There is a cost
associated with preparing an NPDES permit and the associated Storm Water Pollution
Prevention Plan (SWPPP). Some estimate cities the size of Prior Lake will spend
$50,000 every five years for permit preparation. For Prior Lake it is reasonable to
assume that $10 per household will be spent every five years - adding $2 per year to the
individual household's storm water utility bill.
The NPDES permit and SWPPP commit the city to certain activities, including capital
projects, for the purpose of improving the quality of the city's storm water discharge.
The U.S. EPA has estimated that the financial commitments that city's will make may
total $10 per household per year. Others place this figure at $20. Since many of the
activities identified by the SWPPP may already be funded (like street sweeping and pond
maintenance) the $20 figure is probably too high. For the purposes of planning increases
in SWU collection the $10 per year figure should be used. Table 6.5 summarizes the
additional storm water utility charges identified above.
Table 6.6
Future Storm Water Utility Funding
Item Annual Charge to Single Quarterly Charge to Single
Residential Unit Residential Unit
Current commitments $24.00 $6.00
Future pond maintenance $25.00 $6.25
NPDES permit and SWPPP $2.00 $0.50
NPDES permit compliance $10.00 $2.50
Total $61.00 $15.25
The estimate of stormwater utility funding needs does not include City participation in
TMDL processes nor does it include preparation by the City ofa non-degradation
analysis as currently required in the draft of the new Phase II NPDES Permit. And the
estimate of funding needs does not include any mitigation that may occur due to the
TMDL or non-degradation processes.
A $61.00/residential unit charge would be close to one of the highest rates among Metro
Area cities.
n City of Prior Lake
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lit Local Surface Water Management Plan
6-18
6.8 Design Standards
The City of Prior Lake has produced and regularly updates a Public Works Design
Manual. The latest version of this manual is titled Public Works Design Manual. City of
Prior Lake, January 2002. This manual, as revised, is adopted by reference into this
Surface Water Management Plan as the applicable design standard for surface water
management.
6.9 Watershed Implementation Priorities
Among the two watershed districts that cover the City, only the Prior Lake Spring Lake
Watershed District has developed a detailed list of implementation priorities. At present
the primary implementation priority for Scott WMO is implementation of its Rules,
adopted on May 10,2005.
The Prior Lake-Spring Lake Watershed District has adopted a Water Resources
Management Plan that includes programs and projects focused on water quality and
runoff management, land management to improve water quality and reduce runoff
volumes, and management of the Prior Lake Outlet System. Additional information
about the District's implementation priorities, programs and projects can be found at
www.plslwd.org or by contacting the District office at (952) 447-4166.
6.10 City of Prior Lake Implementation Priorities
Downtown Redevelopment
In 2007 the City will propose a downtown stormwater management study that will
provide a plan for the stormwater management within a downtown overlay. This plan
will spell out site specific BMPs for rate and volume control, as well as proposed
centralized facilities that will meet City and District rules. Through cooperative effort in
creating and reviewing this plan, the City and District can promote further partnership in
the implementation phase. Currently reconstruction of a portion ofthe downtown area is
scheduled to begin in 20 II. When this reconstruction begins, the approved downtown
stormwater management study will be implemented.
Other implementation priorities for the City as it adopts this Plan and begins the
implementation phase of the Plan include:
I. Assisting the PLSL WD in implementing its retention storage program. Specific
areas with high potential for City implementation are indicated on the system
maps and within the body of this Plan.
2. Increasing Storm Water Utility Funding so that the City can meet its current and
future obligations toward pond maintenance, NPDES compliance, and mitigation
that may come out of the City's non-degradation analysis.
n City of Prior Lake
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1il1 Local Surface Water Management Plan
6-19
3. Application ofthe revised area charge outlined in this report and update of the
area charge based on increases in land value and construction costs.
4. Implementation of the rate control targets as outlined in the appendices and
stormwater modeling that supports this plan.
5. Application of the wetland susceptibility criteria in determining how wetlands are
used for flood storage, retention, and rate control.
6. Working with the PLSL WD regarding the feasibility of augmenting storage in
Buck Lake.
7. Working with the City of Shako pee toward redefining rate control objectives from
their Sand Creek drainage which will ultimately enter the City of Prior Lake
system through its Louisville Swamp system.
8. Gaining equivalency with Scott WMO and PLSL WD rules.
9. Working in partnership with WMO and WD to minimize for downstream impacts
due to urbanization.
10. Passing ordinance revisions consistent with rules regarding buffer widths by
August 9,2006.
II. Implement the City NPDES permit and SWPPP.
6.11 Amendment Procedures
The Prior Lake LSWMP is intended to extend through the year 2016. For the plan to
remain dynamic, an avenue must be available to implement new information, ideas,
methods, standards, management practices and any other changes that may affect the
intent and/or results of the LSWMP. The amendment procedure for the LSWMP is
presented below.
Request for Amendment
Written request for plan amendment is submitted to City staff. The request shall
outline the need for the amendment as well as additional materials that the City will
need to consider before making its decision.
Staff review of Amendment
A decision is made as to the validity of the request. Three options exist: 1) reject the
amendment, 2) accept the amendment as a minor issue, with minor issues collectively
added to the plan at a later date, or 3) accept the amendment as a major issue, with
major issues requiring an immediate amendment. In acting on an amendment request,
City staff shall recommend to City Council whether or not a public hearing is
warranted.
Council Consideration
The amendment and the need for a public hearing shall be considered at a regular or
special Council meeting. Staff recommendations should also be considered before
decisions on appropriate action(s) are made.
n City of Prior Lake
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lit Local Surface Water Management Plan
6-20
Public Hearine: and Council
This step allows for public input based on public interest. Council shall determine
when the public hearing should occur in the process. Based on the public hearing, the
City Council could approve the amendment.
Council Adoption
Final action on an amendment is City Council adoption. However, prior to the
adoption, an additional public hearing could be held to review the plan changes and
notify the appropriate stakeholders.
Coordination with WMO and WD
To the extent and manner required by the Scott WMO all amendments to the LSWMP
shall be submitted to the WMO for review and approval in accordance with applicable
state rules and statutes. (Section 103B and Rules)
To the extent and manner required by the PLSL WD all amendments to the LSWMP
shall be submitted to the WD for review and approval in accordance with applicable
state rules and statutes. (Sections I03B, 103D and Rules)
6.11.1 Minor amendments:
Changes required for TMDL's, Nondegradation Planning, and Ground Water Protection
plans will be considered minor amendments to this document.
6.12 Annual Report to Council
A brief annual report will be made by City staff summarizing development changes,
capital improvements, and other water management-related issues that have occurred
over the past year. The review will also include an update on available funding sources
for water resource issues. Grant programs are especially important to review since they
may change annually. These changes do not necessarily require individual amendments.
The report can, however, be considered when the plan is brought up to date. The annual
report should be completed by July 1 st to allow implementation items to be considered in
the normal budget process.
The City's LSWMP will remain in effect through 2016. The City will then review the
LSWMP for consistency with current water resource management methods. At that time,
all annual reports and past amendments will be added to the document. Depending on the
significance of changes, a new printing of the LSWMP may be appropriate. At a
minimum, the Capital Improvement Program should be amended every five years.
a City of Prior Lake
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1J1 Local Surface Water Management Plan
6-21
7. SUMMARY AND RECOMMENDATIONS
7.1 Summary
The Prior Lake Surface Water Management Plan has a dual purpose: it will serve as a
guide for the construction of storm drainage facilities and provide a basis for a consistent
approach to water resource protection. The following themes have been incorporated
into this LSWMP:
I. Division of the City into drainage districts and subdistricts;
2. Modeling of storm water runoff under ultimate land use conditions;
3. General layout and sizing of trunk storm sewers and open channels;
4. Tributary areas, storage volumes, and high water levels of all required ponding
areas;
5. Development of wetland management policies to ensure compliance with local,
state, and federal wetland regulations;
6. Estimated construction and implementation costs of the Surface Water
Management Plan; and
7. Storm Water Utility funding needs;
8. Recommendations for education of City residents, staff, and development
community .
While providing for public safety through planning for and mitigating flood potential, the
stormwater management systems also functions to minimize economic loss and
inconvenience of periodic flooding and provides water quality and volume management
to retain the high quality of water in the wetlands, lakes and streams.
To provide flood protection for adjacent property, the design storm interval for ponding
areas is a 100-year storm as compared to a 5-year or 10-year storm for design of storm
sewer piping. To provide an additional safety factor, it is recommended that the lowest
exposed opening of a structure in a development should be at least 2 feet above the
calculated high water level of an adjacent pond.
The numerous natural wetlands and depressions found throughout Prior Lake have been
incorporated into the Surface Water Management Plan as ponding areas. The effective
use of ponding areas enables the installation of outflow sewers with reduced capacities
since the design storm duration is effectively increased over the total time required to fill
a City of Prior Lake
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lit Local Surface Water Management Plan
7-1
and empty the ponding reservoirs. Storm sewers represent a sizable investment for the
community and this investment can be more efficiently utilized by ponding stormwater in
designated ponding areas and allowing smaller diameter pipes to be used as outfall lines.
Equally as important as flood control and cost considerations is the use of ponding areas
to:
I. Improve water quality;
2. Return stormwater to the groundwater table; and
3. Increase water amenities in developments for aesthetic, recreational and wildlife
purposes.
For water quality ponds, the wet volume is the most important consideration. It can be
assumed that water quality ponds, which reduce phosphorus loadings by 60% under
standard runoff concentrations, will generally reduce heavy metal concentrations by 70%
and sediments by 90%.
The trunk storm sewer system alignments shown in this LSWMP are conceptual in nature
since future development and the floodplain management requirements of the PLSLWD
will determine the exact location of channels or storm sewers.
All storm sewer facilities, especially those conveying large quantities of water at high
velocities, should be designed with efficient hydraulic characteristics. Special attention
should be given during final design to those lines, which have extreme slopes and create
high hydraulic heads. The Best Management Practices (BMPs) recommended by the
MPCA should be followed wherever necessary.
7.2 Recommendations
The following recommendations are presented for the City Council's consideration based
upon the data compiled in this report:
I. The Surface Water Management Plan as presented herein be adopted by the City
of Prior Lake.
2. Standard review procedures be established to ensure all new development or
redevelopment within the City is in compliance with the grading and stormwater
management controls determined by this Plan.
3. Detailed hydrologic analyses be required or all development and redevelopment
activities.
4. Final high water levels governing building elevations adjacent to ponding areas
and floodplains be established as development occurs or when drainage facilities
are constructed.
5. Overflow routes be established and maintained to provide relief during extreme
storm conditions, which exceed design conditions.
6. A surface water system maintenance program be established to ensure the
successful operation of the system.
n City of Prior Lake
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lit Local Surface Water Management Plan
7-2
7. The erosion and sedimentation control criteria for new developments be enforced.
8. An education program for City residents, staff, and development community be
implemented.
9. Amendments to the plan be adopted and implemented as warranted by future
standards or regulations.
10. That the plan be updated in 2010 or earlier if needed.
II. Promote the use of small-site/distributed BMPs to help achieve water quality and
volume control goals.
12. Pursue partnerships with watershed management organizations and other agencies
to incorporate volume control BMPs into re-development projects, including City
projects.
13. Ordinances be revised to be consistent with rules detailed in the PWDM regarding
water resource management.
n City of Prior Lake
lit Local Surface Water Management Plan
7-3
City of Prior Lake
Surface Water
Management Plan
INDEX OF
AREA MAPS
..'
.
February 2005
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[!J Index Map Designation
Dra"- District Nama
D BU<. Bud< Lake
D CALK. Campbell Laka
D CD13. County Dltch 13
D CR . CradiI River
D CRBA - QystaI Bay
C CRlK. ClystaI Lake
[:::' ERlK - East Rice Lake
C FLK - FISh Lake
D GU< - Geis lake
o HU<. Howard Laka
D JP - JeIlenl Pond
o LPPL. Lower Prior Lake
C LSW. Louisville Swamp
,
o
D ML - Markley Lake
o MLK - Mystic Lake
. D PL - Pika Lake
o RLK - Rice Lake
o SC . Hess Lake
o SCLK - Schneider Lake
o SH - Shakopea
o SPC . Spring Canetral
o SPLK - Spring Lake
o SPW - Spring West
o SULK - Sutton Laka
o SWLK - Swamp Lake
o UPPL - Upper Prior Laka
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[!J Index Map Designation (I J1 " l.~
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City of Prior Lake Drainage District Name '>'" k',. ,
Surface Water ~;I BLK - Buck Lake o ML - Markley Lake ~ i"- t-- ~ ~l4+' I> riT/<~.;; . .>, !, " Li'"
. CALK - Campbell Lake f'.;':f~; I MLK - Mystic Lake I~ I C
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Management Plan [i;~1!l CD13 - County Ditch 13 o PL - Pike Lake . , -
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_ CR - Credit River o RLK - Rice Lake (~ I" 1 -s:::: ,',C,' -f=.
o CRBA - Crystal Bay o SC - Hass Lake f-- SC ~,'L
INDEX OF ,
_ CRLK - Crystal Lake D SCLK - Schneider Lake "]3' ,". .
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AREA MAPS _ ERLK - East Rice Lake D SH - Shakopee f- L ~ [J j'
_ FLK - Fish Lake _ SPC - Spring Canetral \ ---' I/' v \ .
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o GLK - Geis Lake I'" '.",'1 SPLK - Spring Lake 1
N ~1 HLK - Howard Lake .. SPW - Spring West '-'" -..... v ~,
W*E o JP - Jeffers Pond o SULK - Sutton Lake ~
s o LPPL - Lower Prior Lake _ SWLK - Swamp Lake :;:::r
February 2005 _ LSW - Louisville Swamp _ UPPL - Upper Prior Lake
d~r~; Ie:; Feet JP
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llecerriler 2004
1000
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