HomeMy WebLinkAbout7B Impervious Surface Info
4646 Dakota Street S.E.
Prior Lake, MN 55372-1714
PLANNING REPORT
PREPARED BY:
PUBLIC HEARING:
CASE FILE #:
DATE:
6B
DISCUSSION REGARDING POSSIBLE MITIGATION
ALTERNATIVES FOR IMPERVIOUS SURFACES
JEFF MATZKE, PLANNER
_YES X NO-N/A
08-124
MAY 27,2008
AGENDA ITEM:
SUBJECT:
INTRODUCTION:
As part of our Zoning Ordinance Update project City Staff has been directed by
the City Council to review the current impervious surface requirements and
research possible mitigation alternatives. The following information includes
details regarding the current ordinance requirements, a memorandum from City
Staff related to possible Best Management Practice alternatives for impervious
surfaces, and comments from the recent Lake Advisory Committee discussion.
ANALYSIS:
Current Ordinance Requirements
The current definition of impervious surface is as follows:
Impervious Surface. The portion of the buildable parcel which has a covering which
does not permit water to percolate into the natural soil. Impervious surface shall include,
but not be limited to, all driveways and parking areas, whether paved or not, sidewalks
greater than 3 feet in width, patios, tennis and basketball courts, swimming pools,
covered decks and other structures. Decks open to the sky and having open joints of at
least 1/4 inch, areas beneath overhangs less than 2 feet in width, and sidewalks 3 feet in
width or less shall be exempted from the calculation of impervious surface. The
impervious surface of a lot shall be documented by a certificate of survey unless
exempted from this requirement by the Zoning Administrator.
Section 1104.306 of the Shoreland Ordinance states the following:
Impervious Surface Coverage: Impervious surface coverage for lots in all Use
Districts shall not exceed 30% of the lot area, except as provided in the following
sections. Such impervious surface coverage shall be documented by a certificate
of survey at the time of any zoning or building permit application, according to the
definitions of impervious surface as listed in subsection 1101.400.
(1) An existing site which is being altered, remodeled, or expanded without
expanding the existing impervious surface may be allowed, provided that where
appropriate and where necessary, structures and practices for the treatment of
storm water runoff are in compliance with the Prior Lake Storm Water
Management Plan and approved by the City Engineer.
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Phone 952.447.9800 / Fax 952.447.4245
(2) In all "C" and "I" Use Districts and for Park/Recreation uses, new construction on
conforming lots or an existing site being altered, remodeled, or expanded which
expands the existing impervious surface coverage may be allowed where
necessary, provided the site conforms to the Prior Lake Storm Water
Management Plan, Best Management Practices and is approved by the City
Engineer, provided the impervious surface coverage does not exceed 75% of the
total lot and provided the following stipulations are met:
y All structures, additions or expansions shall meet setback and other
requirements of this Ordinance.
y The lot shall be served by municipal sewer and water.
y Storm water is collected and treated in compliance with the City Storm
Water Management Plan and Best Management Practices. Such
treatment may be comprised of either on-site control, access to the City's
storm water control system, or a combination of both, to be approved by
the City Engineer.
(3) Impervious surface coverage for all "C" and "I" Use Districts and for
Park/Recreation uses may be allowed to exceed 75% of the total lot or exceed
existing conditions on the lot which are over 75% provided the following
stipulations are met:
y A conditional use permit is submitted and approved as provided for in
subsection 1108.200.
y Improvements that will result in an increased rate of runoff directly
entering a public water shall have all structures and practices in
compliance with the City Storm Water Management Plan in place for the
collection and treatment of storm water runoff or will be constructed in
conjunction with a conditional use permit application.
y All the conditions in subsection 1104.306(2) above are satisfactorily met.
(4) Measures for the treatment of stormwater runoff and/or prevention of stormwater
from directly entering public water include such appurtenances as sediment
basins (debris basins, desilting basins, or silt traps), installation of debris guards
and microsilt basins on storm sewer inlets, oil skimming devices, etc.
Alternative Best ManaQement Practices (BMPs)
The attached memorandum from the City's Water Resource Engineer details the
current statutory limits for impervious surface, as well as an engineering
background and technical explain of various Best Management Practice methods
to handle stormwater. Details regarding possible procedures for monitoring and
managing these installed methods are also included at the end of the
memorandum.
lake Advisory Committee Discussion
The Lake Advisory Committee discussed these mitigation alternatives at their
May 21,2008 meeting. The following are comments from their discussion:
. With high prices of lakeshore properties it would be nice to offer
alternative methods for the small lots that need a greater
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amount of impervious surface to gain the high value of the
property.
. In regards to a possible escrow or letter of credit to be held by
the City on properties with a mitigation system, this could place
a burden on a residential property owner. The mitigation
systems will already incur a large cost for its installation and
maintenance.
. A method of reviewing/regulating maintenance schedules of the
mitigation systems would need to be very clear and objective so
that current and any future residents of a property and the City
Staff are well aware of the requirements.
. Startup and implementation of managing and monitoring
methods of the mitigation systems could be a serious
undertaking for the City both financially and practically.
. Since our ordinance already allows for a 5% more impervious
surface than the DNR regulation of 25% and our water quality
has not really improved over the last few years, the DNR may
not be very receptive to additional impervious surface
allowances.
. Impervious surface is not only looked at from a stormwater
perspective (which the mitigation BMPs may address) but also
its impact on aesthetic values and natural wildlife in our lake
community. It is difficult for these other factors to be addressed
by specific mitigation regulations because they are more
arbitrary than stormwater quality and runoff statistics.
CONCLUSION
After reviewing the provided information, City Staff will be addressing the
Planning Commission for discussion, questions, and comments regarding the
possibility of using alternative mitigation techniques for impervious surface
credits. These comments will be presented to the City Council at their review of
any possible ordinance changes.
ACTIONS REQUIRED:
No Formal Action is required.
ATTACHMENTS:
1. Memorandum from the Water Resource Engineer
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3
Engineering Department
City of Prior lake
16200 Eagle Creek Avenue S.E.
Prior Lake, MN 55372
MEMORANDUM
TO:
Danette Walthers-Moore, Community Development and Natural Resources Director
FROM:
Ross Bintner, Water Resources Engineer.
DATE:
April 18, 2008
RE:
BMP Credits for Impervious Surface Requirement.
In a recent meeting with a DNR hydrologist, the possibility was floated of allowing a higher cap in the impervious limit if
equivalent structural practices were put in place. Currently the City of Prior Lake enforces a maximum impervious
surface limit of 30% on residential lots within 1000' of a lake. The purpose of this memorandum is threefold:
I. Statutory background - Explain state planning process and statutory limit.
2. Engineering background - Explain thought process behind requiring impervious limit.
3. Technical exploration - Look into possibil\ty of allowing credit mechanism.
Statutory Back2round
The concept of a 1000 foot "shoreland zone" and its impervious cover limitation are a product of State Rules and Statute.
Minnesota rule 6120.330 Subp II part B sets a limit of 25% maximum impervious on shoreland zone lots. The State
planning processes that lead to this rule creation was complete in 1989. The impervious limit was one of many
mechanisms put into place as a part of statewide rules to mitigate urbanization's detrimental affect on water quality. The
impervious limit was also enacted to provide enhanced wildlife habitat and a natural visual appearance though increased
tree canopy and vegetative cover, however these reasons will not be explored in this memorandum.
EUI!ineerin2 Back2round
The urbanization of a watershed leads to increased
impervious surfaces. A more impervious watershed causes
more direct nutrient transfer, higher peak runoff rates and
larger runoff volumes. This increased pollutant load and
connectivity of the watershed is difficult to manage in the
shoreland zone, due to the proximity of the lake.
As impervious surfaces increase, groundwater recharge,
evaporation and transpiration decrease and runoff
increases. The use of Best Management Practices (BMPs)
is common in water resource management, as engineers
attempt to design a sustainable series of treatment devices
and practices that route and treat stormwater to mimic the
G:\Water Resources\080418 BMP Shoreland Impervious Memo.doc
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hydrology of an area before development. A
practical example of this practice: A forested area
along the Lake is planned to be developed as
housing. In the existing condition only 10 percent of
rainfall will run off the site. In the proposed
condition 30 percent of rainfall will run off the site.
Since housing and roads generate additional runoff,
water quality and rate control ponds are planned
with the development to slow the additional water
runoff and settle out sediments and debris generated
by the increased impervious surfaces.
All BMPs require monitoring and maintenance to
continue functioning and providing a consistent
level of service. While all BMPs provide public and
private benefit, to prevent public degradation of
water, not all are publicly maintained.
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The above diagram shows a pond cut-section with normal, 100-year and sediment volumes pictured. Water quality ponds
are generally development-scale BMPs, In the technical section of the memorandum, private home-scale BMPs will be
explored.
Technical Exploration
This section will explore the possibility of providing a credit system that would allow a homeowner to exceed the 30%
limit by providing structural BMPs, First; potential structural BMPs will be detailed, highlighting benefits, intended use,
maintenance liability, and, monitoring and inspection protocol.. Next; water quality issues will be explored to detail a
potential credit system. Finally; a monitoring and maintenance system will be theorized and issues detailed.
Potential BMPs
Type: Infiltration Trenches
. Description: A trench dug with jilter fabric
placed around the excavation enclosing an
open graded aggregate that provides space
to store water. A observation well is
provided to monitor system function.
· Purpose: To provide suiface water flow a
place to collect and a path to disburse to the
groundwater.
· Benefit: Moderate benefit to volume control
and groundwater recharge. Small benefits
for water quality and rate control exist as
well.
· Suitability: This BM P is not suitable for p,.,....: SChlNtH ,N7
tight soils such as clay and may be a risk to
groundwater quality in sandy soils or wellhead protection areas.
. Maintenance: Sediment frequently clogs the suiface layer and over the long term clogs the storage throughout
the system. Raking and replacement of suiface to maintain flow is required ANNUALLY. Monitoring is required
QUARTERLY.
. Design life: A infiltration trench system can be expected to last 3 to 7 years.
G:\Water Resources\080418 BMP Shoreland Impervious Memo.doc
Type: Underground Storage / Rainwater Cistern
. Description: A storage chamber made of Plastic or
Corrugated Metal provides a place to temporarily
store water.
. Purpose: Water is directed from roofs or driveways
and is stored under lawn or pavement until it is
neededfor irrigation.
. Benefit: Substantial benefit for volume control.
Moderately benefit for groundwater recharge, and
rate control. Small benefit to water quality.
. Suitability: This BM P is suitable for many
applications.
. Maintenance: Tanks must be inspected for sediment
QUARTERLY. Sediment must be cleaned from tanks
BIYEARLY. Pump must be maintained on an as
needed basis.
. Design life: With regular cleaning and pump replacement a system can be expected to last 45 years.
Type: Pervious Pavements
. Description: A permeable pavement slllj'ace that
allows rainwater to soak into the ground. Previous
slllj'aces such as the one pictured to the left can also
be planted with vegetation.
. Purpose: To promote groundwater recharge while
providing a stable pavement slllj'ace for light vehicle
traffic and parking
. Benefit: Moderate benefit to groundwater recharge
and volume control. Some benefit to rate control and
water quality.
. Suitability: This BM P is not suitable for tight soils
such as clay, or may require additional under drains
in such situations.
. Maintenance: Pores need cleaning in situations where sedimentation can occur. ANNUAL sweeping and
reseeding is required. Freeze thaw cycle can cause heaving and damage if drainage is not functioning. Shoveling
and plowing may cause damage.
. Design life: A pervious pavement system can be expected to
last 25 years.
Type: Green Roofs
. Description: A roof with special reinforcement and planting
media that allows plant growth on top of homes, garages, or
sheds.
. Purpose: Provide a unique architectural aesthetic that also
provides benefit to water quality.
G:\Water Resources\080418 BMP Shoreland Impervious Memo.doc
. Benefit: Substantial Benefit to water quality and volume control. Moderate benefit to rate control. Added
benefit of habitat and reduced heat.
· Suitabil ity: This BM P is suitable to a wide variety of situations but can be very cost prohibitive.
. Maintenance: Frequent maintenance is required to establish vegetation. Weeding and replanting can be a
challenge at heights and slopes.
. Design life: A green roof can be expected to last 15 years.
Type: Raingardens
. Description: A shallow depression planted with flowering perennial plants and native grasses.
· Purpose: Raingardens collect water from nearby
impervious slllj'aces and allow it to infiltrate.
Rain gardens use the deep rooted plants to provide
a path for groundwater recharge.
. Benefit: Substantial benefit to volume control and
groundwater recharge. Moderate benefit to water
quality and rate control.
· Suitability: Rain gardens are suitable for a wide
variety of situations but may not work well on tight
soils such as clay, or in excessive sediment
conditions or groundwater sensitive areas.
. Maintenance: Vegetation management is required
MONTHLY. Sediment cleaning is required
ANNUALLY. Tilling and replanting is required
BIANNUALLY.
· Design life: A properly maintained rain garden can be expected to last 20+ years before major soil amendments
are needed.
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Type: Buffer Zones
. Description: A near shore zone planted with deep rooted
native vegetation.
. Purpose: Buffer zones stabilize the shoreline and near
shore area soil by providing a thick cover of native
vegetation and root growth both upland and in the
water.
. Benefit: Substantial benefits to water quality and
shoreline stabilization exist. Small benefits for volume
and rate control also present.
. Suitability: Buffers are suitable for all shorelines.
Heavy tree canopy can reduce ground plants thickness
and limit species, but tree roots serve the same purpose
for soil retention.
. Maintenance: Establishing a buffer zone can take
MONTHLY maintenance in the first 5 years. After vegetation is established, minimal maintenance is required.
· Design life: A diverse plant community is self repairing and can be expected to last indefinitely. Protection is
required from encroachment. Some vegetation management may be desired to promote wiklflower growth.
G:\Water Resources\080418 BMP Shoreland Impervious Memo.doc
Type: Boulevard Trees
. Description: Trees planted near
impervious surfaces with canopy
overlapping planned.
. Purpose: Boulevard Trees intercept
rainwater that would otherwise fall
onto impervious surfaces. This
concept of "canopy intercept and
evaporation" can have a applicable
effect on total runoff in small
storms.
. Benefit: Some volume control
benefit exists.
. Suitability: This is a natllral process and can be taken advantage of in all near-pavement tree plantings.
. Maintenance: This practice depends on the size and health of the tree. Proper tree care is required.
. Design life: Variable based on number of trees. tree age and health.
SHADE TIlIiES
ORNAMENTAL TIlIiES
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In this first portion of the technical exploration, the following potential BMPs were detailed: Infiltration trenches,
underground storage / rainwater cistern, pervious pavements, green roofs, raingardens, buffer zones, and boulevard trees.
In the next section the BMPs will be compared with the potential trade off of increased site impervious balance water
quality issues.
Potential Credit System
If a credit system were allowed, a homeowner would be able to apply for a permit to build roofs, driveways, sheds, patios
and other impervious surfaces that would amount to greater than the current 30% impervious limit. In exchange for this
increased impervious the homeowner would agree to design, construct, monitor and maintain a series of BMPs that would
provide equivalent or greater public benefit to water quality. (Aesthetic and habitat public benefits would be lost, or
mitigated through another, unexplored method)
To craft a credit system and figure out what methods would be required to mitigate the affects of a hypothetical
impervious addition on a small scale basis, first we
must look at what is lost by adding more impervious. RELATlOtIALlRHlDS OF FRESH WATER FISH AOIVITYTO TURBIDITY VALUES AND TIME
The addition of impervious causes the following:
. Increased runoff volumes.
. Increased runoff rates.
. Increased pollutant loads.
o Phosphorus (organic matter)
o Turbidity (suspended sediment)
o Heavy metals (pavement and roof)
o Salts and household chemicals
o Hydrocarbons (cars)
. Decreased soil stability.
. Decreased Groundwater Recharge.
These five items each could be mitigated for using a
series of BMPs such as those described above. To
steam line the planning design and approval process a
fixed credit system with strict design standards could
be created,
G:\Water Resources\080418 BMP Shoreland Impervious Memo.doc
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This fixed credit system would call out how-much and which methods could be used mitigate increased impervious. The
following is a concept:
POTENTIAL CREDIT SYSTEM
Per 1000 SF Requirement Infiltration Cistern Pervious Green roof Raingarden Buffer Trees
Pavement
Volume 104C ICICF ICICF 0.5C I SF 0.5C I SF ICICF N/A IC/ea
Rate 1000C (credits) 10C I CF 10C I CF 5C I SF 2C I SF 20C I CF 0.05C/SF N/A
Pollutant IOOOC (credits) 5C I CF 3C I CF 2C I SF 2C I SF I OC I CF 0.6C/SF 10C/ea
Soil Stabilize Shore N/A N/A N/A N/A N/A IC/SF 15C/ea
and Runoff path
I: I wi IOcrease
( 1000C)
Groundwater 42C 0.5C I CF 0.5CICF .25C/SF N/A 1.5C I CF N/A N/A
In this concept, a hypothetical 1000SF impervious surface would need to
meet the 5 requirements (volume, rate, pollutant, soil, and groundwater) by
mixing the 7 BMPs credit systems; an example.
I OOOSF of new impervious would require the following per storm:
. 104 Cubic Feet of volume control
. 1000 Credits of rate control
. 1000 Credits of Pollutant removal
. 1000 Credits of Soil Stabilization
. 42 Credits of groundwater recharge
Here is an example of a mix of BMPs installed to meet the credit
requirements. (In this example, all BMPs are used, but this is not required)
. Green roof on 80SF shed
. 10CF rain barrels/cistern
. 30CF Infiltration trench
. 30SF Pervious path
. 5 Boulevard/Driveway Trees
. 12CF Raingarden
Requirement Infiltration Cistern Pervious Green Raingarden Buffer Trees Total
(C=credits) Pavement roof
DESIGN 30CF 10CF 30SF 80SF 12CF 925SF 5
Volume 104C 30C 10C 15C 40C 12C 5C 112C
Rate 1000C 300C IOOC 150C 160C 240C 50C 1000C
Pollutant 1000C 150C 30C 60C 160C 120C 555C 50C 1125C
Soil 1000C 925C 75C 1000C
Groundwater 42C 15C 5C 7.5C 18C 45.5C
G:\Water Resources\080418 BMP Shoreland Impervious Memo.doc
Sustainable Monitoring and Maintenance Protocols
Because benefits provided by the homeowner-installed and maintained BMPs would need to be sustainable indefinitely,
there would need to a program to ensure that monitoring and maintenance was taking place a regular intervals. The
following will detail a recommended protocol for successful implementation and monitoring of BMPs:
The recommended protocol has 3 parts: Homeowner agreements, Design review and construction verification, and
Program manager duties,
I. Homeowner Agreements: A legally enforceable tool will be required such that the City can compel current and
future owners of BMPs to maintain all BMPs to an agreed upon level of service. The agreement must be easily
enforceable allowing the City to enter onto private property and must be a part of the property Deed so that future
buyers are bound to the requirements as well.
2. Design Review and Construction Verification: The design and construction must be approved and witnessed by
City Staff to ensure proper methods are used and BMPs will function as planned. The agreement will need to
spell out liability issues, such that a failed BMP that causes damage to property or financial hardship does not
expose the City to liability since Staff will review and approve design and construction.
3. Program Manager: To maintain the promised level of service, City Staff must regularly inspect and test BMP
function. A program manager will be needed to coordinate program documentation, record keeping, notification
correspondence, and other duties.
The startup costs of creating a sustainable monitoring and maintenance protocol would be significant, due to the technical
and legal nature of the agreement and BMPs. This protocol only puts a basic outline to the program and more thought it
needed if a credit system is offered.
G:\Water Resources\080418 BMP Shoreland Impervious Memo.doc