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09 26 2016 Flood Reduction Study
SURFACE WATER MODELING AND FLOOD DAMAGE REDUCTION STUDY JOINT WORKSHOP September 26, 2016, 5:00 PM Prior Lake City Hall, Parkview Conference Room INTRODUCTION The Prior Lake-Spring Lake Watershed District (PLSLWD) and the City of Prior Lake, in coordination with Spring Lake Township, have been working with Barr Engineering to complete the Surface Water Modeling and Flood Damage Reduction Study which includes updating modeling of the watershed, reviewing flood-related issues and historical projects, identifying and evaluating a suite of potential flood reduction strategies, and creating an implementation plan. A joint meeting of the Spring Lake Town Board, Prior Lake Spring Lake Watershed District Board and the Prior Lake City Council was held on February 8, 2016 to review the Surface Water Modeling and Flood Damage Reduction Study and obtain the input to advance the project. At that time, direction was provided on a number of policy questions and additional informational needs were identified. Some common themes included: support for using public dollars to protect public infrastructure; buyout of existing homes within the 100-year floodplain was generally not supported; preliminary support for the upper watershed storage option; and discussion regarding reducing the frequency of the need to deploy temporary protection measures (i.e., sandbagging). The following goals were communicated during the last joint meeting: 1. Protection of public safety – maintain emergency vehicle access at all times 2. Protection of health and safety – protect public utility infrastructure (i.e., sanitary sewer and water distribution) 3. Maintain traffic flow through the County Road 21 corridor 4. Maintain access to private properties CURRENT CIRCUMSTANCES A draft of the study is attached to this report. The study documents the history of flooding of Prior Lake, describes the modeling completed, and summarizes different potential options that were identified to address flooding issues. In particular, please focus on Section 1, which provides the executive summary of the study. Figure EX-1, which summarizes the flood impacts on Prior Lake for each modeled option, and will be referenced during the work session presentation. Section 7 includes a summary of all options considered and provides information on how the final suite of options were determined. Section 7.4.2 will be critical during the joint meeting discussion as it provides additional information on each option and gives a timeline for potential implementation. The final step in the study process is to identify an option or suite of options to meet the study goals and policymaker priorities. Using the goals from above, staff has developed a recommendation using a combination of strategies that provides additional flood protection and seeks to lower the risk of flood damage over the short and long term: 1. Level of Protection a. High priority to reduce the flood level on Prior Lake to 905.5 at the 25 year return or frequency protection level. This will protect 6 out of the 8 public right-of-ways that would normally be inundated with flood water. b. Secondary priority to cost effectively provide additional flood damage reduction above that level based on future assessments as part of adaptive management strategy. 2. Short-term Goal: Of the options that were evaluated, two of the options had a higher feasibility and could be easily implemented on a short timeline, with relatively minimal cost. a. Option A – Enhanced Protection. An interim strategy to address any flooding event, while other permanent options are being developed. See Section 7.4.2.5 for a detailed description of this option. b. Options G – Actively Managed Prior Lake Outlet. A strategy that involves more deliberate operation of the existing low-flow gate. See Section 7.4.2.1 for a detailed description of this option. 3. Long-term Goal: The remaining options have a longer timeline to implement. Staff recommends upper watershed storage (Option G, Section 7.4.2.2) as the most feasible option at this time considering all factors in the decision matrix. In addition to flood damage reduction, this option has the highest water quality improvement potential. All of the long-term options will take significant time to be fully implemented. This option can be implemented incrementally, therefore some benefit will be achieved as each project area is completed. This final joint meeting of the three organizations has been scheduled to review the final study results, reaffirm that goals as identified above, consider staff recommendations, and provide direction needed to complete the implementation plan. A summary of the study will be presented and policymaker questions that were raised during the last joint policymakers meeting will be answered. Staff will also review potential funding mechanisms and opportunities. Specific feedback will be sought on the staff recommendations and ultimately the identification of a suite of options that will achieve the project goals. With this direction an implementation plan will be developed. Once the study documents are finalized a final community meeting will be held to solicit input from the general public. Finally, the City and PLSLWD will consider approval/adoption of the study. 4300 MarketPointe Drive, Suite 200 Minneapolis, MN 55435 952.832.2600 www.barr.com Prior Lake Stormwater Management & Flood Mitigation Study Prepared for Prior Lake-Spring Lake Watershed District and the City of Prior Lake September, 2016 Prior Lake Stormwater Management Flood Mitigation Study report_FINAL DRAFT_all comments i Prior Lake Stormwater Management & Flood Mitigation Study September, 2016 Contents 1.0 Executive Summary ........................................................................................................................................................ 1 1.1 Introduction ................................................................................................................................................................. 1 1.2 Study Goals .................................................................................................................................................................. 2 1.3 Community Planning Process ................................................................................................................................ 2 1.4 Monitoring and Modeling ....................................................................................................................................... 3 1.5 Results Summary ........................................................................................................................................................ 4 2.0 Background ....................................................................................................................................................................... 7 2.1 Study Partners ............................................................................................................................................................. 7 2.2 Watershed Characteristics ....................................................................................................................................... 7 2.3 History .......................................................................................................................................................................... 11 3.0 Goals and Objectives ................................................................................................................................................... 17 3.1 Study Goals ................................................................................................................................................................ 17 3.2 Study Objectives ....................................................................................................................................................... 17 3.3 Community Goals and Priorities ......................................................................................................................... 18 4.0 Community Planning Process ................................................................................................................................... 19 4.1 Public Kick-Off Meeting—February 19, 2015 ................................................................................................. 19 4.2 First Advisory Group Meeting—May 7, 2015 ................................................................................................. 20 4.3 Second Public Meeting—May 28, 2015 ........................................................................................................... 20 4.4 Farmer Listening Session—February 3, 2016.................................................................................................. 21 4.5 Second Advisory Group Meeting—February 4, 2016 .................................................................................. 21 4.6 Joint Policy Group Meeting—February 8, 2016 ............................................................................................. 22 5.0 Watershed Monitoring and Modeling ................................................................................................................... 23 5.1 Monitoring .................................................................................................................................................................. 23 5.2 Model Development and Calibration ................................................................................................................ 28 5.3 Final Modeling Summary ...................................................................................................................................... 29 6.0 Identification of Mitigation Options ....................................................................................................................... 31 7.0 Flood Mitigation Analysis ........................................................................................................................................... 33 7.1 Potential Improvement Options ......................................................................................................................... 33 7.1.1 Option A—Enhanced Protection ................................................................................................................... 33 ii 7.1.2 Option B—Spring Lake Storage ..................................................................................................................... 33 7.1.3 Option C—Prior Lake Outlet Modification ................................................................................................. 34 7.1.4 Option D—Upper Watershed Storage ........................................................................................................ 35 7.1.5 Option E—Combine Options B, C & D ........................................................................................................ 37 7.1.6 Option F—Floodproofing ................................................................................................................................. 37 7.1.7 Option G—Actively Managed Prior Lake Outlet ...................................................................................... 38 7.2 Improvement Options Modeling and Cost Summary ................................................................................. 38 7.3 Permitting Requirements and Easement Acquisition .................................................................................. 42 7.4 Matrix Comparison of Other Considerations ................................................................................................. 43 7.4.1 Rationale for Scoring Detailed Comparison Matrix of Improvement Options .............................. 43 7.4.1.1 Flood Reduction Benefits ....................................................................................................................... 43 7.4.1.2 Water Quality and Natural Resources Benefits ............................................................................... 44 7.4.1.3 Feasibility Issues ......................................................................................................................................... 44 7.4.1.4 Human Impacts .......................................................................................................................................... 45 7.4.1.5 Risk Factors .................................................................................................................................................. 46 7.4.1.6 Cost of Project and Cost Score ............................................................................................................. 46 7.4.2 Results of Detailed Matrix Comparison ....................................................................................................... 46 7.4.2.1 Actively Manage Prior Lake Outlet (Option G) ................................................................................ 48 7.4.2.2 Upper Watershed Storage (Option D) ............................................................................................... 49 7.4.2.3 Prior Lake Outlet Modification (Option C) ........................................................................................ 50 7.4.2.4 Combine Options B, C & D (Option E) ............................................................................................... 51 7.4.2.5 Enhanced Protection (Option A) .......................................................................................................... 52 7.4.2.6 Spring Lake Storage (Option B) ............................................................................................................ 53 7.5 Cost/Benefit Analysis and Conclusions ............................................................................................................. 56 8.0 Preferred Option and Implementation Plan ........................................................................................................ 58 8.1 Recommended Actions .......................................................................................................................................... 58 8.2 Consideration for Future Development ............................................................................................................ 58 8.3 Schedule/Sequencing ............................................................................................................................................. 58 8.4 Responsibilities ......................................................................................................................................................... 58 8.5 Permitting Requirements and Easement Acquisition .................................................................................. 58 8.6 Funding/Financing Options .................................................................................................................................. 58 iii List of Tables Table 2-1 Summary of Existing Prior Lake Outlet Management Policy and Operating Procedures .... 12 Table 5-1 Drainage Characteristics of Watershed Monitoring Sites ............................................................... 23 Table 5-2 2014 Flow Monitoring Summary ............................................................................................................. 27 Table 5-3 Watershed Model 30-Day Duration Design Event Results ............................................................. 30 Table 6-1 Initial Comparison Matrix of Flood Mitigation Options .................................................................. 32 Table 7-1 Upper Watershed Storage Summary ..................................................................................................... 36 Table 7-2 Existing/Option D Peak Discharge Rates for Upper Watershed Storage Areas/Lakes ......... 37 Table 7-3 Simulated Prior Lake Peak Elevations for flood mitigation options ............................................ 39 Table 7-4 Summary of Flood Mitigation Option Costs and Simulated Flooding Impacts ...................... 41 Table 7-5 Detailed Comparison Matrix of Improvement Options ................................................................... 56 List of Figures Figure EX-1 Cost/Benefit Summary of Potential Improvement Options ............................................................. 6 Figure 2-1 Study Area ......................................................................................................................................................... 8 Figure 2-2 Met Council Land Use (2010) ...................................................................................................................... 9 Figure 2-3 Watershed Topography .............................................................................................................................. 10 Figure 2-4 Historical Prior Lake and Spring Lake Levels ....................................................................................... 13 Figure 2-5 Prior Lake and Spring Lake Levels Since 1971 .................................................................................... 14 Figure 2-6 Prior Lake Annual High Water Frequency Analyses With and Without Lake Outlet ............. 15 Figure 2-7 Public and Private Impacts by Prior Lake Level .................................................................................. 16 Figure 5-1 Calibration Flow Monitoring Stations .................................................................................................... 24 Figure 5-2 Calibration Station Flow Data for 2014 ................................................................................................. 25 Figure 5-3 Observed Flow Data for 2014 with Upper Watershed Sites Combined ..................................... 26 Figure 5-4 Prior Lake Watershed Model Calibration .............................................................................................. 29 Figure 7-1 Existing and proposed conditions elevations for Spring Lake with modified outlet ............ 34 Figure 7-2 Simulated Prior Lake flood levels for existing conditions and flood mitigation options ..... 39 Figure 7-3 Cost/Benefit Summary of Potential Improvement Options ........................................................... 57 List of Appendices, Attachments, or Exhibits Appendix A—Modeling Technical Memoranda Appendix B—TP40/Atlas 14 Precipitation Comparison iv Certifications I hereby certify that this plan, specification, or report was prepared by me or under my direct supervision and that I am a duly Licensed Professional Engineer under the laws of the state of Minnesota. September 21, 2016 Gregory John Wilson PE #: 25782 Date v Acronyms Acronym Description CAC Citizens Advisory Committee FEMA Federal Emergency Management Agency FIS Flood Insurance Study LAC Lakes Advisory Committee LiDAR Light Detection and Ranging MSL Mean Sea Level MnDNR Minnesota Department of Natural Resources MnDOT Minnesota Department of Transportation NWL Normal Water Level OHW Ordinary High Water PLOC Prior Lake Outlet Channel PLSLWD Prior Lake-Spring Lake Watershed District SMSC Shakopee Mdewakanton Sioux Community TAC Technical Advisory Committee Glossary Term Definition Access Used to describe extent to which vehicles can safely pass through flood waters to reach residences and business Flashiness Refers to flow rates that quickly rise (and fall) to higher peak levels, with increasing likelihood for flooding Improved habitat Describes wetland conditions that mimic natural wetland benefits for waterfowl habitat Normalized peak discharge Peak discharge rate (in cfs) at a point in the watershed divided by its total drainage area (in acres) Upper Watershed Storage Any storage area upstream of the Spring Lake and Prior Lake basins 1 1.0 Executive Summary 1.1 Introduction In the spring of 2014, the Prior Lake watershed experienced record amounts of precipitation which led to a historic flooding event. This event triggered many questions and highlighted the need to develop watershed modeling and evaluate flood mitigation strategies for future events. Prior Lake Spring Lake Watershed District (PLSLWD) and the City of Prior Lake, in collaboration with Spring Lake Township, retained Barr to complete this study which includes calibrated modeling of the watershed, review of flood-related issues and projects, identification and evaluation of a suite of potential flood reduction strategies and implementation plan recommendations. The study work also included a public input process that has engaged a broad range of stakeholders including local units of government, lake associations, the agricultural community and private landowners. Their input was used to guide the development and evaluation of the available flood mitigation options described herein. The study area included the Prior Lake-Spring Lake watershed upstream of the Lower Prior Lake outlet structure, encompassing approximately 19,000 acres (30 square miles) of land in Scott County, Minnesota with agricultural land predominant in the south and western areas and residential land uses surrounding the study lakes. Lake levels for Upper and Lower Prior Lakes have historically been one of the most important issues for the community, specifically the residents living around the Lakes, since Prior Lake does not have a natural outlet. In 1978 a Flood Insurance Study (FIS) was completed for Prior Lake, which established the regulatory flood zone around Prior Lake and resulted in the calculated 100-year flood elevation of 908.9 feet mean sea level (MSL). After significant study, public process and agency coordination the establishment of the Prior Lake Outlet and Channel (PLOC) was selected as the first flood mitigation effort by the PLSLWD and the outlet system was first used in 1983. In 1987, an operating plan was adopted for outlet control structure which set operating procedures and allowable discharges. The FIS was updated in 1997, but the modeling did not account for the benefit of the outlet structure because the channel capacity downstream of the control structure and legal constraints with the adjoining communities limit the discharge the City of Prior Lake can pass through the control structure (FEMA, 1997). Beginning in 2004, PLSLWD pursued improvements to the outlet structure of the PLOC which included a fixed weir set at an elevation of 902.5 feet MSL and a slide gate to allow manual discharge of water between lake level elevations of 902.0 and 902.5 feet MSL. The current outlet configuration will not allow the outflow rate to exceed 65 cfs. Even with the lake outlet in operation, the 2014 flood level of 906.2 feet MSL for Prior Lake is significantly higher than any other flood event since 1915 (PLSLWD, 2003). Without the lake outlet, the 2014 flood level would have been more than 6 feet higher. There is a significant incremental increase in the number of homes that are at risk of flooding for each foot above a lake level of 906.5 feet MSL, while there are linear increases in other types of infrastructure at risk with increasing flood levels. 2 1.2 Study Goals The goals of this study were as follows: 1. Update the flood model 2. Compile and review historical studies and projects to summarize efforts already accomplished toward flood damage reduction 3. Identify a universe of flood damage reduction strategies 4. Evaluate the cost/benefit of the strategies and define community goals for flood protection 5. Develop an implementation plan that identifies a suite of projects or mechanisms to reduce flood damage potential 1.3 Community Planning Process This study included a process for gathering input from the community, including advisory groups, study partners, and decision-making bodies that needed to be part of the process (see Section 4). The following describes the results of the community planning process with the meetings listed in chronological order: 1. Public Kick-Off Meeting—February 19, 2015. This open house format meeting was intended to inform the public of the study goals and objectives, receive input on what they experienced during the 2014 flooding and to obtain their ideas for flood mitigation options. The meeting included specific survey questions to provide feedback, which was also made available on the PLSLWD website. 2. First Advisory Group Meeting—May 7, 2015. This meeting was used to present preliminary modeling results and gather feedback on potential flood mitigation options and the draft criteria for a selection matrix. 3. Second Public Meeting—May 28, 2015. Attendees were provided with a study update, including a summary of the watershed modeling and an overview of the matrix. Thirty attendees (with 18 indicating that they live on the lakeshore, eight in the city, and three in rural areas) completed a survey that rated important factors in selecting a potential flood-mitigation project and their feelings about use of public funding for flood mitigation efforts. The survey results indicated the following: a. Water quality was most important factor, followed closely by cost/benefit b. Half of the respondents thought it was important, and another 36% thought it was very important that options should protect properties and public roads below the 100-year FEMA flood level c. Project readiness was the least important factor d. 69% supported use of public money to protect individual homes/businesses e. 60% supported increasing property taxes to finance the options f. City of Prior Lake has obligation to maintain emergency access to properties g. General sentiment that public funds should be used to benefit entire community 4. Farmer Listening Session—February 3, 2016. This meeting was held to gain a better understanding of how farmers were impacted during the 2014 flood event and receive feedback 3 regarding the proposed flood mitigation. The meeting was hosted by the PLSLWD Farmer-led Council. 5. Second Advisory Group Meeting—February 4, 2016. This meeting was held to gather feedback on the up-to-date information from the analysis of flood mitigation scenarios as well as responding to public policy questions that included: a. Should public dollars be used to protect public infrastructure (such as sanitary sewer and water utilities)? There was consensus that this should be maintained b. To what degree should access (from streets) be provided during flood events? There was not unanimity on this, except that emergency vehicle access should be maintained c. To what degree should public dollars be used to protect or assist in the protection of private property? There was general consensus that it was a government’s responsibility to provide access to properties through public rights-of-way, however not necessarily protect the property itself. However, there was not specific feedback on the allowable frequency and duration of access disturbances. Some felt that public dollars should not be spent on specific improvements to private property (such as floodproofing or buyouts) but did support strategies such as upper watershed storage. Some felt that less frequent flooding with a longer period between events was okay, but we should consider protecting homes that get flooded on a greater frequency (such as 25 years). 6. Joint Policy Group Meetings—February 8, 2016 and September 26, 2016. The policy makers provided direction on the community goals to be used to complete the study. Those goals and priorities are as follows: a. Protection of Public Safety - Maintain emergency vehicle access at all times b. Protection of Health and Safety - Protect public utility infrastructure (i.e. sanitary sewer and water distribution.) c. Maintain traffic flow through the County Road 21 corridor d. Maintain access to private properties 1.4 Monitoring and Modeling PLSLWD collected lake stage and stream flow data for several sites within the Prior Lake watershed during the 2014 monitoring season which were used for watershed model calibration. More than seven inches of rainfall fell across most of the watershed between June 15th and the 20th, which contributed to the peak discharge rates observed at all five monitoring sites. The peak Spring Lake elevation occurred three days after the peak discharge rates occurred in the upper watershed, while Prior Lake did not reach its peak flood level until 11 days after the peak discharge occurred in the upper portion of the Spring Lake watershed. The County Ditch 13 watershed, which represents 29% of the Prior Lake watershed, contributed more than 53% of the flow volume that discharged from the Prior Lake outlet in 2014. In addition, watershed yield and peak discharge rates (normalized to drainage area) from the County Ditch 13 watershed were more than 30% higher than any of the monitoring stations used in the model calibration. Barr created a PCSWMM computer model capable of simulating the complexity of watershed runoff from the various types of land surfaces based on the available climate data, land use/land cover characteristics, 4 soil type, topography and imperviousness, and then subsequently route the runoff through stream and ditch channels as well as storage areas and storm sewer/culverts, including the lake basins based on the physical constraints of the individual outlets and conveyances. Model development and preliminary use included calibration to observed flow rates and lake elevations throughout the watershed for the June- July 2014 flood event and running the model for design events intended to simulate flood levels for the 2, 10, 25, 50, 100 and 500-year return period with Atlas 14 rainfall amounts for the critical duration (30-days) event. The 100-year design event modeling produces higher lake levels (approximately one-foot higher) than the peak levels that were observed in each lake in 2014. However, the predicted flood level for Prior Lake is 1.8 feet lower than the FEMA 100-year elevation of 908.9 feet MSL, as it accounts for the effect of the existing outlet while the FEMA modeling did not.. Analysis of the monitoring and modeling data indicated that enhanced upper watershed storage – particularly in the County Ditch 13 watershed – and better control of peak discharge from the Spring Lake outlet had the highest potential for reducing peak elevations in Prior Lake. Upper watershed storage is defined as any storage area upstream of Spring Lake and the Prior Lake basins. To have a noticeable effect on the peak Prior Lake elevation, detention storage areas would need to detain water until after Prior Lake had reached its peak. 1.5 Results Summary Based on feedback from the public a list of flood mitigation options was developed. Each mitigation option was then rated according to its relative merits for criteria that were weighted to correspond with the study objectives and what the public viewed as important factors in selecting potential flood- mitigation options. The results of this analysis showed that improvement options involving upper watershed and Spring Lake storage, sandbagging and modifications to the Prior Lake outlet (including both increased capacity and proactive outlet management) scored highly. The results also indicated that floodproofing and/or buyouts could also be considered as part of the overall solution. As a result of relative comparisons of the merits of the universe of options the flood mitigation analysis was narrowed down to the following seven options: Option A—Enhanced Protection (coordinated temporary protection measures) Option B—Spring Lake Storage Option C—Prior Lake Outlet Modification (increase capacity) Option D—Upper Watershed Storage Option E—Combine Options B, C & D Option F—Floodproofing (for at-risk primary structures) Option G—Actively Manage Prior Lake outlet (low-flow gate) Figure EX-1 shows how each of the potential improvement options are expected to improve the flood impacts for each of the flood frequency events, including summary information pertaining to the total estimated costs and number of primary structures and inaccessible properties at each flood level. The figure shows that, if the conservative cost estimates for securing Spring Lake drainage easements are accurate, then the Spring Lake storage option will not be as cost effective (from a flood control perspective) as increasing the Prior Lake outlet capacity or increasing upper watershed storage. Options involving an increase to the Prior Lake outlet capacity and increasing upper watershed storage are comparable at cost-effectively controlling flooding on Prior Lake. Upper watershed 5 storage provides better flood control for the larger flood events than any of the other individual options. For the 100-year event, the upper watershed storage option would protect an additional 30-35 primary structures and maintain accessibility to an additional 50 properties. Implementing a combination of the first three options would drop the 100-year flood level to within a half-foot of the OHW for Prior Lake (see Figure EX-1), which is likely more protection than would be necessary for this event. However, the predicted 500-year flood level for Option E would still approach the high water level experienced during 2014. Implementation of some combination of upper watershed storage (Option D), increased Prior Lake outlet capacity (Option C) and actively managing the Prior Lake outlet low-flow gate (Option G) is expected to meet the study goals in the most cost-effective manner. The biggest limitation of this combination of options is that it may take several years for full implementation. As a result, it is expected that some combination of Options G and A will need to represent the short-term implementation measures. A scaled-down version of Option B, that involves less inundation on Spring Lake and less easement cost, may also represent a more cost-effective and viable short-term implementation measure. It is also expected that floodproofing (Option F) and/or buyouts will be a cost-effective measure for the lowest primary structures. 7 2.0 Background 2.1 Study Partners The study is sponsored by the PLSLWD and the City of Prior Lake, in collaboration with Spring Lake Township. The Minnesota Department of Natural Resources (MnDNR) has also provided input for permitting and review of potential mitigation options. The advisory committees also included representatives from Scott County, Scott Soil and Water Conservation District, PLSLWD’s Citizen Advisory Committee (CAC), City of Prior Lake’s Lakes Advisory Committee (LAC), Lake Associations and elected officials. It is expected that future implementation projects may also require coordination and/or partnerships with the City of Shakopee, the Shakopee Mdewakanton Sioux Community and the Lower Minnesota Watershed District, in addition to oversight and approvals from permit authorities. Barr Engineering Co. was retained by the study partners—the City of Prior Lake and PLSLWD—to provide assistance in completing this study, which includes updated modeling of the watershed, review of flood- related issues and projects, identification and evaluation of a suite of potential flood reduction strategies and implementation plan recommendations. A public input process that engaged a broad range of stakeholders and included local units of government, lake associations, the agricultural community and private landowners has been used to guide the development and evaluation of the available flood mitigation options. This report describes the results of this study. 2.2 Watershed Characteristics The Prior Lake Spring Lake Watershed is located in Scott County on the southwest edge of the Twin Cities metro area. The Watershed encompasses 42 square miles of land, primarily agricultural. Water flows from primarily agricultural areas north into Spring Lake, through Upper and Lower Prior Lake (located in the City of Prior Lake), and ultimately discharges to the Minnesota River through the PLOC. The study area included the Prior Lake-Spring Lake watershed upstream of the Lower Prior Lake outlet structure, encompassing approximately 19,000 acres (30 square miles) of land in Scott County, Minnesota (Figure 2-1). The watershed area was divided into 202 subwatersheds for this study. Figure 2-2 shows how the current (2010) land use varies across the watershed with agricultural land predominant in the south and western areas and residential land uses surrounding the study lakes. Figure 2-3 shows that there are several natural depressions and wetlands in the upper watershed topography that would have been landlocked under pre-settlement conditions. Some of the depressions in the eastern portion of the watershed are still landlocked under current conditions. 10 Fi g u r e 2 - 3 W a t e r s h e d T o p o g r a p h y 11 2.3 History Lake levels for Upper and Lower Prior Lakes have historically been one of the most important issues for the community, specifically the residents living around the Lakes, since Prior Lake does not have a natural outlet. Residents have observed wide lake level variations dating to the early 1900s, with elevations reportedly varying by up to 34 feet, according to PLSLWD reports from the late 1970s. Levels were likely below 890 during the dustbowl era of the 1930s, based on historic aerial photography. In 1965 and 1969 there were significant flooding events that impacted the entire region, including Prior Lake. Unfortunately, the official MnDNR record contains no water level data for Prior Lake for 1965 and only one reading for 1969. According to records for the Minnesota River, the 1965 flood is considered to be approximately equal to the 1-percent annual chance (or 100-year) flood, and was followed by the 1969 flood which was a 2.5-percent chance (40-year) flood. This regional flooding prompted significant discussion regarding potential mitigation of future events on Prior Lake and a petition from the property owners (resident freeholders) in June 24, 1969, to form a watershed district to address these concerns. The Prior Lake- Spring Lake Watershed District (PLSLWD) was established in March 1970 by order of the Minnesota Water Resources Board (MWRB) under the authority of the Minnesota Watershed Act (Minnesota Statutes, Chapter 112). Much of the early efforts of the PLSLWD focused on lake level issues and the development of an outlet system. After significant study, public process and agency coordination, the establishment of the Prior Lake Outlet and Channel (PLOC) was selected as the first flood mitigation effort by the PLSLWD. In 1979 the MnDNR issued a permit to the PLSLWD for the PLOC. The cities of Prior Lake and Shakopee and the PLSLWD entered into a Joint Powers Agreement (JPA) regarding the PLOC in 1981 and the outlet system was first used in 1983. This JPA guided the operation of the PLOC and originally provided the City of Shakopee the authority to require the outlet to be closed during certain events. In 1987 a Management Policy and Operating Procedures for the Outlet Control Structure for Prior Lake (Operating Plan) was adopted to set management goals and policy, operating procedures including allowable discharges; and review and amendment procedures. In parallel to the creation of the Watershed District, on a federal level the National Flood Insurance Program was being developed. This program established an insurance funding mechanism with the primary goal of reducing risks within the now defined 100-year flood risk zone. In 1978 a Flood Insurance Study (FIS) was completed for Prior Lake, which established the regulatory flood zone around Prior Lake and resulted in the calculated 100-year flood elevation of 908.9 feet mean sea level (MSL). For property owners now identified to be in the 100-year flood risk zone to make use of the program, the community needed to establish an ordinance and administer the program on a local level. Prior Lake has administered the program since 1995. The local ordinance restricts the development of structures within the 100-year flood risk zones and seeks to eliminate flood risk of structures in these zones by requiring removal or floodproofing if significant improvements are completed. The FIS was updated in 1997, but the modeling did not account for the benefit of the outlet structure because the channel capacity downstream of the control structure and legal constraints with the adjoining communities limit the discharge the City of Prior Lake can pass through the control structure (FEMA, 1997). The current Flood 12 Insurance Rate Maps (FIRMS), and thus regulatory requirements are based on this 1997 study. Actual flood levels are thought to be less due to the benefit of the PLOC. Beginning in 2004, PLSLWD pursued improvements to the outlet structure of the PLOC which had started to show signs of aging and monitoring showed that it was inefficient in maximizing the use of the 36” outlet pipe. The DNR approved proposed structure update plans in 2005 and the new outlet box was completed in 2011. It includes a fixed weir set at an elevation of 902.5 feet MSL and a slide gate to allow manual discharge of water between lake level elevations of 902.0 and 902.5 feet MSL. In conjunction with these improvements, revisions to the Operating Plan were made. These revisions allow continual discharge over the weir without legal limitation by downstream jurisdictions. The outlet structure was replaced in 2010, but maintained the same fixed weir and slide (low flow) gate features at the same elevations, subject to the same Operating Plan procedures (further described in Table 2-1). The current outlet configuration will not allow the outflow rate to exceed 65 cfs. Table 2-1 Summary of Existing Prior Lake Outlet Management Policy and Operating Procedures Lake Level Discharge Policy Low Flow Gate Operation Below 902.0 No discharge allowed Closed 902.0 to 902.5 During March and April low flow gate discharge is allowed above elevation 902.0 (with MnDNR approval), based on an analysis of expected lake level rise due to snowmelt and upstream reserves. Otherwise, low flow gate should remain closed when lake levels are at or below 902.5. PLSLWD may also request permission on a case by case basis to discharge in the fall of the year under extraordinary wet conditions with a significant amount of flow coming into Prior Lake by November 1st. March-April: Open (with to MnDNR approval) May-February: Closed 902.5 to 903.5 Outlet structure discharge regulated by fixed weir Closed 903.5 & above Outlet structure may operate at full capacity Closed Notes: The MnDNR generally recommends that the low flow gate remain closed with the exception of the March and April drawdown period. There could be unique cases where a discharge increase would be beneficial and the lower gate could be opened provided that the lake was not drawn down below 902.5, subject to MnDNR approval. In the spring of 2014, the Prior Lake watershed experienced record amounts of precipitation that led to a historic flooding event, which resulted in extensive sandbagging, pumping, inundation of seven public roads including partial or full closure of Scott County Highway 21 (the main arterial road that bisects the City of Prior Lake) for nearly five weeks, prolonged restrictions (up to six weeks) on property access, no wake restrictions on Prior and Spring Lakes throughout most of the 2014 boating season and extensive damages throughout the watershed, including crop/soil damage and low yields, impacts to nearly 50 private structures and approximately $955,000 of damage to the Prior Lake Outlet Channel (PLOC). The City of Prior Lake spent $190,000 to mitigate the effects of the flooding during the flood event. This event triggered many questions and highlighted the need to update watershed modeling and use it to evaluate flood mitigation strategies for future events. The results of these analyses can, in turn, help inform decisions and policies of the organizations within the watershed. 13 MnDNR historical Prior Lake and Spring Lake level records are presented graphically in Figure 2-4, and since 1971 in Figure 2-5. The Prior Lake outlet began operation in 1983 and Figure 2-5 shows how much higher Prior Lake flooding would have been in the recent past without the 65 cfs outlet discharge capacity. Figure 2-5 also shows that, with the exception of 2009, there has been discharge from the Prior Lake outlet every year since 1992. The lake level data shown in Figure 2-4 shows that, even with the lake outlet in operation, the 2014 flood level of 906.2 feet MSL for Prior Lake is significantly higher than any other flood event on record with the exception of the highest known flooding events from 1906 and 1915 that reached respective high water levels of 907.6 and 907.0 feet MSL (PLSLWD, 2003). The 2014 flood level of 913.3 feet MSL for Spring Lake was approximately one foot higher than any other flood event on record. Figure 2-4 Historical Prior Lake and Spring Lake Levels 14 Figure 2-5 Prior Lake and Spring Lake Levels Since 1971 Using the annual high water level data from the 32 years of Prior Lake levels since the outlet was brought on-line, the recurrence of various lake levels can be analyzed based on the frequency with which the respective lake levels have been exceeded during the recent period of record (since the lake outlet was brought on-line). Figure 2-6 shows the frequency analysis for the Prior Lake high water level observations since 1983, with 2014 representing the highest point on the graph. Figure 2-6 shows that the high-water levels plotted against the recurrence period follows a straight-line when graphed this way. The figure also shows that there were five years where the Prior Lake level remained below the outlet control elevation or Normal Water Level (NWL) of 902.5 feet MSL. Since 1983 PLSLWD has been estimating how high the annual Prior Lake level maximum levels would have been without the influence of the lake outlet (shown in Figure 2-5). The resulting estimates of the annual high water levels without the lake outlet can be combined with 20 years of lake level data from Prior Lake and the recurrence of various lake levels can be re-analyzed based on the frequency with which the respective lake levels have been exceeded during the entire period of record. Figure 2-6 shows the frequency analysis for the 52 years of combined Prior Lake high water level observations and estimates without the lake outlet. 2014 represents the second highest point on the graph in Figure 2-6, which shows that without the lake outlet, the flood level would have been more than 6 feet higher. Without the lake outlet, the highest estimated lake level would have been 913.9 feet MSL in 2011. Figure 2-6 also shows that the Prior Lake level remained below the NWL during ten of the 52 years with lake level readings. 15 Figure 2-6 Prior Lake Annual High Water Frequency Analyses With and Without Lake Outlet The City of Prior Lake used recent Scott County LiDAR elevation data (MnDNR, 2011) to estimate the number of primary (homes) and secondary residential structures, street segments and manhole openings at-risk by incremental lake level elevations surrounding the Prior Lake basins (shown graphically in Figure 2-7). Figure 2-7 shows that there is a significant uptick in the number of homes that are at risk of flooding above the lake levels of 906.5 feet MSL, while there are linear increases in other types of infrastructure at risk with increasing flood levels. 16 Figure 2-7 Public and Private Impacts by Prior Lake Level 17 3.0 Goals and Objectives 3.1 Study Goals The flood of 2014 triggered many questions regarding flooding on Spring Lake and the Prior Lake basins and highlighted the need to update tools that help inform decisions and policies of the organizations within the watershed. Study partners identified updating the existing watershed model and evaluating flood damage reduction strategies as major study priorities. Stakeholder integration was identified as being key to the success of the study and implementation portion of the plan and public input was incorporated throughout the study process. The goals of this study were as follows: 1. Update the flood model 2. Compile and review historical studies and projects to summarize efforts already accomplished toward flood damage reduction 3. Identify a universe of flood damage reduction strategies 4. Evaluate the cost/benefit of the strategies and define community goals for flood protection 5. Develop an implementation plan that identifies a suite of projects or mechanisms to reduce flood damage potential 3.2 Study Objectives To achieve the study goals, the main objectives of this study involved the following: 1. Updating the existing watershed model with new and up to date inputs including Atlas 14 precipitation data, monitoring data, as-built information, and topographic data (County LIDAR). Calibrating the watershed model to accurately simulate the 2014 flood event. 2. Compiling historical information and reviewing flood-related issues and projects to increase understanding of current flood damage risks 3. Identifying potential flood mitigation measures that will reduce the risk of flood damage. 4. Developing a decision matrix to evaluate the options that include cost/benefit factors and secondary benefits including water quality and natural resource benefits, restoring natural hydrology, improving groundwater recharge and drought tolerance, feasibility issues, upstream/downstream and human impacts. 5. Evaluating flood damage reduction strategies based upon the decision matrix. 6. Using the modeling and cost benefit, define community goals for flood damage reduction as approved by the policymakers. 7. Using the evaluation matrix and community goals develop an implementation plan that includes a potential suite of flood damage reduction strategies, defines partner roles and potential funding sources. 18 3.3 Community Goals and Priorities Based on discussion at the policymakers joint meetings on February 8, 2016 and September 26, 2016, the policymakers provided direction on the community goals to be used to complete the study. Those goals and priorities are as follows: 1. Protection of Public Safety - Maintain emergency vehicle access at all times 2. Protection of Health and Safety - Protect public utility infrastructure (i.e. sanitary sewer and water distribution.) 3. Maintain traffic flow through the County Road 21 corridor 4. Maintain access to private properties 19 4.0 Community Planning Process Many residents were impacted by the flood of 2014 and expressed a desire to be involved in future flood damage reduction planning. It is key for the success of the implementation plan that stakeholders be integrated into the process to provide input and ultimately comment on the approved plan. This study included a process for gathering input from the community, including other jurisdictions that are located within the watershed. There were also several advisory groups, study partners, and decision making bodies that needed to be part of the process. The following groups had involvement at key times with their respective roles throughout the process: Technical Advisory Committee (TAC) – This group included key staff from PLSLWD, City of Prior Lake, and Spring Lake Township who have regulatory authority within the watershed, as well as representatives from Scott County and Scott Soil and Water Conservation District to ensure coordinated results. The technical working group was intended to drive the modeling and planning process. (12 Meetings). In addition, the staffs of the City of Prior Lake, PLSLWD and Spring Lake Township met regularly to guide the TAC and the final drafts of the Study. Advisory Groups – PLSLWD’s Citizen Advisory Committee (CAC), City of Prior Lake’s Lakes Advisory Committee (LAC), Prior Lake and Spring Lake Associations, and representation from policy groups: Spring Lake Township, Scott Soil and Water Conservation District and City of Prior Lake (elected and appointed officials) and the Scott Water Management Organization. This stakeholder group provided feedback and ideas from representatives that remain actively involved in surface water issues. Functions of this group included communicating ideas to and from the public, communicating planning status, and helping to educate the public using factual information. (Two Meetings) Policy Group – An ad hoc group including the Prior Lake City Council, Spring Township Supervisors and the Board of Managers of the Prior Lake-Spring Lake Watershed PLSLWD. This group provided policy direction, and plan development guidance. Public/Stakeholders - Generate flood mitigation ideas, stay informed on the process and assist in reaching out to inform the wider community regarding the status of this planning effort. (Three Public Meetings) PLSLWD’s Farmer-led Council – A group of local farmers that PLSLWD has previously engaged to develop and guide implementation strategies that will accomplish nutrient reduction goals. This group provided information regarding impacts experienced by the agricultural producers as a result of the historic rainfalls in 2014. The following sections describe the results of the community planning process with the meetings listed in chronological order. 4.1 Public Kick-Off Meeting—February 19, 2015 This open house format meeting was intended to inform the public of the study goals and objectives, receive input on what they experienced during the 2014 flooding and to obtain their ideas for flood mitigation options. The meeting included informal small group discussions following short presentations, 20 as well as specific survey questions to provide feedback, which was also made available on the PLSLWD website. The survey responses and open house comments were compiled and summarized according to the major considerations and used to guide the development of mitigation options that should be considered in the process. 4.2 First Advisory Group Meeting—May 7, 2015 This meeting was used to present preliminary modeling results and gather feedback on potential flood mitigation options and the draft criteria for a selection matrix. It was explained that the preliminary modeling was being calibrated to accurately replicate the 2014 flood event before it could in turn be used to evaluate the following flood mitigation strategies: Changes to upper watershed storage/detention Changes to storage/detention in Spring Lake Increases to the Prior Lake outlet capacity Floodproofing and/or property buyouts/removals Potential criteria to evaluate the flood mitigation strategies that could be considered discussed included: Costs of projects (design/prep, construction, loss of property/use Benefits/Impacts (ecological, water quality, etc.) Feasibility (regulations, available funding, etc.) Inconvenience factors (loss of use, accessibility, etc.) Timing (how long to implement) 4.3 Second Public Meeting—May 28, 2015 Attendees were provided with a study update, including a summary of the watershed model that has been created using the most current data available which allowed Barr to simulate what will happen during significant flood events for possible mitigation scenarios, and an overview of the matrix that has been developed to help guide decision makers through the project selection/implementation process and help weigh the costs against the benefits of different stormwater management strategies. Receiving public feedback at this meeting was important to consider as the modeling was being finalized and alternatives were being studied. Thirty attendees (with 18 indicating that they live on the lakeshore, eight in the city, and three in rural areas) completed a survey that asked participants about the location of their properties, what they viewed as important factors in selecting a potential flood-mitigation project, and their feelings about use of public funding for flood mitigation efforts. The important factors rated by each survey respondent included cost-benefit ratio, water quality benefits, protection below the 100-year FEMA flood level and project readiness. The survey results indicated the following, based on 30 total responses: Water quality was most important factor, followed closely by cost/benefit 21 Half of the respondents thought it was important, and another 36% thought it was very important that options should protect properties and public roads below the 100-year FEMA flood level Project readiness was least important factor 69% supported use of public money to protect individual homes/businesses 60% supported increasing property taxes to finance the options City of Prior Lake has obligation to maintain emergency access to properties General sentiment that public funds should be used to benefit entire community 4.4 Farmer Listening Session—February 3, 2016 This meeting was held to gain a better understanding of how farmers were impacted during the 2014 flood event and receive feedback regarding the proposed flood mitigation options. Paul Krueger, a local dairy farmer and a member of the District’s Farmer-led Council, helped facilitate the meeting. Prior to the meeting, farmers and operators within the upper watershed were sent a questionnaire to help gather information about the impacts of the flood on their fields, buildings and crops, and to share their comments or concerns about the Flood Study. Twenty-three participants attended the session and shared flood damage experiences. Attendees were prompted with questions concerning the impacts of the 2014 on their farming operations and property. The seven potential flood mitigation options were explored with a focus on the Upper Watershed Storage Option. The questionnaire results and feedback during the meeting can be summarized by the following: Upstream farmers also suffered unheard losses during the 2014 flood such as: o Many farmers tried to re-plant after the heavy rains, only to get hit again and lose the 2nd crop. Crop insurance only covers the first. o If fields are flooded for a certain period of time, the microbes in the soil die off. Farmers need time to re-build the soil before they can plant crops again. Some flood damaged fields will take two or more years to recover. o For dairy farmers, crop insurance does not cover the purchase of replacement silage for their cows. Buying silage is much more expensive than growing your own, and many farmers took a hard hit in 2014. o Farmers experienced a loss of valuable soil that washed away during flood events. o For some, the rain washed away a 2-year application of fertilizer. Not only did they lose the year’s fertilizer, but the following as well. Concerns about the creation of upper watershed storage areas affecting tile lines upstream. Farm acreage in the Twin Cities area is extremely valuable as the amount of tillable land decreases every year with the expansion of the urban areas. This should be considered when identifying upper watershed storage areas. 4.5 Second Advisory Group Meeting—February 4, 2016 This meeting was held to gather feedback on the up-to-date information from the analysis of flood mitigation scenarios as well as responding to the following public policy questions: 22 Should public dollars be used to protect public infrastructure (such as sanitary sewer and water utilities)? There was fairly consistent support that the City of Prior Lake has a responsibility to protect and minimize damage to public infrastructure. To what degree should access (from streets) be provided during flood events? Three street/access priorities were identified—in descending order, these priorities include access for emergency services, major transportation routes (such as CR21), and local access. To what degree should public dollars be used to protect or assist in the protection of private property? There were a range of opinions on this topic. Some felt it was a government’s responsibility to provide access to properties through public rights-of-way, however not necessarily protect the property itself. However, there was not specific feedback on the allowable frequency and duration of access disturbances. Some felt that public dollars should not be spent on specific improvements to private property (such as floodproofing or buyouts) but did support strategies such as upper watershed storage. Some felt that less frequent flooding with a longer period between events was okay, but we should consider protecting homes that get flooded on a greater frequency (such as 25 years). 4.6 Joint Policy Group Meeting—February 8, 2016 This was the first meeting of the Joint Policy Group. The participants reviewed the most recent modeling results and discussed possible mitigation options, as well as the same policy questions that were introduced at the last advisory group meeting. The questions were: Should public dollars be used to protect public infrastructure? To what degree should access be provided? To what degree should public dollars be used to protect or assist in the protection of private property? How would the options be funded? Common themes from the discussion included: Public access should be protected Buying out homes below the Prior Lake level of 907.1 feet MSL is generally not supported Some support for upper watershed storage option Homeowners should take responsibility for their properties Significantly more discussion regarding the mitigation options, funding and policy questions occurred, and it was decided that another meeting should be held to allow more time for discussion. 23 5.0 Watershed Monitoring and Modeling PLSLWD has been collecting stream stage and flow data for several sites within the Prior Lake watershed. Continuous (15-minute interval) stage monitoring data collected at five sites during the 2014 monitoring season – two major tributaries and one minor tributary to Spring Lake, were used for model calibration along with continuous data collected from the Spring Lake and Prior Lake outlets (Figure 5-1) Most of the available monitoring data covered the time period from March 28th through October, 2014, with some individual measurements prior to March 28, 2014. Table 5-1 summarizes drainage characteristics for five PLSLWD monitoring sites considered for use in the watershed model calibration. The combined drainage area of the Buck Lake and County Ditch 13 sites accounts for 76 percent of the Spring Lake watershed and 52 percent of the Prior Lake watershed. Table 5-1 Drainage Characteristics of Watershed Monitoring Sites Monitoring Site—Site # Watershed Area (acres)1 Percent of Spring Lake Watershed Area Percent of Prior Lake Watershed Area Marshall Road Crossing—Site 19 401 3% 2% Buck Lake Outlet—Site 14 4,036 32% 21% County Ditch 13—Site 7 5,526 44% 29% Spring Lake Outlet—Site 21 12,703 100% 66% Prior Lake Outlet 19,239 -- 100% 1 Watershed areas include potentially landlocked or non-contributing areas. 5.1 Monitoring Figure 5-2 shows the measured hydrograph data for the five watershed monitoring sites for the period between the end of April and July, 2014. More than seven inches of rainfall fell across most of the watershed between June 15th and the 20th, which contributed to the peak discharge rates observed at all five monitoring sites. While all three of the tributary stations experienced peak flow on June 19th, the flow monitoring results show that the tributary stations experienced varying levels of flashiness and relative magnitudes of peak discharge. All of the storm flow at Site 19 discharged within two days, while flow at Sites 7 and 14 returned to pre-storm levels within six days after peak discharge. For the Spring Lake outlet, peak discharge occurred on June 22nd – three days after the peak discharge rates occurred at the three tributary monitoring stations. Flow out of the Spring Lake outlet returned to pre-storm levels within 14 days of the peak discharge. The peak Prior Lake level and outlet discharge rate occurred on June 30th and Prior Lake did not return to pre-storm levels for more than 40 days. 25 Figure 5-2 Calibration Station Flow Data for 2014 It is important to note that flow out of the Prior Lake outlet did not begin until April 29, 2014 because the beginning lake elevation was 900.10 feet MSL on March 28th, which would have required approximately 3,460 acre-feet of inflow to raise the Prior Lake elevation to the control elevation of 902.5 feet MSL. In addition, lake level and outflow estimates from the Spring Lake outlet do not begin until April 29th, when the estimated outflow rate already exceeded 50 cfs. Figure 5-3 shows how the combined hydrograph of the three monitored tributaries compares to the flow discharging from the Spring Lake and Prior Lake outlets. This figure allows for a more direct comparison of the magnitude and timing of the Spring Lake inflow hydrographs to the resulting discharges from each of the lake outlets. The peak Spring Lake elevation occurred three days after the peak discharge rates occurred in the upper watershed, while Prior Lake did not reach its peak flood level until 11 days after the peak discharge occurred in the upper portion of the Spring Lake watershed. 26 Figure 5-3 Observed Flow Data for 2014 with Upper Watershed Sites Combined Table 5-2 summarizes the measured flow volume through September 10th, expressed both in acre-feet and as the yield (volume divided by watershed area) in inches, as well as two expressions of the peak discharge rates at the five monitoring stations (with the last column normalized to watershed area). 27 Table 5-2 2014 Flow Monitoring Summary Monitoring Site—Site # Measured Flow Volume—thru 9/10/14 (acre-feet) Watershed Yield (inches) Peak Discharge Rate (cfs) Area-Normalized Peak Discharge Rate (cfs/acre) Marschall Road Crossing—Site 19 468 13.5 62 0.155 Buck Lake Outlet—Site 14 3,732 10.8 71 0.018 County Ditch 13—Site 7 8,259 17.6 207 0.038 Combined Upper Watershed— Sites 7, 14 & 19 12,458 14.6 318 0.032 Spring Lake Outlet—Site 21 12,0211 11.4 222 0.018 Prior Lake Outlet 15,4852 9.7 64 0.003 1 Does not include unmonitored volume associated with the Spring Lake outlet discharge prior to April 29, 2014. 2 Includes water volume associated with lake level rise in advance of discharge from the Prior Lake outlet. The following conclusions can be drawn from the 2014 monitoring data: The upper Spring Lake watershed (combining drainage to Sites 7, 14 and 19), which represents 52% of the Prior Lake watershed, contributed more than 80% of the flow volume that discharged from the Prior Lake outlet in 2014 The County Ditch 13 watershed, which represents 29% of the Prior Lake watershed, contributed more than 53% of the flow volume that discharged from the Prior Lake outlet in 2014 The Buck Lake watershed, which represents 21% of the Prior Lake watershed, contributed 24% of the flow volume that discharged from the Prior Lake outlet in 2014 Watershed yield from the County Ditch 13 watershed was significantly higher (more than 30% higher) than any of the other watershed monitoring stations Watershed yield from the Buck Lake tributary was significantly lower than the other upper Spring Lake watershed tributaries, but higher than the areas draining directly to Prior Lake The normalized peak discharge rate for the County Ditch 13 watershed is an order of magnitude higher than the normalized peak discharge rate for the Prior Lake outlet in 2014 The available lake and wetland storage in the Buck Lake Outlet watershed results in less than half the normalized peak discharge rate computed for the County Ditch 13 watershed Our analysis of the 2014 monitoring data indicated that enhanced upper watershed storage – particularly in the County Ditch 13 watershed – and better control of peak discharge from the 28 Spring Lake outlet would have had the highest potential for reducing peak elevations in Prior Lake during the 2014 storm events. 5.2 Model Development and Calibration Barr created a PCSWMM computer model that is capable of simulating the complexity of watershed runoff from the various types of land surfaces based on the available climate data, land use/land cover characteristics, soil type, topography and imperviousness, and then subsequently route the runoff through stream and ditch channels as well as storage areas and storm sewer/culverts, including the lake basins based on the physical constraints of the individual outlets and conveyances. Model development and preliminary use involved the following steps: 1. Subdividing watersheds to road crossings of drainageways and major storage areas 2. Identifying road culvert, storm sewers, and surface channels that collectively route flow to Prior Lake 3. Creating a watershed model to simulate flow rates and lake elevations throughout the study area 4. Calibrating the watershed model to observed flow rates and lake elevations throughout the watershed for the June 2014 -July 2014 flood event 5. Running the calibrated watershed model for design events intended to simulate flood levels for the 2, 10, 25, 50, 100 and 500-year return period Atlas 14 rainfall amounts for the critical duration event, which was determined to be 30-days. Detailed discussion regarding the watershed modeling approach and methodology is included in Appendix A. Barr initially used data from the tributary monitoring sites, except for the Marshall Road Crossing (Site 19), for model calibration. Following calibration to the observed flows at the upstream stations, the modeling was calibrated to the watersheds contributing directly to Upper Prior Lake and Lower Prior Lake. The resulting model generally matched the Prior Lake elevation, but over-predicted the monitored peak elevation that occurred on June 30, and predicted a slower recession rate than the shown by the monitoring data. Adjustments were made to account for groundwater seepage from Prior Lake under high water levels and the final watershed modeling was calibrated to match the peak elevation of Prior Lake and the recession limb of that peak. Figure 5-4 compares the calibrated watershed modeling results, with and without seepage, to the Prior Lake level monitoring data. 29 Figure 5-4 Prior Lake Watershed Model Calibration 5.3 Final Modeling Summary Barr used the calibrated watershed model to determine the critical duration 100-year event, i.e. the 100- year event that resulted in the highest lake elevation for the 2014 existing conditions. Barr evaluated rainfall events ranging from the 100-year, 10-day event through the 100-year, 60-day event. The 30-day event was selected as the critical event for Prior Lake as it produced a peak flood level that was comparable or higher than any of the other durations. The calibrated watershed model to evaluate the 30-day design event for several return recurrences in addition to the 100-year event, including the 2-, 10-, 25-, 50-, and 500-year events. Table 5-3 shows the peak elevations of Spring Lake and Prior Lake for each of the 30-day events. The 100-year results show that the design event modeling produces higher lake levels (approximately one-foot higher) than the peak levels that were observed in each lake in 2014. However, the predicted flood level for Prior Lake is 1.8 feet lower than the FEMA 100-year elevation of 908.9 feet MSL, as it accounts for the effect of the existing outlet while the FEMA modeling does not. The current FEMA 100-year elevation for Spring Lake is 914.4 feet MSL, which is consistent with the results shown in Table 5-3. 30 Table 5-3 Watershed Model 30-Day Duration Design Event Results Event Return Recurrence Rainfall Depth (in) Spring Lake Peak Elevation (ft) Prior Lake Peak Elevation (ft) 2-year 8.31 912.4 903.6 10-year 10.8 913.3 904.8 25-year 12.3 913.7 905.6 50-year 13.5 914.1 906.3 100-year 14.6 914.5 907.1 500-year 17.1 915.3 908.4 The peak 100-year Prior Lake elevation determined by this study occurred approximately 14 days after the peak rainfall intensity of the 100-year, 30-day Atlas 14 rainfall event. To have a noticeable effect on the peak Prior Lake elevation, detention storage areas would need to detain water until after Prior Lake had reached its peak. 31 6.0 Identification of Mitigation Options Based on feedback from the public kickoff meeting (February 19, 2015) and the second public meeting (May 28, 2015), the TAC developed a list of flood mitigation options. Each mitigation option was then rated according to its relative merits for the following criteria, which were weighted to correspond with the study objectives and what the public viewed as important factors in selecting potential flood- mitigation options: Stormwater management benefits, including flood reduction Water quality and natural resource benefits Legal authority, including access for construction equipment without have to secure additional permissions or easements from other landowners Project readiness Human impacts Cost The results of this analysis were compiled in Table 6-1, which showed that improvement options involving upper watershed and Spring Lake storage, sandbagging and modifications to the Prior Lake outlet (including both increased capacity and proactive outlet management) scored highly. The results also indicated that floodproofing and/or buyouts should also be considered as part of the overall solution. As a result of relative comparisons of the merits of the universe of options shown in Table 6-1, and more detailed analysis including preliminary modeling of potential structural measures, the TAC narrowed the flood mitigation analysis down to the following seven options: Option A—Enhanced Protection (coordinated temporary protection measures) Option B—Spring Lake Storage Option C—Increase Prior Lake outlet capacity Option D—Upper Watershed Storage Option E—Combine Options B, C & D Option F—Floodproofing (for at-risk primary structures) Option G—Actively Manage Prior Lake outlet (low-flow gate) 33 7.0 Flood Mitigation Analysis 7.1 Potential Improvement Options As discussed in the previous section, the calibrated watershed model was used to evaluate several potential flood-reduction measures. Each measure was evaluated separately (i.e. as if it was the only measure that was implemented) and then all of the potential flood-reduction measures were implemented together (i.e. as if they were all implemented). Those measures that were judged to be reasonable and to have a measurable impact on the peak flood elevation of Prior Lake were combined with the existing conditions modeling and other potential mitigation options from Table 6-1 to narrow the analysis down to the following seven options: Option A—Enhanced Protection Option B—Spring Lake Storage Option C—Prior Lake Outlet Modification Option D—Upper Watershed Storage Option E—Combine Options B, C & D Option F—Floodproofing Option G—Actively Manage Prior Lake Outlet (low-flow gate) With the exception of Option F (which was based on the existing conditions), all of the above options were modeled to estimate the flood control benefits. In addition to the potential benefits (including flood reduction, water quality and natural resources benefits), detailed cost estimates and evaluations of other selection criteria were developed for each of the seven options. 7.1.1 Option A—Enhanced Protection This option primarily involves coordinated temporary protection measures, similar to flood control efforts that were utilized during high water in 2014, and is consistent with the calibrated existing conditions modeling described in Section 5 (i.e., this option does not change current flood levels, it simply improves resiliency). As a result, it is expected that some form of enhanced protection measures may need to be implemented several times within a 100-year period. At a minimum, it is also expected that this option would incorporate structural measures to prevent storm sewer backflow at the Highway 21 crossing. 7.1.2 Option B—Spring Lake Storage The existing Spring Lake Outlet structure is a concrete sill with a bottom width of 16 feet and invert elevation of 909.9 feet MSL. During extreme Prior Lake flooding events, temporary sandbagging at the Spring Lake outlet has been implemented to increase the available storage within Spring Lake. Barr evaluated a permanent embankment with a weir overflow and a 24-inch diameter outlet pipe that would maintain the existing Spring Lake normal water elevation of 909.9 feet MSL during typical flows while allowing for additional storage during high flows. It was estimated that 261 landowners would be impacted from the implementation of this option. Figure 7-1 shows the predicted Spring Lake levels under existing conditions for the 100-year, 30-day design event and the predicted water levels for the 2-, 34 5-, 10-, 50-, 100-, and 500-year, 30-day design event for proposed conditions with this improvement option. Figure 7-1 Existing and proposed conditions elevations for Spring Lake with modified outlet 7.1.3 Option C—Prior Lake Outlet Modification The Prior Lake outlet structure is currently limited to a maximum flow of approximately 65 cfs through its design and through legal agreements with downstream communities. A key factor in the 2014 flooding was the occurrence of several consecutive rainstorms separated by intervals that were too short for Prior Lake to return to its normal water elevation before the next storm. To achieve a higher discharge rate, this option assumes that the existing outlet pipe system would be supplemented with the construction of an additional parallel pipe, gate valve and submerged inlet that would be independent of the existing outlet structure. An additional pipe system consisting of a 24-inch diameter pipe was modeled that would be constructed with an upstream invert elevation of 898.5, or four feet below Prior Lake’s normal water elevation of 902.5. The proposed pipe system would be approximately 2,600 feet long and would be equipped with a gate valve that would be left closed during normal flow conditions and opened during flood flow conditions, subject to MnDNR permit approval and an agreement with downstream communities regarding its operation. This conceptual design would allow for a flow rate of approximately 15 cfs through the new pipe with the headwater at 902.5, while the pipe size would limit the peak flow to approximately 20 cfs at higher water elevations. With both outlets operating, the maximum operating 35 discharge during high water elevations would be approximately 85 cfs. It was estimated that this option would require easement acquisition on four parcels of land to implement. For this improvement option, the watershed modeling was revised to account for the combined discharge capacity of both the existing and proposed outlet pipes. It was assumed that the existing low-flow gate, which is manually operated and housed within the Prior Lake Outlet Structure, would be opened prior to the start of the simulation (“actively managed discharge”) and would remain open throughout the event. 7.1.4 Option D—Upper Watershed Storage Working with the TAC, Barr identified a number of potential upper watershed storage areas. These sites were selected solely on their storage potential, based upon model and topographic data, and no contact with landowners was made nor were there any assurances made that any of these areas will be available for use in the upper watershed storage option. The sites selected for further evaluation included locations where modeling showed temporary ponding already occurred under existing conditions, locations where the existing topography allowed for significant increased storage by adding limited additional infrastructure, and other locations where local interest had previously been shown. Barr screened the potential sites using the 100-year, 30-day design event modeling and selected ten sites for more-detailed modeling, nine of which include the development of additional storage volume. An option including Little Prior Lake was included in the analysis to account for additional storage that could be provided with existing infrastructure, but wasn’t utilized in 2014. Barr modeled each area with a restrictive outlet that would decrease its discharge rate and assumed that berms would be constructed as needed to increase the area’s storage volume in order to achieve the required detention time. Table 7-1 summarizes the existing and proposed outlet characteristics for each of the upper watershed storage areas. It was estimated that 120 landowners would be impacted by longer floodwater inundation times due to implementation of this option with several of these landowners also experiencing higher flood levels (that would not threaten structures). Figures showing the location of the detention areas and comparing the existing and proposed conditions 100-year inundation extents are shown in Appendix A. Appendix A includes modeling and discussion of an extra storage option that would have combined the Spring Lake storage option with extensive modifications to the Arctic Lake outlet. While this option would provide good flood control benefits for Prior Lake, it was eliminated from further consideration due to the high cost and negative impacts on the current efforts to restore the water quality and integrity of Arctic Lake. As discussed in Section 5, our analysis of the 2014 monitoring data indicated that enhanced upper watershed storage, particularly in the County Ditch 13 watershed, has the highest potential for reducing peak elevations in Prior Lake. Table 7-1 shows two expressions of the modeled existing and estimated (Option D) peak discharge rates for each of the upper watershed storage areas as well as the resulting effects at the Spring Lake and Prior Lake outlets for the 100-year, 30-day design event. The peak discharge rates were normalized to the respective watershed (contributing) areas, as shown in the last two columns of Table 7-2, to provide an impartial way to compare how well the predicted flood flows are or can be controlled in various areas of the watershed. The results show that the upper watershed 36 storage envisioned for Option D will bring the normalized peak discharge rates to levels that are comparable to what would occur at the Spring Lake outlet for several of the potential storage sites, most notably Buck Lake, S-SPL-054, S-SPL-080, S-SPL-094 and Sutton Lake. Except for S-SPL-046 and S-SPL-078, which would experience 10 to 13 percent reductions in the peak discharge rate with Option D, all other upper watershed storage sites would reduce their respective peak discharge rates by more than 68 percent. Table 7-1 Upper Watershed Storage Summary Storage Site Name Tributary Watershed Existing Outlet Proposed Restrictive Outlet and Overflow Structure S-LPL-048 (Little Prior Lake) Lower Prior Lake 1.5-ft diameter storm sewer pipe with skimmer (gate open) 1.5-ft diameter storm sewer pipe with skimmer (gate closed) S-BL-001 (Buck Lake) Buck Lake Open channel 2-ft diameter orifice 6 x 6 ft box S-BL-020 Buck Lake Open channel 0.5-ft diameter orifice 6 x 6 ft box S-SPL-046 County Ditch 13 30-ft weir 2-ft diameter orifice 30-ft weir S-SPL-054 County Ditch 13 Open channel 0.3-ft diameter orifice 6 x 6 ft box S-SPL-059 County Ditch 13 Open channel 0.5-ft diameter orifice 6 x 6 ft box S-SPL-078 County Ditch 13 4-ft diameter culvert 0.5-ft diameter orifice 6 x 6 ft box S-SPL-080 County Ditch 13 2-ft diameter culvert (estimated) 0.5-ft diameter orifice 6 x 6 ft box S-SPL-094 County Ditch 13 5-ft diameter culvert 2-ft diameter orifice 6 x 6 ft box S-SUL-001 (Sutton Lake) County Ditch 13 4-ft diameter culvert 1-ft diameter orifice 6 x 6 ft box 37 Table 7-2 Existing/Option D Peak Discharge Rates for Upper Watershed Storage Areas/Lakes Location Contributing Area (acres) Peak Discharge Rate (cfs) Area-Normalized Peak Discharge Rate (cfs/acre) Existing Conditions Option D Existing Conditions Option D S-BL-001 (Buck Lake) 4,021 194.4 61.4 0.048 0.015 S-BL-020 1,186 301 81.6 0.254 0.069 S-LPL-048 (Little Prior Lake) 131 12.1 0 0.092 0 S-SPL-046 5,526 271.7 242.9 0.049 0.044 S-SPL-054 226 183.5 0.8 0.814 0.004 S-SPL-059 255 306.9 18.0 1.206 0.071 S-SPL-078 339 140.3 123.0 0.414 0.363 S-SPL-080 216 34 3.0 0.157 0.014 S-SPL-094 3,127 132.6 40.1 0.042 0.013 S-SUL-001 (Sutton Lake) 1,546 117.6 25.8 0.076 0.017 Spring Lake Outlet 12,703 286.8 170.3 0.023 0.013 Prior Lake Outlet 19,239 67.7 62.6 0.004 0.003 During the 2014 open water period it was estimated that approximately 2,200 pounds of phosphorus in the flow from County Ditch 13 would have bypassed the ferric chloride treatment system and entered Spring Lake. This phosphorus load is approximately 375 pounds more than MPCA’s total maximum daily load (TMDL) loading capacity for all of the sources of phosphorus entering Spring Lake on an annual basis (Wenck, 2011). Based on the peak flow reductions associated with Option D, it is estimated that the ferric chloride treatment system could have treated approximately 690 pounds of additional phosphorus from the County Ditch 13 flows, not including the additional treatment that would have been realized within each one of the upper watershed storage areas during 2014. 7.1.5 Option E—Combine Options B, C & D This option combines all of the previously described features of the Spring Lake storage, Prior Lake outlet modification and upper watershed storage options into one option for the purposes of modeling the potential for combined benefits and cost effectiveness. It was estimated that a total of 385 landowners would be impacted from the implementation of this option. 7.1.6 Option F—Floodproofing This option primarily involves the development of permanent protection measures for at-risk primary structures, including buyouts where floodproofing is infeasible or not cost-effective, and is consistent with the calibrated existing conditions modeling described in Section 5 (i.e., this option does not change current flood levels, it simply improves resiliency). At a minimum, it is also expected that this option would need to incorporate structural measures to prevent storm sewer backflow at the Highway 21 crossing. 38 7.1.7 Option G—Actively Managed Prior Lake Outlet For this improvement option, it was assumed that the existing low-flow gate, which is manually operated and housed within the Prior Lake Outlet Structure, would be opened in advance of large inflows and would remain open throughout the event. It further assumes that full outlet capacity above NWL would be the same as it currently exists (in Option A) and that the proposed gate controlling the proposed additional outlet pipe would be closed if the Prior Lake elevation fell to 902.0 or below. Simulation of this option resulted in Prior Lake being drawn down to 902.0 in most cases at the start of the simulation. Modeling of the actively managed discharge option while maintaining the current Prior Lake outlet configuration resulted in an estimated Prior Lake flood level of 906.8 feet MSL for the 100- year, 30-day rainfall event, which is 0.3 feet lower than the predicted flood level under existing conditions (with the existing, unmanaged outlet structure). As a stand-alone option, it is also expected that this option would need to incorporate structural measures to prevent storm sewer backflow at the Highway 21 crossing. 7.2 Improvement Options Modeling and Cost Summary As previously discussed, each potential flood mitigation measure was evaluated by modeling it with the calibrated watershed model using the 2-, 5-, 10-, 25-, 50-, 100-, and 500-year, 30-day Atlas 14 rainfall events. Barr also evaluated all of the potential upper watershed storage areas together, and all of the flood mitigation options together. Table 7-3 and Figure 7-2 summarize the resulting simulated peak water surface elevations for Prior Lake. The results show that implementation of Options B and C would provide similar but significant benefits for all storm frequencies, while Option D provides more benefit during the 100-and 500-year events and less benefit during the more frequent storms. Option E would result in significant flood reductions across the board with predicted flood levels that are lower than the OHW for all but the 100- and 500-year events. Since the OHW is typically attained in a lake at a ten year frequency, Option E would likely alter the OHW elevation and represents a greater flood level reduction than what should be considered or recommended in the implementation plan. 39 Table 7-3 Simulated Prior Lake Peak Elevations for flood mitigation options Simulation Peak Prior Lake Water Surface Elevation (ft) (Change from Existing Conditions in ft) 2-year 5-year 10-year 25-year 50-year 100-year 500-year Existing Conditions/Enhanced Protection/Floodproofing (Options A & F) 903.6 904.2 904.8 905.6 906.3 907.1 909.0 Spring Lake Storage (Option B) 903.1 (-0.5) 903.4 (-0.8) 903.9 (-0.9) 904.8 (-0.8) 905.6 (-0.7) 906.3 (-0.8) 908.3 (-0.8) Prior Lake Outlet Modification (Option C) 902.8 (-0.8) 903.3 (-0.9) 903.9 (-0.9) 904.8 (-0.9) 905.5 (-0.8) 906.2 (-0.9) 908.1 (-1.0) Upper Watershed Storage (Option D) 903.3 (-0.3) 903.7 (-0.5) 904.1 (-0.7) 904.8 (-0.9) 905.3 (-1.0) 905.8 (-1.2) 907.5 (-1.5) Combine Options B, C & D (Option E) 902.5 (-1.1) 902.6 (-1.7) 902.7 (-2.1) 902.9 (-2.7) 903.5 (-2.8) 904.2 (-2.9) 905.9 (-3.1) Figure 7-2 Simulated Prior Lake flood levels for existing conditions and flood mitigation options Table 7-4 summarizes the peak elevations of Spring Lake and Prior Lake, along with the associated planning level cost estimates (which are presented in the appendices), as well as the associated cost- benefit based on the predicted reductions in the Prior Lake peak flood level for each of the potential improvement options. If the conservative cost estimates for securing Spring Lake drainage easements are accurate, then Table 7-4 indicates that the Spring Lake storage option will not be as cost effective 40 (from a flood control perspective) as increasing the Prior Lake outlet capacity or increasing upper watershed storage. A scaled-down version of Option B, that involves less inundation on Spring Lake and less easement cost, may represent a more cost-effective and viable short-term implementation measure. Table 7-4 indicates that the options involving an increase to the Prior Lake outlet capacity and increasing upper watershed storage are comparable at cost-effectively controlling flooding on Prior Lake. The table also shows that some upper watershed storage locations are significantly more cost-effective for Prior Lake flood reduction than others with the Sutton Lake and S-SPL-094 (airport) sites being the best. Upper watershed storage at Buck Lake and S-BL-020 and S-SPL-080 is also cost-effective, while the relative benefit associated with storage at S-SPL-046, S-SPL-054 and S-SPL-059 does not appear to offset the high cost that was assumed for purchase of the additional inundation area. As discussed in Section 7.1.4, the peak flow reductions associated with Option D have good potential for increasing the total annual phosphorus load from the County Ditch 13 watershed that can be treated by the ferric chloride treatment system as well as the additional treatment that would be realized within each one of the upper watershed storage areas. Actively managing the Prior Lake outlet low-flow gate (Option G) represents the most cost-effective option, and may provide a solution for some of the more frequent flood events (10-year or less), but would likely need to be combined with other options to meet the study goals for larger storm events. Option C assumed a modest (20 cfs or about 30%) increase in the current discharge capacity of the Prior Lake outlet which resulted in moderate cost-effectiveness for reductions to the Prior Lake flood level. It is expected that more significant increases in discharge capacity would be more cost-effective based on the relative infrastructure costs, but may also be more difficult to permit and rely on during high flow events that do not have downstream capacity. Table 7-4 shows that the cost-effectiveness of Option E is a composite of Options B, C & D and does not represent an overly economical or expensive option in relation to the others. However, it appears there is good potential to cost-effectively optimize aspects of Options B, C and/or D, and combine it with Option G, to implement an overall solution that results in a peak flood level reduction of at least two feet for Prior Lake during critical 100-year event. 41 Table 7-4 Summary of Flood Mitigation Option Costs and Simulated Flooding Impacts Name Type of Flood Reduction Measure Spring Lake Peak Elevation Change1 (ft) Prior Lake Peak Elevation Change1 (ft) Estimated Implementation Cost2,3,4 Cost per Prior Lake Peak Elevation Reduction1,4 ($/ft) Enhanced Protection (Option A) Existing (Temporary) Management No Change No Change $ 1,000,000 ∞ Spring Lake Storage (Option B) Outlet Modification and Detention Storage + 0.9 - 0.8 $ 4,100,0005 $ 5,130,000 Prior Lake Outlet Modification (Option C) Outlet Modification No Change - 0.9 $ 2,800,000 $ 3,110,000 Upper Watershed Storage (Option D) Upper Watershed Storage - 1.1 - 1.2 $ 3,900,000 $ 3,250,000 S-BL-001 (Buck Lake) Upper Watershed Storage - 0.4 - 0.3 $ 670,0005 $ 2,230,000 S-BL-020 Upper Watershed Storage - 0.1 - 0.1 $ 260,0005 $ 2,600,000 S-LPL-048 (Little Prior Lake) Gated Detention Storage (Existing Infrastructure) < - 0.1 < - 0.1 N/A $ 0 S-SPL-046 Upper Watershed Storage - 0.2 - 0.1 $ 860,0005 $ 8,600,000 S-SPL-054 Upper Watershed Storage < - 0.1 < - 0.1 $ 390,0005 $ 7,800,000 S-SPL-059 Upper Watershed Storage < - 0.1 < - 0.1 $ 260,0005 $ 8,670,000 S-SPL-078 Upper Watershed Storage - 0.1 - 0.1 $ 300,0005 $ 3,000,000 S-SPL-080 Upper Watershed Storage - 0.1 - 0.1 $ 270,0005 $ 2,700,000 S-SPL-094 Upper Watershed Storage - 0.1 - 0.5 $ 710,000 $ 1,420,000 S-SUL-001 (Sutton Lake) Upper Watershed Storage < - 0.1 - 0.3 $ 130,000 $ 430,000 Combine Options B, C & D (Option E) Combines upper watershed storage, Prior and Spring Lake outlet modifications < - 0.1 - 2.9 $ 10,800,000 $ 3,720,000 Floodproofing (Option F) Buyouts/Floodproofing (Permanent Management) No Change No Change $ 35,000,000 ∞ Actively Manage Prior Lake Outlet (Option G) Existing Outlet Management No Change - 0.3 $ 100,000 $330,000 1) Rounded to the nearest 0.1 feet. 2) Planning level cost estimate 3) 2015 Dollars 4) Rounded to nearest $10,000 5) Assumes full purchase value for any additional inundated area 42 7.3 Permitting Requirements and Easement Acquisition It is expected that, at a minimum, most of the proposed improvement options will be subject to the MnDNR Public Waters Permit requirements and associated drainage easement implications. MnDNR staff were contacted to discuss the permit requirements and provided the following feedback based on their interpretation of the applicable rules and statutes: Changing the size of an outlet for a public water triggers the public waters permit requirements (as well as construction, repair, reconstruction, or abandonment of any water level control structure) While the rule regarding water level controls doesn’t explicitly state this, it is DNR policy that flowage easements or consent forms from all affected property owners be obtained prior to any significant change in lake level control elevations [MN Rule 6115.0220 Subp. 2 A. (2)] While the improvement options do not propose changes to runout or control elevations, the riparian landowner consent requirement applies since the options involve storing more water and increasing water level bounce, which may shift the OHW level up over time and increase flooding to a greater depth or frequency, which would likely require the purchase of flowage easements The permit applicant must own or have easements for the site of the water level control structure The MnDNR will also need to approve proposed changes to the Operating Plan (previously described in Table 2-1). This permit requirement pertains to the improvement option that involves active management of the existing Prior Lake low-flow gate. It is also expected that some of the improvement options may need to undertake, or be subject to, the following permit activities as a part of implementing the proposed/recommended plan: Fish & Wildlife review/public noticing/meeting Corps of Engineers Section 404 permitting EAW requirements based on 1 or more acres of public water or public waters wetland area impacted, for projects that will change or diminish the course, current, or cross-section Nonpublic waters will be subject to Wetland Conservation Act requirements, with EAW requirements for projects that will change or diminish the course, current, or cross-section of 40 percent or more or five or more acres of types 3 through 8 wetland of 2.5 acres or more if any part of the wetland is within a shoreland area or delineated flood plain If the Spring Lake weir structure is of “State Significance”, then it will be subject to Historic Preservation Act requirements (Section 106) MPCA Construction Activities NPDES permit for projects that result in an acre or more of disturbance City of Prior Lake Conditional Use Permit (CUP) and/or grading permit 43 7.4 Matrix Comparison of Other Considerations As previously discussed, the potential mitigation options were narrowed down for further analysis and rated in terms of potential benefit (using flood reduction modeling results and qualitative assessments of potential water quality and natural resources benefits), cost and other selection criteria (such as human impacts, feasibility issues and risk factors). 7.4.1 Rationale for Scoring Detailed Comparison Matrix of Improvement Options This section describes the rationale for completing the scoring criteria ratings for each of the following improvement options contained in the Detailed Comparison Matrix of Improvement Options, given the basis for rating each criterion: Option A—Enhanced Protection (coordinated temporary protection measures) Option B—Spring Lake Storage (increases existing Spring Lake 100-year flood level by 0.9 feet) Option C—Prior Lake Outlet Modification (increases capacity to 85 cfs, or 20 cfs greater than current conditions) Option D—Upper Watershed Storage (combined effect of storage at 9 upper watershed priority sites) Option E—Combine Options B, C & D Option F—Floodproofing (for at-risk primary structures) Option G—Actively Manage Prior Lake Outlet low-flow gate (assumes full capacity above NWL) 7.4.1.1 Flood Reduction Benefits Increases upper watershed stormwater storage &/or moderates runoff rates [yes (1) -or- no (0)]—Weight Factor=10 The ratings for this scoring criterion are based on whether the runoff rates into Prior Lake area reduced in comparison to existing conditions for any of the improvement options. Upper watershed storage is defined as any storage area upstream of Prior Lake. Reduction of peak Prior Lake 100-year flood level [# of inches]—Weight Factor=10 Using the modeling results, the ratings for this scoring criterion correspond with the 100-year flood level reduction for Prior Lake, expressed in inches, which reflect the difference between the existing and proposed conditions watershed modeling results for each of the respective options. Improved level of Prior Lake flood protection [# of inches]—Weight Factor=10 The ratings for this scoring criterion correspond with the improved level of 100-year flood protection for Prior Lake, expressed in inches, which reflect the difference between the existing and proposed conditions watershed modeling results for all of the options, except Options A and F. It should be noted that only primary structures are protected in Option F and streets/public access may or may not be maintained with this category in Option A. 44 7.4.1.2 Water Quality and Natural Resources Benefits Mimics natural hydrology [yes (1) -or- no (0)]—Weight Factor=5 Extent to which the options create conditions that are expected to provide benefits similar to natural hydrology. Provides opportunity for stormwater infiltration &/or improves drought tolerance in upper watershed storage areas [yes (1) -or- no (0)]—Weight Factor=5 This will keep water in upper watershed storage and will enhance the ability to manage soil moisture conditions upstream of Prior Lake. Minimizes downstream pollution impacts &/or deposition of sediments [yes (1) -or- no (0)]—Weight Factor=5 The ratings for this scoring criterion reflect the expectation that the options that will detain the flood flows in the upper watershed will minimize pollution and deposition impacts, while the other options will not have that effect. Expands wetland areas [yes (1) -or- no (0)]—Weight Factor=5 Currently, many of the wetlands in the upper watershed are channelized and/or subject to limited fluctuations. Options that promote more water level fluctuations are expected to result in larger areas that would be delineated as a wetland. Enhances the effectiveness of existing water quality BMPs (improves upon prior investment) [yes (1) -or- no (0)]—Weight Factor=10 Existing BMPs include the ferric chloride treatment system in the County Ditch 13 tributary as well as other ponds/wetlands/lakes that may be providing some water quality treatment under existing conditions. This criterion was scored in the matrix based on whether the improvement option would be expected to provide a significant water quality improvement for the downstream lakes. Provides additional/improved habitat for fish and/or wildlife [yes (1) -or- no (0)]—Weight Factor=5 Improved habitat for this criterion was based on the ability to maintain higher and/or flowing water for extended durations, which would be expected to improve habitat for waterfowl. 7.4.1.3 Feasibility Issues Level of effort required to secure permits and necessary approvals [hi (-1)/med.(0)/low (1)]—Weight Factor=5 45 The types of permits and approvals that may be needed are listed/described in Section 7.3, depending on the types of improvements under consideration. Is there current access to the property in order to implement the project? [yes (1) -or- no (0)]—Weight Factor=5 Access for this criterion is based on the ability for construction equipment to access the proposed site without having to secure additional permissions or easements from other landowners. Will we need to secure property rights? [# properties affected]—Weight Factor=-0.5 To implement those options that will need to secure property rights, the number of affected landowners were obtained from the GIS parcel database and entered into the rating column for the respective options. Project implementation [less than one year (2), 1-10 years (1) -or- 10+ years (0)]—Weight Factor=15 Based on expectations for whether implementation of each option could occur within a 1 to 10- year window after accounting for period of time it takes for project funding to become available, along with other factors, such as permitting, feasibility study and property/easement acquisition. 7.4.1.4 Human Impacts Improves access to area roadways & businesses [no effects (0), minimal effects (1), large potential effects (2)]—Weight Factor=10 It is expected some options will have minimal effects to area roadways and businesses, while others will greatly improve access to all areas. Minimal effects would include improved access on up to half of the roadways that would otherwise experience flooding under the current condition, while large potential effects would “greatly improve” access and ensure that almost all roadways are not significantly affected during a 100-year flood event. No effects are for those options that would not represent a significant change from the current condition. Stakeholder groups impacted [unaffected (0), positive effects (1), negative effects (-1)]—Weight Factor=10 for each group This scoring criterion is intended to describe/address human impacts that aren’t already being mitigated by compensation for easements and other loss of property usage (not including access issues). There are four separate entries for this section of the spreadsheet which are specific to the respective stakeholder groups that have the potential to be impacted by each individual option. Impacts summarized for each of the respective criteria are primarily intended to represent an inconvenience factor as it is expected that other, more significant impacts will be mitigated and/or compensated through the purchase of easements or other negotiated forms compensation. Impacts for residents near Prior Lake could include lack of access to properties, road closures, prolonged water pumping, damaged secondary structures, sandbagging, etc. 46 Impacts for residents near Spring Lake could include inundated land, closure of primary streets, damage to secondary structures, etc. Impacts for upper watershed storage area residents could include extra time and effort spent on cleanup from longer inundation time, damage to agricultural buildings and administration of upper watershed storage agreements, etc. Impacts for residents downstream of Prior Lake could include longer inundation time for high flows. 7.4.1.5 Risk Factors Potential for the project to cause low lake levels during drought conditions [no effects (0), minimal effects (1), large potential effects (2)]—Weight Factor=-10 It is expected that implementation of some options will cause lower lake levels (and associated impacts on recreation, shoreline erosion, etc.) unless the higher discharge rate is limited to periods of flooding. Potential for the project to cause detrimental effects downstream/upstream [no effects (0), minimal effects (1), large potential effects (2)]—Weight Factor=-10 It is expected that implementation of some options will have detrimental effects (i.e., impacts on channel erosion, inundation time, etc.) either upstream or downstream of Prior Lake. 7.4.1.6 Cost of Project and Cost Score There is one entry for the total cost of the project, in millions. With the exception of Option A, current entries in the spreadsheet are based on independent estimates of the capital costs for each option, which include construction and easement purchase costs (combined with estimates for engineering/design/permitting/contingencies) based on the descriptions and assumptions listed in the appendices. The cost for Option A is an assumed order of magnitude estimate to cover the cost of sandbagging over the course of a 100-year timeframe and upfront installation of backflow prevention on storm sewer outfalls along the Highway 21 crossing of Prior Lake. With the possible exception of Option D, it is expected that all of the remaining options will experience similar lifespans. As a result, operation and maintenance costs have not been estimated, but are expected to represent a small fraction of the capital costs associated with each option. Also, it is unclear at this time whether any of the projects will qualify for other outside funding sources, except for Option D which may qualify for BWSR implementation funds (see Section 8.6). 7.4.2 Results of Detailed Matrix Comparison Table 7-5 shows that Option G (actively manage Prior Lake outlet low-flow gate) had the highest overall score while Option F (floodproofing) and Option B (Spring Lake storage) had the lowest overall scores. Of the remaining structural measures, Option D (upper watershed storage) and Option C (Prior Lake outlet modification) scored the highest, followed by Option E (combine Options B, C & D). Option A (enhanced protection) had the third lowest overall score in Table 7-5. 47 The results of the scoring for each individual mitigation option can be summarized as follows (in descending order of the final project scores): #1: Actively Manage Prior Lake Outlet - Option G [850 score]—represents a good, low-cost option for reducing the risk of flooding for more frequent events, but will not significantly reduce flood damages for a 100-year event on its own #2: Upper Watershed Storage - Option D [74]—structural measure that scored higher due to more significant flood level reductions and water quality benefits without significantly higher construction costs (in relation to the benefits) #3: Prior Lake Outlet Modification - Option C [64]—structural measure that scored higher due to more significant flood level reductions without significantly higher costs, but was limited by relative upstream and downstream benefits #4: Combine Options B, C & D - Option E [46]—composite score was elevated by inclusion of the cost-benefit of upper watershed storage and increased PLO capacity, but was limited by the inclusion of the Spring Lake storage option #5: Enhanced Protection - Option A [30]—relatively low cost option that provides temporary flood protection, but does not provide flood level reductions #6: Spring Lake Storage - Option B [17]—structural measure that scored lower due to higher costs (in relation to the benefits) and number of properties affected #7: Floodproofing - Option F [16]—the highest cost alternative and does not provide flood level reductions. 48 7.4.2.1 Actively Manage Prior Lake Outlet (Option G) Rank out of 7 Options: #1 Summary: The existing low-flow gate, which is manually operated and housed within the Prior Lake Outlet Structure, would be opened in advance of large inflows and would remain open throughout the event. This option represents a good, low-cost option for reducing the risk of flooding for more frequent events, but will not significantly reduce flood damages for a 100-year event on its own. Flood Reduction Benefits: Matrix Score (0 to 690): 60 # of inches reduced in a 100-year flood event: 3 This option would reduce the flooding on Prior Lake by three inches and provide improved level of flood protection. Relative to the other options, this option has minimal flood reduction benefit but overall is more cost effective. Water Quality and Natural Resource Benefits: Matrix Score (0 to 35): 0 This option does not provide water quality or natural resource benefits. Feasibility Issues: Matrix Score (-196 to 40): 35 # of properties to acquire rights to: 0 The low-flow gate has already been installed and the property secured. No additional permitting or property acquisitions are necessary. However, in order to change current procedures to more aggressively manage the Prior Lake Outlet, the DNR must approve a revised Management Policy and Operating Procedures. The current document was approved by the DNR in 2005. Human Impacts: Matrix Score (-10 to 20): 20 Although minimal, this option would provide a small reduction in lake level which would improve access to area roadways and businesses and provide a benefit to Prior Lake landowners by reducing flooding impacts. There will be no modeled impacts to downstream communities. No impact to landowners on Spring Lake or in the Upper Watershed. Risk Factors: Matrix Score (-40 to 0): -30 As this option would lower lake levels in anticipation of storm events, it has potential risk of causing low lake levels during drought conditions should the predicted rain not come to fruition. This option also has the potential to cause detrimental effects downstream, as more water is sent downstream. Cost of Project: Estimated Cost of Project: $100.000 The cost of the project includes staff time to amend the operating manual for the Prior Lake Outlet Structure and to receive approval from the DNR and support by local partners. The cost also includes the additional staff time required to monitor lake levels and predicted weather, and to open and close the low flow gate as necessary over a 100-year timeframe. Potential Outside Funding Sources: None Total Matrix Score = Score/Cost Factor (0-850): 850 Implementation Timeline: 1-3 years 0 years 30 years 10 years 20 years 49 7.4.2.2 Upper Watershed Storage (Option D) Rank out of 7 Options: #2 Summary: Potential upper watershed storage sites were identified in locations where modeling showed temporary ponding already occurred, the existing topography allowed for significant storage, and other locations of prior local interest, allowing water to be held back in the upper watershed. This option scored higher due to more significant flood level reductions and water quality benefits without significantly higher costs or number of Prior Lake or Spring Lake properties affected. Flood Reduction Benefits: Matrix Score (0 to 690): 310 # of inches reduced in a 100-year flood event: 15 This option would significantly reduce the flooding on Prior Lake and provide improved level of flood protection. Relative to Spring Lake Storage (Option B) and PLO Modification (Option C), this has relatively similar flood reduction benefits at the 25-year flood event, but has superior flood reduction at the 100-year event. In addition, this option can be implemented incrementally, while Options B and C are “all or nothing” options. Water Quality and Natural Resource Benefits: Matrix Score (0 to 35): 35 This option will hold back water, allowing time for sediment and nutrients to settle out of the stormwater. It also includes the restoration and expansion of wetlands. This option provides all of the water quality or natural resource benefits considered, including: mimics natural hydrology, provides stormwater infiltration &/or improves drought tolerance, minimizes downstream pollution &/or deposition of sediments, expands wetland areas, enhances effectiveness of existing water quality BMPs, and provides habitat for fish &/or wildlife. Feasibility Issues: Matrix Score (-196 to 40): -65 # of properties to acquire rights to: 120 The storage sites were identified solely on storage potential and did not include contacting landowners. Identifying, contacting, educating, and negotiating with landowners will be part of the process which will likely take some time. Permits will also need to be acquired for construction of the water control structures. Note: new, alternative storage sites may be identified during the implementation phase. Human Impacts: Matrix Score (-10 to 20): 10 This option would reduce the flood potential on both Prior Lake and Spring Lake, providing a benefit to those landowners. The impact to the upper watershed will be determined on a site-by-site basis. Through negotiations affected landowners will be compensated for the impacts associated with the use of their land. Risk Factors: Matrix Score (-40 to 0): -10 This option has the potential to cause detrimental effects upstream by causing additional areas to pool with water along upstream tile lines, although these effects would be considered during feasibility studies. Cost of Project: Estimated Cost of Project: $3,900.000 The cost of the project are based on independent estimates of the capitol costs for installing upstream storage, including engineering design, construction and easement/fee title purchase costs. The feasibility studies (which are not included in the Cost of Project) will take a closer look at how much storage can be achieved and will also include outreach to landowners to assess willingness to participate in the process and provide a more refined estimate per site. Note: staff time, title costs, appraisal cost and survey work are not included in the Cost. Potential Outside Funding Sources: CCRP, EQIP, RIM Reserve, WRP, WHIP, BWSR Clean Water Fund, MnDNR Conservation Partners Legacy, Flood Hazard Mitigation, MPCA Clean Water Partnership, Section 319, USACOE Flood Programs Total Matrix Score = Score/Cost Factor (0-850): 74 Implementation Timeline: 30+ years Every storage area will need a feasibility study that will each take 3-4 years to complete. In addition, negotiations with landowners and implementation will take 2-10 years each. 0 years 30 years 10 years 20 years 50 7.4.2.3 Prior Lake Outlet Modification (Option C) Rank out of 7 Options: #3 Summary: The Prior Lake outlet structure is currently limited to a maximum flow of approximately 65 cfs. This option includes an additional, parallel outlet structure, allowing a flow rate of 15-20 cfs, increasing the maximum flow to approximately 85 cfs. This option scored higher due to significant flood level reductions without significantly higher costs, but was limited by potential impacts upstream/downstream. Flood Reduction Benefits: Matrix Score (0 to 690): 200 # of inches reduced in a 100-year flood event: 10 This option would significantly reduce the flooding on Prior Lake and provide improved level of flood protection. Relative to Spring Lake Storage (Option B) and Upper Watershed Storage (Option D), this has relatively similar flood reduction benefits at the 25-year flood event, but has inferior flood reduction at the 100-year event than Options D or E. Water Quality and Natural Resource Benefits: Matrix Score (0 to 35): 0 This option does not provide water quality or natural resource benefits. Feasibility Issues: Matrix Score (-196 to 40): 8 # of properties to acquire rights to: 120 This option will require extensive efforts to secure permits and necessary approvals. Modifying the Prior Lake Outlet will trigger the public waters permit requirements (as well as construction, repair, reconstruction, or abandonment of any water level control structure). This option will also require extensive conversations and negotiations with downstream partners as part of the Joint Powers Agreement. In addition, Minnesota River flooding concerns will need to be addressed as well as the Pike Lake TMDL. Property rights will need to be secured on four parcels. Human Impacts: Matrix Score (-10 to 20): 10 This option will improve access to area roadways & businesses, and have positive impacts on Prior Lake landowners. However, there may be impacts to people/communities downstream of Prior Lake. Risk Factors: Matrix Score (-40 to 0): -40 By allowing more water to exit Prior Lake, this options risks causing low lake levels during drought conditions. This project also has the potential to cause detrimental effects downstream of Prior Lake. Cost of Project: Estimated Cost of Project: $2,800.000 The Cost of the Project includes engineering, construction, and land acquisition payments. However, feasibility studies, staff time (including negotiation, administration, etc.), title work, appraisals, and survey work are not included. Potential Outside Funding Sources: USACOE Flood Programs Total Matrix Score = Score/Cost Factor (0-850): 64 Implementation Timeline: 10+ years A feasibility study would be completed first along with discussions with downstream partners and landowners. Installing the pipe (including easements) could take 3-4 years. 0 years 30 years 10 years 20 years 51 7.4.2.4 Combine Options B, C & D (Option E) Rank out of 7 Options: #4 Summary: This option combines Spring Lake Storage (Option B), Prior Lake Outlet Modification (Option C) and Upper Watershed Storage (Option D) to obtain maximum flood reduction benefits. The composite score was elevated by inclusion of the cost-benefit of upper watershed storage and increased PLO capacity, but was limited by the inclusion of the Spring Lake storage option. Flood Reduction Benefits: Matrix Score (0 to 690): 690 # of inches reduced in a 100-year flood event: 34 This option combines the three most beneficial options, providing the most significant flood reductions benefits of any option. Water Quality and Natural Resource Benefits: Matrix Score (0 to 35): 35 The upstream storage portion of this option will hold back water, allowing time for sediment and nutrients to settle out of the stormwater. It also includes the restoration and expansion of wetlands. This option provides all of the water quality or natural resource benefits considered, including: mimics natural hydrology, provides stormwater infiltration &/or improves drought tolerance, minimizes downstream pollution &/or deposition of sediments, expands wetland areas, enhances effectiveness of existing water quality BMPs, and provides habitat for fish &/or wildlife. Feasibility Issues: Matrix Score (-196 to 40): -195 # of properties to acquire rights to: 381 This option includes the feasibility issues for all three options it combines, including permitting and land acquisition of 381 parcels. See specific options for more information about feasibility issues. Human Impacts: Matrix Score (-10 to 20): 10 This option would reduce the flood potential on both Prior Lake and Spring Lake, providing a benefit to those landowners. However, with the Prior Lake Outlet Modification there may be some impacts to downstream landowners. Although they will be compensated for any loss in usable land, the impact to upper watershed landowners was also taken into account here. Risk Factors: Matrix Score (-40 to 0): -40 This option has both the potential to cause low lake levels during drought conditions and to cause detrimental effects downstream/upstream. Cost of Project: Estimated Cost of Project: $10,800.000 The cost of the project includes feasibility studies and then implementation for all three options. Limited by staff capacity, the combination of all three options will likely take a long time to implement fully. Potential Outside Funding Sources: CCRP, EQIP, RIM Reserve, WRP, WHIP, BWSR Clean Water Fund, MnDNR Conservation partners Legacy, Flood Hazard Mitigation, MPCA Clean Water Partnership, Section 319, USACOE Flood Programs Total Matrix Score = Score/Cost Factor (0-850): 46 Implementation Timeline: 30+ years 0 years 30 years 10 years 20 years 52 7.4.2.5 Enhanced Protection (Option A) Rank out of 7 Options: #5 Summary: This Enhanced Protection Option involves coordinated temporary protection measures, similar to flood control efforts that were utilized during high water in 2014. This is a relatively low cost option that is easier to implement, but does not provide flood level reductions. This option, along with Option G, are the two most feasible short-term options to address flooding issues. Flood Reduction Benefits: Matrix Score (0 to 690): 0 # of inches reduced in a 100-year flood event: 0 This option would not provide any flood reduction benefits. Water Quality and Natural Resource Benefits: Matrix Score (0 to 35): 0 This option does not provide water quality or natural resource benefits. Feasibility Issues: Matrix Score (-196 to 40): 40 # of properties to acquire rights to: 0 Permits, approvals, and property rights are not required to implement this option. Enhanced protection measures will be implemented on public property only (road right-of-way, public easements, etc.). Human Impacts: Matrix Score (-10 to 20): -10 This option would not impact people/communities downstream of Prior Lake, Spring Lake landowners, or upper watershed landowners. However, there is an inconvenience factor to the Prior Lake landowners for having to coordinate private sandbagging efforts with enhanced protection measures. Risk Factors: Matrix Score (-40 to 0): 0 As this project will not affect the lake levels, there is no potential to cause low lake levels during droughts or detrimental effects downstream/upstream. Cost of Project: Estimated Cost of Project: $1,000,000 The cost of the project includes staff time, supplies, and other resources necessary to implement enhanced protection. This option can be implemented immediately, but costs are included are relative to a 100-year lifespan. Potential Outside Funding Sources: None Total Matrix Score = Score/Cost Factor (0-850): 30 Implementation Timeline: 1 year 0 years 30 years 10 years 20 years 53 7.4.2.6 Spring Lake Storage (Option B) Rank out of 7 Options: #6 Summary: This option includes installing a permanent embankment with a weir overflow and a 24-inch diameter outlet pipe that would maintain the existing Spring Lake normal water elevation of 909.9 feet MSL during typical flows while allowing for additional storage during high flows. This option is scored lower due to higher costs (in relation to the benefits) and the high number of properties affected. Flood Reduction Benefits: Matrix Score (0 to 690): 190 # of inches reduced in a 100-year flood event: 9 This option would significantly reduce the flooding on Prior Lake and provide improved level of flood protection. Relative to PLO Modification (Option C) and Upper Watershed Storage (Option D), this has relatively similar flood reduction benefits at the 25-year flood event, but has inferior flood reduction at the 100-year event compared to Options D or E (Combination). Water Quality and Natural Resource Benefits: Matrix Score (0 to 35): 10 This option will hold back water, allowing time for infiltration. It will also provide additional fish & wildlife habitat, expanding the shoreline of Spring Lake. However, this option scored lower because it does not provide the other water quality or natural resource benefits. Feasibility Issues: Matrix Score (-196 to 40): -120.5 # of properties to acquire rights to: 261 This project will require MnDNR, US Army Corps of Engineers, and MPCA permits, US Fish & Wildlife review, and permission from all 261 landowners around Spring Lake. In addition, the MnDNR has indicated that it will not allow this option to be installed until greater efforts have been made to secure storage in the upper watershed. Human Impacts: Matrix Score (-10 to 20): -10 This option would reduce the flood potential on both Prior Lake, but would increase flood potential on Spring Lake. Although no primary structures on Spring Lake will be impacted, this option could potentially decrease the amount of dry land available for use by landowners. Risk Factors: Matrix Score (-40 to 0): -20 This project could cause low lake levels on Prior Lake in drought conditions, and could potentially cause detrimental effects upstream by holding back water. Cost of Project: Estimated Cost of Project: $4,100.000 The cost of the project includes feasibility studies and then implementation, including engineering, construction, and land acquisition payments. However, staff time (including negotiation, administration, etc.), title work, appraisals, and survey work are not included. Potential Outside Funding Sources: USACOE Flood Programs Total Matrix Score = Score/Cost Factor (0-850): 17 Implementation Timeline: 10+ years A feasibility study would first be completed for the project. The most time-consuming and challenging part of the project will be obtaining permission from the 261 landowners on Spring Lake. 0 years 30 years 10 years 20 years 54 7.4.2.7 Floodproofing (Option F) Rank out of 7 Options: #7 Summary: This option primarily involves permanent protection measures for at-risk primary structures only, including buyouts where floodproofing is not feasible or cost-effective. This is the highest cost alternative and does not provide any flood level reductions. Flood Reduction Benefits: Matrix Score (0 to 690): 535 # of inches reduced in a 100-year flood event: 53.5 This option would improve the level of Prior Lake 100-year protection by removing primary structures below that level. However, this option did not receive any points for reduction of peak Prior Lake flood levels. Water Quality and Natural Resource Benefits: Matrix Score (0 to 35): 0 This option does not provide any water quality or natural resource benefits. Feasibility Issues: Matrix Score (-196 to 40): 35 # of properties to acquire rights to: 0 This project would require construction permits for the installation of floodproofing measures. The project would also require landowners to sell or alter their property for floodproofing. Human Impacts: Matrix Score (-10 to 20): 10 This option would have positive impacts on Prior Lake landowners as it removes primary structures from the 100- year floodplain. Risk Factors: Matrix Score (-40 to 0): 0 This project will have no impact on flood levels on the lakes, and therefore will have no impacts during drought conditions or to downstream/upstream homes. Cost of Project: Estimated Cost of Project: $35,000,000 The cost of the project includes the purchase of properties below the 100-year flood level and/or floodproofing homes which is estimated to be similar costs. Potential Outside Funding Sources: USACOE Flood Programs Total Matrix Score = Score/Cost Factor (0-850): 16 Implementation Timeline: 10+ years This option will require cooperation from landowners to install floodproofing measures or to purchase their property for buyouts. 0 years 30 years 10 years 20 years 56 7.5 Cost/Benefit Analysis and Conclusions Figure 7-3 shows how each of the potential improvement options are expected to improve the flood impacts for each of the flood frequency events, including summary information pertaining to the total estimated costs and number of primary structures and inaccessible properties at each flood level. The figure shows that, if the conservative cost estimates for securing Spring Lake drainage easements are accurate, then the Spring Lake storage option will not be as cost effective (from a flood control perspective) as increasing the Prior Lake outlet capacity or increasing upper watershed storage. Figure 7-3 also indicates that the options involving an increase to the Prior Lake outlet capacity and increasing upper watershed storage are comparable at cost-effectively controlling flooding on Prior Lake. Upper watershed storage provides better flood control for the larger flood events than any of the other individual options. For the 100-year event, the upper watershed storage option would protect an additional 30-35 primary structures and maintain accessibility to an additional 50 properties. Implementing a combination of the first three options would drop the 100-year flood level to within a half-foot of the OHW for Prior Lake (see Figure 7-3), which is likely more protection than would be necessary for this event. However, Figure 7-3 shows that the predicted 500-year flood level for Option E would still approach the high water level experienced during 2014. Implementation of some combination of upper watershed storage (Option D), increased Prior Lake outlet capacity (Option C) and actively managing the Prior Lake outlet low-flow gate (Option G) is expected to provide the greatest flood protection in the most cost-effective manner. The biggest limitation of this combination of options is that it may take several years for full implementation. As a result, it is expected that some combination of Options G and A will need to represent the short- term implementation measures. A scaled-down version of Option B, that involves less inundation on Spring Lake and less easement cost, may also represent a more cost-effective and viable short- term implementation measure. It is also expected that floodproofing (Option F) and/or buyouts will be a cost-effective measure for the lowest primary structures. 58 8.0 Preferred Option and Implementation Plan 8.1 Recommended Actions 8.2 Consideration for Future Development 8.3 Schedule/Sequencing 8.4 Responsibilities 8.5 Permitting Requirements and Easement Acquisition 8.6 Funding/Financing Options Appendix A—Modeling Technical Memoranda DRAFT Technical Memorandum To: Prior Lake-Spring Lake Watershed District Managers From: Greg Fransen and Greg Wilson, Barr Engineering Subject: Prior Lake Watershed Model Development and Calibration Date: November 23, 2015 Project:23701048.00 1.0 Introduction The purpose of this memorandum is to document the methodology used by Barr Engineering (Barr) to build and calibrate a hydrologic and hydraulic model of the Prior Lake-Spring Lake Watershed in Scott County, Minnesota, and to summarize the effects of design storm simulations on the predicted peak water surface elevations of Spring Lake and Prior Lake. The study area included the Prior Lake-Spring Lake watershed upstream of the Lower Prior Lake outlet structure, encompassing approximately 19,000 acres (30 square miles) of mostly agricultural and residential land in Scott County, Minnesota (Figure 1-1). The watershed is managed by the Prior Lake- Spring Lake Watershed District (District), in cooperation with the City of Prior Lake (City) and other local government units. Record amounts of precipitation fell on the watershed during the spring of 2014, which led to a historic flooding event for Prior Lake. The event prompted the District to seek an updated surface water model for the watershed that could be used to evaluate flood mitigation strategies for future events. Barr and the District chose PCSWMM, developed by Computational Hydraulics, International (CHI), as the software platform for the updated watershed model. PCSWMM consists of a proprietary graphical user interface combined with the publicly-available EPA SWMM engine. Models developed using PCSWMM can be opened, modified, and re-run using the EPA SWMM software. Barr constructed a PCSWMM model that combined culvert and storm sewer data provided by the City of Prior Lake, Scott County, and the Minnesota Department of Transportation (MNDOT) with stream and ditch cross sections and stage-storage curves developed from the Scott County LiDAR dataset. The scope for constructing the model included: 1. Subdividing watersheds to road crossings and major storage areas 2. Identifying road culvert, storm sewers, and surface channels that were required to route runoff to Lower Prior Lake DRAFT To: Prior Lake-Spring Lake Watershed District Managers From: Greg Fransen and Greg Wilson, Barr Engineering Subject: Prior Lake Watershed Model Development and Calibration Date: November 23, 2015 Page: 2 P:\Mpls\23 MN\70\23701048 Prior Lake Flood Damage Reduct\WorkFiles\Tech Memos\Model_Development_TechMemo_v03.docx 3. Creating a PCSWMM computer model to analyze flow rates and lake elevations throughout the study area 4. Calibrating the PCSWMM model to observed flow rates and lake elevations for the June 2014 - July 2014 calibration event. D RAFT Lo w e r P r i o r L a k e Sp r i n g L a k e Fi s h L a k e Su t t o n L a k e Up p e r P r i o r L a k e Ri c e L a k e Bu c k L a k e Sw a m p L a k e Pi k e L a k e Cr y s t a l L a k e Ca t e ' s o r Hi d d e n L a k e Li t t l e P r i o r L a k e PR I O R L A K E - S P R I N G L A K E £¤16 9 28 2 13 41 4 5 6 7 15 4 5 6 7 10 4 5 6 7 21 4 5 6 7 8 4 5 6 7 14 4 5 6 7 23 4 5 6 7 78 4 5 6 7 83 4 5 6 7 16 4 5 6 7 91 4 5 6 7 27 4 5 6 7 42 4 5 6 7 17 4 5 6 7 18 4 5 6 7 12 4 5 6 7 82 4 5 6 7 68 4 5 6 7 44 4 5 6 7 69 4 5 6 7 8 4 5 6 7 15 4 5 6 7 16 4 5 6 7 15 4 5 6 7 18 ¨©79 ¨©81 ¨©69 ¨©87 ¨©70 ¨©73 ¨©77 ¨©89 ¨©81 ¨©87 ¨©81 ¨©79 Sa v a g e Pr i o r L a k e Sh a k o p e e Sp r i n g L a k e To w n s h i p Sa n d C r e e k To w n s h i p Cr e d i t R i v e r To w n s h i p Lo u i s v i l l e To w n s h i p Ja c k s o n To w n s h i p Ja c k s o n To w n s h i p !;N B a r r F o o t e r : A r c G I S 1 0 . 3 , 2 0 1 5 - 0 7 - 2 1 1 2 : 0 9 F i l e : I : \ P r o j e c t s \ 2 3 \ 7 0 \ 1 0 4 8 \ M a p s \ R e p o r t s \ T e c h _ M e m o _ J u l y 2 0 1 5 \ F i g 0 1 - 1 S t u d y A r e a . m x d U s e r : m b s 2 101Miles Figure 1 -1 STUDY AREA PCSWMM Modeling Technical Memo Prior Lake-Spring Lake Watershed District Scott County, MinnesotaPublic Water I nventory Basin Civil Township Watershed Management Districts Municipal Boundary 1.5 0 1.5Kilometers !( DRAFT To: Prior Lake-Spring Lake Watershed District Managers From: Greg Fransen and Greg Wilson, Barr Engineering Subject: Prior Lake Watershed Model Development and Calibration Date: November 23, 2015 Page: 4 P:\Mpls\23 MN\70\23701048 Prior Lake Flood Damage Reduct\WorkFiles\Tech Memos\Model_Development_TechMemo_v03.docx 2.0 Model Development 2.1 Hydrologic Model Parameters 2.1.1 Subwatershed Divides Subwatershed divides for portions of the Prior Lake watershed lying outside the City of Prior Lake were delineated using Scott County LiDAR (Minnesota DNR, 2011) to identify subwatershed boundaries and likely road culvert locations. Subwatershed divides within the City of Prior Lake were based on a City- provided subwatershed layer, which was modified by merging subwatersheds to form larger subwatersheds that terminated at potential flow restrictions or large storage areas. A draft subwatershed layer was provided to the project’s Technical Advisory Committee (TAC) for comments, and suggestions from the TAC for additional subwatershed divides were incorporated into the final version, which contained 202 subwatersheds (Figure 2-1). D RAFT Lo w e r _ P r i o r _ L a k e Su t t o n _ L a k e Sp r i n g _ L a k e Bu c k _ L a k e SP L - 42 BL - 26 Up p e r _ P r i o r _ L a k e Ar c t i c _ L a k e BL - 27 BL - 13 Fi s h _ L a k e SP L - 63 SP L - 10 Ri c e _ L a k e BL - 09 SP L - 45 SP L - 17 SP L - 33 SP L - 32 SP L - 40 SP L - 39 BL - 23 LP L - 06 SP L - 03 Sw a m p _ L a k e BL - 07 SP L - 18 LP L - 17 LP L - 14 SP L - 31 LP L - 13 UP L - 16 SP L - 04 SP L - 53 SP L - 52 BL - 22 RL - 04 SP L - 67 SP L - 38 SP L - 13 SP L - 49 SP L - 50 RL - 18 SP L - 55 BL - 15 SU L - 02 SP L - 47 SP L - 28 BL - 20 LP L - 02 RL - 02 SW L - 02 BL - 19 SP L - 61 BL - 24 SP L - 21 FL - 04 BL - 28 SU L - 08 SP L - 24 BL - 14 SU L - 06 SP L - 68 FL - 06 FL - 03 BL - 06 SP L - 27 SP L - 20 BL - 02 BL - 16 BL - 08 RL - 06 LP L - 10 RL - 20 UP L - 15 SP L - 23 Cr y s t a l _ L a k e SP L - 09 BL - 43 BL - 25 SP L - 02 BL - 38 RL - 07 SP L - 12 SP L - 07 SP L - 73 SU L - 04 FL - 07 SP L - 72 UP L - 17 SP L - 44 SP L - 34 AL - 04 UP L - 08 BL - 37 SP L - 48 SP L - 62 SP L - 57 RL - 21 LP L - 19 UP L - 25 UP L - 12 BL - 03 BL - 34 SP L - 36 SU L - 07 SP L - 58 LP L - 18 RL - 05 b UP L - 07 RL - 17 BL - 41 LP L - 20 CL - 03 BL - 40 UP L - 23 SP L - 06 LP L - 03 RL - 01 SU L - 09 SP L - 60 SP L - 46 UP L - 24 SU L - 10 SP L - 16 SP L - 35 UP L - 14 BL - 17 SP L - 74 SP L - 64 SU L - 05 UP L - 05 FL - 02 SP L - 25 SP L - 30 SP L - 19 SP L - 43 UP L - 11 LP L - 12 BL - 39 UP L - 22 BL - 11 FL - 05 RL - 15 LP L - 04 UP L - 06 BL - 04 RL - 05 a CL - 02 SP L - 56 LP L - 05 SU L - 11 RL - 14 RL - 09 LP L - 16 SP L - 37 SP L - 11 SP L - 51 UP L - 02 BL - 42 UP L - 21 AL - 03 RL - 10 UP L - 27 SP L - 65 RL - 11 BL - 21 SP L - 22 RL - 08 FL - 08 UP L - 13 RL - 19 SP L - 71 SU L - 03 FL - 10 SP L - 66 BL - 33 RL - 22 SP L - 70 RL - 16 SP L - 75 SP L - 05 UP L - 04 RL - 03 UP L - 10 LP L - 08 UP L - 09 SP L - 41 BL - 35 SP L - 15 BL - 32 BL - 12 LP L - 11 BL - 18 LP L - 07 LP L - 21 SP L - 29 SP L - 59 BL - 36 UP L - 19 BL - 29 UP L - 26 SP L - 69 SP L - 26 SP L - 08 LP L - 09 UP L - 18 RL - 13 BL - 10 FL - 09UP L - 20 LP L - 15 AL - 02 28 2 Pr i o r L a k e Sa v a g e Sh a k o p e e !;N B a r r F o o t e r : A r c G I S 1 0 . 3 , 2 0 1 5 - 0 7 - 2 1 1 2 : 2 2 F i l e : I : \ P r o j e c t s \ 2 3 \ 7 0 \ 1 0 4 8 \ M a p s \ R e p o r t s \ T e c h _ M e m o _ J u l y 2 0 1 5 \ F i g 0 2 - 1 S u b w a t e r s h e d B o u n d a r i e s . m x d U s e r : m b s 2 4,500 0 4,500Feet Figure 2 -1 SUBWATERSHED BOUNDAR I ES PCSWMM Modeling Technical Memo Prior Lake-Spring Lake Watershed District Scott County, MinnesotaSubwatershed Boundary Public Water I nventory Basin Watershed Management Districts Municipal Boundary 1,500 0 1,500Meters DRAFT To: Prior Lake-Spring Lake Watershed District Managers From: Greg Fransen and Greg Wilson, Barr Engineering Subject: Prior Lake Watershed Model Development and Calibration Date: November 23, 2015 Page: 6 P:\Mpls\23 MN\70\23701048 Prior Lake Flood Damage Reduct\WorkFiles\Tech Memos\Model_Development_TechMemo_v03.docx 2.1.2 Runoff parameters In XP-SWMM, surface runoff from subwatersheds is routed to the stormwater system via the nonlinear reservoir methodology. During each time-step XP-SWMM calculates the surface runoff from the subwatershed as shown in Figure 2-2. In the equation, “Q” is the flow rate from the subwatershed (cfs), “n” is the Manning’s roughness coefficient, “d” is the depth of water (ft), “dp” is the depression storage (ft), and “s” is the slope (ft/ft). Figure 2-2 Nonlinear Reservoir Schematic of a Subwatershed Used in XP-SWMM The flow rate from a subwatershed is directly related to the watershed slope, overland flow surface roughness, depression storage, and width parameter. As the watershed width increases, the flow rate from the subwatershed also increases. With a higher runoff rate, less runoff is stored within the subwatershed and less infiltration occurs. This increases the runoff volume for a given rainfall event. However, as the watershed width decreases the opposite occurs; the flow rate from the subwatershed decreases, infiltration increases, and less runoff volume is generated. 2.1.2.1 Impervious Area Land-cover within a watershed affects the quantity and timing of runoff. Each land use generates a different quantity of runoff due, primarily, to the amount of impervious area within that land-cover. The impervious area input into the PCSWMM model is, by definition, hydraulically connected to the drainage systems being analyzed. This directly connected impervious percentage includes driveways, rooftops, and parking areas that are directly connected to the stormwater collection system. Runoff from the portion of a rooftop draining onto adjacent pervious areas was not treated as connected impervious area. DRAFT To: Prior Lake-Spring Lake Watershed District Managers From: Greg Fransen and Greg Wilson, Barr Engineering Subject: Prior Lake Watershed Model Development and Calibration Date: November 23, 2015 Page: 7 P:\Mpls\23 MN\70\23701048 Prior Lake Flood Damage Reduct\WorkFiles\Tech Memos\Model_Development_TechMemo_v03.docx The percent of directly connected impervious area associated with each land-use type was calculated using 2010 land-use data developed by the Metropolitan Council. The land-use data set is shown in Figure 2-3, and the impervious percentages assigned to each land-use classification are listed in Table 2-1. Table 2-1 Met Council Land Use and Associated Impervious Percentage Land Use Impervious Percentage Agricultural 0 Extractive 0 Park, Recreational, or Preserve 0 Undeveloped 0 Open Water 100 Golf Course 2 Farmstead 8 Seasonal/Vacation 8 Single Family Detached 16 Single Family Attached 16 Manufactured Housing Parks 30 Institutional 35 Major Highway 45 Railway 45 Multifamily 65 Mixed Use Residential 65 Office 70 Mixed Use Industrial 70 Industrial and Utility 70 Retail and Other Commercial 80 Mixed Use Commercial 80 Airport 80 D RAFT Pr i o r L a k e Sa v a g e Sh a k o p e e !;N B a r r F o o t e r : A r c G I S 1 0 . 3 , 2 0 1 5 - 1 1 - 2 3 1 8 : 5 7 F i l e : I : \ P r o j e c t s \ 2 3 \ 7 0 \ 1 0 4 8 \ M a p s \ R e p o r t s \ T e c h _ M e m o _ J u l y 2 0 1 5 \ F i g 0 2 - 3 M e t C o u n c i l L a n d U s e ( 2 0 1 0 ) . m x d U s e r : g d f 4,500 0 4,500Feet Figure 2 -3 MET COUNC I L LAND USE (2010 )PCSWMM Modeling Technical Memo Prior Lake-Spring Lake Watershed District Scott County, MinnesotaSubwatershed Boundary Prior Lake-Spring Lake Watershed District Municipal Boundary Met Council Land Use (2010 )Farmstead Seasonal/Vacation Single Family Detached Single Family Attached Multifamily Retail and Other Commercial Office Mixed Use Residential Mixed Use I ndustrial Mixed Use Commercial and Other I ndustrial and Utility Extractive I nstitutional Park, Recreational or Preserve Golf Course Major Highway Airport Agricultural Undeveloped Water 1,500 0 1,500Meters DRAFT To: Prior Lake-Spring Lake Watershed District Managers From: Greg Fransen and Greg Wilson, Barr Engineering Subject: Prior Lake Watershed Model Development and Calibration Date: November 23, 2015 Page: 9 P:\Mpls\23 MN\70\23701048 Prior Lake Flood Damage Reduct\WorkFiles\Tech Memos\Model_Development_TechMemo_v03.docx 2.1.2.2 Watershed Slope Watershed slope was calculated by using ArcGIS to calculate an area-weighted slope percentage for each subwatershed using a slope grid that was based on a National Elevation Dataset 30-meter DEM. 2.1.2.3 Watershed Width The SWMM user’s manual (Storm Water Management Model; Version 4 User’s Manual, U.S. EPA 1988) suggests estimating the watershed width for a given subwatershed by dividing the watershed area by the longest flow path. The longest flow path was calculated for each subwatershed using the ArcHydro Tools package within ArcGIS. The watershed width was calculated by dividing the area of each watershed (square feet) by the longest flowpath length (feet). 2.1.2.4 Depression Storage Depression storage is a volume that must be filled with water prior to generating runoff from both pervious and impervious areas. It represents losses from processes including surface ponding, surface wetting, interception and evaporation (reference SWMM4 manual). Pervious surface depression storage is subject to infiltration as well as evaporation, while impervious surface depression storage is subject only to evaporation. Depression storage can be calculated as an area -weighted parameter based on published values for various land uses, or it can be used as a calibration parameter. For this study, pervious depression storage was assumed to be 0.17 inches based on values published in the U.S. EPA SWMM Version 5.0 User’s Manual, and impervious depression storage was assumed to be 0.06 inches which is also within the range of values published in the U.S. EPA SWMM Version 5.0 User’s Manual. These values were not adjusted during model calibration. Water captured in depression can evaporate over time. Barr used PCSWMM’s internal functions to calculate daily surface water evaporation from daily air temperature using Hargreaves’ method. Daily temperature data were obtained from the NWS weather station at Flying Cloud Airport, located approximately six miles north of Prior Lake. Evaporation was allowed only during “dry” time-steps, i.e. precipitation and evaporation could not happen simultaneously. 2.1.2.5 Overland Flow Roughness Overland flow is surface runoff that occurs as sheet flow over land surfaces prior to concentrating into defined channels. In order to estimate the overland flow or runoff rate a modified version of Manning’s equation is used by PCSWMM. A key parameter in the Manning’s equation is the roughness coefficient. The shallow flows typically associated with overland flow result in substantial increases in surface friction. As a result, the roughness coefficients typically used in open channel flow calculations are not applicable DRAFT To: Prior Lake-Spring Lake Watershed District Managers From: Greg Fransen and Greg Wilson, Barr Engineering Subject: Prior Lake Watershed Model Development and Calibration Date: November 23, 2015 Page: 10 P:\Mpls\23 MN\70\23701048 Prior Lake Flood Damage Reduct\WorkFiles\Tech Memos\Model_Development_TechMemo_v03.docx to overland flow estimates. These differences can be accounted for by using an effective roughness parameter instead of the typical Manning’s roughness parameter. Typical values for the effective roughness parameter are published in the U.S. COE HEC-1 User’s Manual, June 1998; and EPA SWMM Manual, October 2005. After reviewing the above references, Barr used a default pervious roughness coefficient of 0.20 and a default impervious roughness coefficient of 0.015. 2.1.2.6 Zero Detention Area Barr modeled the open water areas of lakes and wetlands as zero detention area. Zero detention area is defined in PCSWMM as impervious area that is assigned zero depression storage in order to promote immediate runoff. Barr calculated zero detention areas in GIS using the “hydro breaklines” layer from the Scott County LiDAR dataset. 2.1.2.7 Infiltration Barr used the Green-Ampt infiltration method was used to simulate the variation in infiltration rate with time and soil moisture conditions. Green-Ampt input parameters include the saturated hydraulic conductivity (K), initial moisture deficit fraction (θ), and the average capillary suction (psi). Infiltration is most rapid at the start of a rainfall event, when the initial moisture deficit fraction is applied to the entire soil profile. During a rainfall event, the infiltration rate decreases and eventually approaches the saturated hydraulic conductivity. In 1982, Rawls et al presented mean saturated hydraulic conductivity values for eleven USDA soil texture classes, based on a limited survey of literature (Rawls, 1982). Later, Rawls et al assembled a national database of observed saturated hydraulic conductivities (nearly 1,000 values) and summarized the mean and range of saturated hydraulic conductivities for fourteen USDA soil texture classes (Rawls, 1998). Barr calculated area-weighted Green-Ampt infiltration parameters based on the SSURGO hydrologic soil group (HSG) attribute. To help determine suitable model input parameters for each HSG, a specific soil texture was chosen to represent each HSG. For HSG A, infiltration parameters were selected to reflect a sandy loam soil texture. For groups B, C, and D, infiltration parameters were selected based on loam, sandy clay loam, and silty clay, respectively. Figure 2-4 shows the SSURGO hydrologic groups within the Prior Lake watershed. The Green-Ampt infiltration parameters used for the modeling analysis are shown in Table 2-2. D RAFT 28 2 13 Pr i o r L a k e Sa v a g e Sh a k o p e e !;N B a r r F o o t e r : A r c G I S 1 0 . 3 , 2 0 1 5 - 1 1 - 2 3 1 9 : 0 5 F i l e : I : \ P r o j e c t s \ 2 3 \ 7 0 \ 1 0 4 8 \ M a p s \ R e p o r t s \ T e c h _ M e m o _ J u l y 2 0 1 5 \ F i g 0 2 - 4 S o i l H y d r o l o g i c G r o u p s . m x d U s e r : g d f 4,500 0 4,500Feet Figure 2 -4 SO I L HYDROLOG I C GROUPS PCSWMM Modeling Technical Memo Prior Lake-Spring Lake Watershed District Scott County, MinnesotaSubwatershed Boundary Public Water I nventory Basin Prior Lake-Spring Lake Watershed District Municipal Boundary Soil Hydrologic Groups A A/D B B/D C C/D D 1,500 0 1,500Meters DRAFT To: Prior Lake-Spring Lake Watershed District Managers From: Greg Fransen and Greg Wilson, Barr Engineering Subject: Prior Lake Watershed Model Development and Calibration Date: November 23, 2015 Page: 12 P:\Mpls\23 MN\70\23701048 Prior Lake Flood Damage Reduct\WorkFiles\Tech Memos\Model_Development_TechMemo_v03.docx Table 2-2 Green-Ampt Infiltration Parameters for Hydrologic Soil Groups Hydrologic Soil Group Representative Soil Texture Saturated Hydraulic Conductivity1 (in/hr) Initial Moisture Deficit2 Average Capillary Suction3 (in) A Sandy Loam 0.90 0.20 4.33 B Loam 0.20 0.13 8.00 C Sandy Clay Loam 0.14 0.10 8.60 D Silty Clay 0.06 0.09 11.50 1 Rawls, 1998 2 Rawls, 1998 3 Maidment, 1993 Once a rainfall event has ended, PCSWMM gradually resets the initial moisture deficit fraction. The rate at which the moisture deficit is reset can be adjusted using a monthly factor to reflect seasonal changes in the soil drying rate. To modify the default PCSWMM soil recovery rate, Barr used a monthly adjustment factor based on average daily evaporation rates recorded at the University of Minnesota Saint Paul campus during the 30-year normal period from 1980 to 2010, normalized to the April evaporation rate. Table 2-1 shows the soil recovery rate factors used for the simulation. Table 2-3 Soil Recovery Rate Factors Month Days Monthly Evaporation Depth Daily Average Evaporation Depth Normalized Soil Recovery Rate April 10 1.85 0.185 1.00 May 31 6.66 0.215 1.16 June 30 7.32 0.244 1.32 July 31 7.87 0.254 1.37 August 31 6.56 0.212 1.14 September 30 4.66 0.155 0.84 October 10 1.22 0.122 0.66 DRAFT To: Prior Lake-Spring Lake Watershed District Managers From: Greg Fransen and Greg Wilson, Barr Engineering Subject: Prior Lake Watershed Model Development and Calibration Date: November 23, 2015 Page: 13 P:\Mpls\23 MN\70\23701048 Prior Lake Flood Damage Reduct\WorkFiles\Tech Memos\Model_Development_TechMemo_v03.docx 2.1.3 Groundwater The groundwater module of PCSWMM was used to simulate the fate of water that fell as precipitation and then infiltrated into the soil. While soil-water is relatively unimportant for short-duration (e.g. 24-hour) design event simulations, during long-term simulations it can represent a significant proportion of the total water yield, particularly in watersheds with dense stream networks, or watersheds with extensive agricultural drainage ditches and closed-conduit drain-tile. This subsurface drainage volume is important for calibrating the Prior Lake watershed because the June 2014 Prior Lake flooding event was primarily a volume-driven event rather than a peak runoff-driven event. PCSWMM implements “aquifer” objects to simulate soil water storage within the runoff layer. These aquifer objects can be shared by multiple subwatersheds, or a separate aquifer object can be created for each subwatershed. Barr chose to create individual aquifer objects for each subwatershed where groundwater was modeled. Water volume that infiltrates into the soil is passed to the aquifer object during each time step. Water deposited into the aquifer can have a variety of fates, including storage within the soil, lateral drainage to a PCSWMM hydraulic node, loss to deep percolation, and loss to evapotranspiration. 2.1.3.1 Soil Water Storage Characteristics used to simulate soil water storage in PCSWMM include porosity, wilting point, field capacity, saturated hydraulic conductivity, conductivity slope, and tension slope. Porosity is the total proportion of the soil volume that is porous and available for water storage. When the soil pores have been completely filled, the soil is termed to be “saturated”. Water can drain from saturated soil vertically to groundwater aquifers, or laterally to lakes, streams, or ditches. Saturated soil can drain by gravity until it reaches its field capacity, defined as the proportion of the soil volume that can hold water against the pull of gravity. Water then can be removed through evaporation or evapotranspiration until the soil’s wilting point is reached. The wilting point represents the proportion of the soil volume that can retain water through surface tension with the soil particles. This water cannot be removed under typical physical conditions. Porosity, wilting point, field capacity, and saturated hydraulic conductivity were calculated as area- weighted averages based on SSURGO soil textures, using typical values for each soil texture as presented by Rawls, et. al. (1982). Figure 2-5 shows the SSURGO soil texture classes found in the Prior Lake watershed. Table 2-4 shows the values used to calculate area-weighted soil water parameters. Barr used the PCSWMM default values of 10 (dimensionless) for conductivity slope and 15 (inches) for tension slope. D RAFT 28 2 13 Pr i o r L a k e Sa v a g e Sh a k o p e e !;N B a r r F o o t e r : A r c G I S 1 0 . 3 , 2 0 1 5 - 1 1 - 2 3 1 9 : 0 7 F i l e : I : \ P r o j e c t s \ 2 3 \ 7 0 \ 1 0 4 8 \ M a p s \ R e p o r t s \ T e c h _ M e m o _ J u l y 2 0 1 5 \ F i g 0 2 - 5 S o i l T e x t u r e C l a s s i f i c a t i o n . m x d U s e r : g d f 4,500 0 4,500Feet Figure 2 -5 SO I L TEXTURE CLASS I F I CAT I ON PCSWMM Modeling Technical Memo Prior Lake-Spring Lake Watershed District Scott County, MinnesotaSubwatershed Boundary Public Water I nventory Basin Prior Lake-Spring Lake Watershed District Municipal Boundary Soil Surface Texture Classification Muck Mucky silty clay loam Clay loam Silty clay loam Silt loam Loam Sandy clay loam Sandy loam Sand Gravelly sandy loam 1,500 0 1,500Meters DRAFT To: Prior Lake-Spring Lake Watershed District Managers From: Greg Fransen and Greg Wilson, Barr Engineering Subject: Prior Lake Watershed Model Development and Calibration Date: November 23, 2015 Page: 15 P:\Mpls\23 MN\70\23701048 Prior Lake Flood Damage Reduct\WorkFiles\Tech Memos\Model_Development_TechMemo_v03.docx Table 2-4 Soil Water Properties Used to Compute Area-weighted Aquifer Parameters1 Texture Total porosity (fraction) Moisture Content at 1/3 bar4 (fraction) Moisture Content at 15 bar5 (fraction) Saturated Hydraulic Conductivity (cm/hr) Sand 0.437 0.091 0.033 21.000 Sandy loam 0.453 0.207 0.095 2.590 Gravelly sandy loam2 0.453 0.207 0.095 2.590 Loam 0.463 0.270 0.117 1.320 Silt loam 0.501 0.330 0.133 0.680 Sandy clay loam 0.398 0.255 0.148 0.430 Clay loam 0.464 0.318 0.197 0.230 Silty clay loam 0.471 0.366 0.208 0.150 Muck3 0.471 0.366 0.208 0.150 Mucky silty clay loam3 0.471 0.366 0.208 0.150 1 Reproduced from Rawls, et. al, (1982) except as noted. 2 Not included in Rawls. Sandy loam characteristics were assumed for this class. 3 Not included in Rawls. Silty clay loam characteristics were assumed for this class. 4 Defined as field capacity. 5 Defined as wilting point. 2.1.3.2 Lateral Drainage Lateral drainage represents shallow groundwater flow from the saturated zone to a receiving ditch, stream, or other water body. The SWMM groundwater equation is: ܳ௪ ൌ ܣͳሺܪ௪ െܧሻଵ െ ܣʹሺܪ௦௪ െܧሻଶ ܣ͵ܪ௪ ܪ௦௪ where: = groundwater flow (cfs per acre) = elevation of groundwater table (ft) = elevation of surface water at receiving node (ft) = threshold groundwater elevation or node invert elevation (ft) and DRAFT To: Prior Lake-Spring Lake Watershed District Managers From: Greg Fransen and Greg Wilson, Barr Engineering Subject: Prior Lake Watershed Model Development and Calibration Date: November 23, 2015 Page: 16 P:\Mpls\23 MN\70\23701048 Prior Lake Flood Damage Reduct\WorkFiles\Tech Memos\Model_Development_TechMemo_v03.docx A1, A2, B1, B2, and B3 are coeffecients that determine the rate of groundwater flow between the aquifer and the receiving node. Barr used PCSWMM’s built-in flow equation to simulate groundwater flow rates according to the Dupuit- Forchheimer approximation for groundwater flow to a ditch. The Dupuit-Forchheimer approximation assumes that the groundwater flow rate is proportional to the difference between the groundwater and surface water heads, which is calculated using only the first term of the above equation. Therefore A2 and A3 were set to zero, and B1 was set to 2 as recommended in the SWMM5 manual. The A1 coefficient was calculated as: ܣͳൌͶ݇Ȁܮଶ where: k = soil lateral saturated hydraulic conductivity L = ½ * Length Length = the subcatchment flow length Barr calculated the soil lateral saturated hydraulic conductivity based on the subcatchment area-weighted vertical saturated hydraulic conductivity. Barr multiplied the vertical conductivity by a factor of 1.5 according to guidance from Sands (2014) as presented by others. Barr calculated the subcatchment flow length by manually digitizing a representative cross-section in ArcGIS, perpendicular to the main flow path of each subcatchment. The cross-section length was used to represent the “Length” variable. The groundwater module requires values for the initial elevation of the saturated zone and the average surface elevation of the subcatchment, from which is derived the initial depth of the unsaturated zone, and the water content of the unsaturated zone. The elevation of the saturated zone was initially set equal to the receiving node invert, and thereafter increased as needed during calibration to match early-season discharge rates. The unsaturated zone water content was set equal to field capacity. The subcatchment surface elevation of the subcatchment was calculated as the average elevation of the representative subcatchment cross-section using ArcGIS. 2.1.3.3 Deep Percolation Deep percolation in PCSWMM represents the movement of water downward from the saturated zone of soil water to regional aquifers, where it is lost from the model. Deep percolation rates can be set independently for each aquifer. Barr used the deep percolation rate as a calibration parameter for the groundwater component of PCSWMM. DRAFT To: Prior Lake-Spring Lake Watershed District Managers From: Greg Fransen and Greg Wilson, Barr Engineering Subject: Prior Lake Watershed Model Development and Calibration Date: November 23, 2015 Page: 17 P:\Mpls\23 MN\70\23701048 Prior Lake Flood Damage Reduct\WorkFiles\Tech Memos\Model_Development_TechMemo_v03.docx 2.1.3.4 Evaporation and Evapotranspiration Subsurface evapotranspiration in PCSWMM is simulated by removing water from the unsaturated (upper) and saturated (lower) zones of the groundwater aquifers. Evaporation from the unsaturated zone is calculated as a percentage of the remaining daily evaporation allowance following evaporation from surface depression storage. For example, if the evaporation depth for a given date is 0.2 inches, and there are 0.2 inches of water in depression storage, there will be zero inches of water left for evapotranspiration. However, if there only 0.1 inches of water are in depression storage, and the allowable fraction of remaining evaporation is set at 100 percent, an additional 0.1 inches would be removed from the aquifer. Correspondingly, if the allowable fraction of remaining evaporation is set at 50 percent, 0.05 inches would be removed from the aquifer. Barr set the allowable upper zone fraction of remaining evaporation at 15 percent for all aquifers, and used monthly adjustment factors to simulate variation of seasonal evapotranspiration due to plant growth and maturation, according to an empirical relationship described by Gruber (1997). The monthly evaporation adjustment factors are shown in Table 2-5. Table 2-5 Allowable Upper Zone Evaporation Fraction Adjustment Factors Month Adjustment Factor April 1.00 May 1.20 June 3.50 July 6.15 August 5.83 September 3.03 October 0.93 November 0.00 Evapotranspiration from the lower zone is calculated internally in PCSWMM as a function of the upper zone depth and can be restricted to a user-defined lower depth limit. Barr set the evapotranspiration lower limit at three feet of depth below the soil surface, corresponding to the typical lower limit of corn rooting depth. DRAFT To: Prior Lake-Spring Lake Watershed District Managers From: Greg Fransen and Greg Wilson, Barr Engineering Subject: Prior Lake Watershed Model Development and Calibration Date: November 23, 2015 Page: 18 P:\Mpls\23 MN\70\23701048 Prior Lake Flood Damage Reduct\WorkFiles\Tech Memos\Model_Development_TechMemo_v03.docx 2.2 Hydraulic Model Parameters 2.2.1 Stream Network Barr digitized ditch and stream network for the Prior Lake watershed using ArcGIS to trace flow lines that appeared on LiDAR and aerial photography. Stream cross sections used in the PCSWMM model were developed using the GIS interface of PCSWMM. The most restrictive cross section for a given reach was selected and station points and elevations were assigned using the Scott County LiDAR data. 2.2.2 Closed Conduits Barr added closed conduit data to an ArcGIS database using as-built and design plan information supplied by Scott County, the Minnesota Department of Transportation, and the City of Prior Lake. Storm sewer information was added wherever it was needed to route water between storage areas. Culvert information was added at road crossings. Culvert information was not available for township road crossings and for some County and State Highway crossings. Where information was not available, conduit dimensions were estimated based on aerial photos and the diameters of nearby culverts of known dimensions. Figure 2-6 shows the hydraulic network used to simulate the Prior Lake watershed. 2.2.3 Pond Volume The Prior Lake watershed contains numerous lakes and wetland basins where water can temporarily pond before being either conveyed to the watershed outlet or infiltrated into the soil. Barr developed stage- area curves for ponding areas using ArcGIS and Scott County LiDAR data. For ponding areas where the pond invert was lower than its outlet invert and where aerial photography showed no surface water, Barr allowed infiltration for the storage area in PCSWMM. For pond areas where aerial photography showed surface water, infiltration was not allowed. Water that infiltrates into the soil through storage areas does not become part of the groundwater pool and therefore this volume is lost from the PCSWMM system. Evaporation rates from storage units can be turned off or varied as a percentage of the calculated evaporation rate. For the model calibration Barr turned on evaporation in every storage unit and set the evaporation rate to 100% of the calculated evaporation rate. 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As per the PCSWMM program guidance, Barr converted temperature units from degrees Celsius to degrees Fahrenheit and wind speed units from meters per second to miles per hour before importing the time series into PCSWMM. 2.3.2 Historic Precipitation Accurate temporal and spatial distributions of rainfall depth across a watershed are critical for creating a useful, calibrated watershed model. To simulate the 2014 flooding event, Barr used the digital precipitation rate (DPR) dataset collected by the National Weather Service (NWS) Next-Generation Radar (NEXRAD) station located in Chanhassen, Minnesota. NEXRAD data is collected by a rotating radar dish that rotates 360-degrees once every four to six minutes. The DPR dataset is an estimate of precipitation rate in units of millimeters per hour at any location within range of the radar at the time of the sweep. For this study Barr downloaded the DPR data for April 1, 2014 through July 31, 2014, a time period that encompasses the rainfall event that resulted in flooding at Prior Lake. Because the DPR dataset is an estimate of rainfall intensity inferred by measuring radar scatter from raindrops, and not a physical measurement, it is necessary to verify the rainfall estimates using observed data from within or nearby the study area. The PCSWMM software includes utilities to process raw NEXRAD data and adjust the estimated precipitation rate to better match gage data. Barr used the bias- correction utility of PCSWMM to adjust the data. Bias correction is performed using the assumption that for any radar sweep, the precipitation rate estimation error at any one point is the same for other nearby points. By comparing rain gage data collected at a single point to the NEXRAD estimated rate collected at the same point and time period, an adjustment factor is calculated and applied to all the data collected during that sweep. Thirty-minute precipitation depths from a station just west of Prior Lake were provided by the Shakopee Mdewakanton Sioux Community (SMSC). Barr obtained 5-minute precipitation data for two of the closest available airport rain gages lying in the same southeasterly direction from the radar station as the Prior Lake watershed, located at Lakeville and Faribault, respectively (Figure 2-7). NEXRAD data is referenced in Coordinated Universal Time (UTC), and was converted internally by PCSWMM to Central Daylight Time (CDT), the local time zone for the project site. Barr adjusted the SMSC gage data from Central Standard Time (CST) to CDT and the airport gages from UTC to CST. Barr made an additional subtraction of 30 minutes from the SMSC gage data because the gage recorded rainfall accumulation at the end of each time step, delaying the intensity reading compared to the shorter duration time step accumulations of the NEXRAD and airport gages. Barr completed bias-correction in PCSWMM using a three-hour rainfall DRAFT To: Prior Lake-Spring Lake Watershed District Managers From: Greg Fransen and Greg Wilson, Barr Engineering Subject: Prior Lake Watershed Model Development and Calibration Date: November 23, 2015 Page: 21 P:\Mpls\23 MN\70\23701048 Prior Lake Flood Damage Reduct\WorkFiles\Tech Memos\Model_Development_TechMemo_v03.docx accumulation moving window, with the requirement that at least one of the three gage locations had to have recorded rainfall for both the NEXRAD and the gage time series. The minimum adjustment factor was set at 0.5, and the maximum adjustment factor was set at 2.5 (the PCSWMM default values). D RAFT #* #* #* #* #* SM S C ( 30 -m i n u t e ) La k e v i l l e A i r p o r t Fa r i b a u l t A i r p o r t KM P X N E X R A D S t a t i o n Di s t r i c t O f f i c e ( D a i l y ) So u r c e s : E s r i , H E R E , D e L o r m e , I n t e r m a p , i n c r e m e n t P C o r p . , G E B C O , U S G S , F A O , N P S , N R C A N , G e o B a s e , I G N , K a d a s t e r N L , Or d n a n c e S u r v e y , E s r i J a p a n , M E T I , E s r i C h i n a ( H o n g K o n g ) , s w i s s t o p o , M a p m y I n d i a , © O p e n S t r e e t M a p c o n t r i b u t o r s , a n d t h e G I S Us e r C o m m u n i t y !;N B a r r F o o t e r : A r c G I S 1 0 . 3 , 2 0 1 5 - 1 1 - 2 3 1 9 : 1 5 F i l e : I : \ P r o j e c t s \ 2 3 \ 7 0 \ 1 0 4 8 \ M a p s \ R e p o r t s \ T e c h _ M e m o _ J u l y 2 0 1 5 \ F i g 0 2 - 7 R a i n f a l l M o n i t o r i n g S t a t i o n s . m x d U s e r : g d f 1012345Miles Figure 2 -7 RA I NFALL MON I TOR I NG STAT I ONS PCSWMM Modeling Technical Memo Prior Lake-Spring Lake Watershed District Scott County, Minnesota#*Rainfall Station Public Water I nventory Basin Prior Lake-Spring Lake Watershed District 20246810Kilometers DRAFT To: Prior Lake-Spring Lake Watershed District Managers From: Greg Fransen and Greg Wilson, Barr Engineering Subject: Prior Lake Watershed Model Development and Calibration Date: November 23, 2015 Page: 23 P:\Mpls\23 MN\70\23701048 Prior Lake Flood Damage Reduct\WorkFiles\Tech Memos\Model_Development_TechMemo_v03.docx Barr used PCSWMM to calculate area-weighted 5-minute rainfall depths from the bias-adjusted NEXRAD data for each subwatershed in the study area. Cumulative area-weighted subwatershed rainfall depth for the period from April 1, 2015 to July 25, 2014 ranged from 21.8 inches to 42.3 inches, with an area-weighted average rainfall depth for the Prior Lake watershed of 30.4 inches. Barr obtained daily rainfall data from daily monitoring gages within the Prior Lake watershed and compared the daily gage cumulative rainfall depth to the NEXRAD area-weighted cumulative depth for the subwatershed in which it lay (Figure 2-8, Figure 2-9). The comparisons showed that the NEXRAD cumulative depths were typically slightly greater than the daily station cumulative depths, but that these differences were confined to specific rainfall events rather than accumulated over the entire period. Figure 2-8 Comparison of Cumulative Rainfall Recorded at HiDEN Network Station TRS 114-22-3 and NEXRAD-detected Rainfall for Subwatershed Upper_Prior_Lake DRAFT To: Prior Lake-Spring Lake Watershed District Managers From: Greg Fransen and Greg Wilson, Barr Engineering Subject: Prior Lake Watershed Model Development and Calibration Date: November 23, 2015 Page: 24 P:\Mpls\23 MN\70\23701048 Prior Lake Flood Damage Reduct\WorkFiles\Tech Memos\Model_Development_TechMemo_v03.docx Figure 2-9 Comparison of Cumulative Rainfall Recorded at HiDEN Network Station TRS 114-22-4 and NEXRAD-detected Rainfall for Subwatershed SPL-55 DRAFT To: Prior Lake-Spring Lake Watershed District Managers From: Greg Fransen and Greg Wilson, Barr Engineering Subject: Prior Lake Watershed Model Development and Calibration Date: November 23, 2015 Page: 25 P:\Mpls\23 MN\70\23701048 Prior Lake Flood Damage Reduct\WorkFiles\Tech Memos\Model_Development_TechMemo_v03.docx 2.3.3 Design Event Precipitation Previous hydrologic models of the Prior Lake watershed were run using rainfall depths from the U.S. Weather Bureau’s Technical Paper No. 40 (TP40), published in 1961. In 2013 the National Oceanic and Atmospheric Administration (NOAA) released Atlas 14, Volume 8, which revised precipitation frequency estimates for 11 Midwestern states, including Minnesota. The estimates serve as an update to TP40. Atlas 14 rainfall estimates were developed for individual rainfall monitoring stations with long, consistent periods of record. After estimates were developed for individual stations, they were interpolated between stations to account for the spatial variability of rainfall depths. The Atlas 14 dataset includes the Jordan 2E precipitation monitoring station (NWS Cooperative Station ID 21-4176), located approximately two miles west of the Prior Lake watershed. Atlas 14 rainfall depths for selected durations and return recurrences are shown in Table 2-6. As shown in Figure 2-10, rainfall depths for the 30-day, 100-year event vary by approximately 0.1 inches (14.5 inches to 14.6 inches) across the Prior Lake watershed. Table 2-6 Atlas 14 Rainfall Depths for the Jordan 2E Precipitation Monitoring Station Return Interval (years) 1 2 10 25 50 100 500 Event Duration Rainfall Depth (inches) 10-day 4.41 5.02 6.94 8.22 9.26 10.3 13.0 20-day 6.04 6.77 8.96 10.3 11.4 12.5 15.0 30-day 7.45 8.31 10.8 12.3 13.5 14.6 17.1 45-day 9.25 10.3 13.4 15.2 16.4 17.6 20.1 60-day 10.8 12.1 15.8 17.8 19.2 20.5 23.1 D RAFT 1 4 . 5 1 4 . 5 Sh a k o p e e Sa v a g e La k e v i l l e Bu r n s v i l l e Pr i o r L a k e Ch a s k a Ca r v e r Jo r d a n Bl o o m i n g t o n Ch a n h a s s e n !;N B a r r F o o t e r : A r c G I S 1 0 . 3 , 2 0 1 5 - 0 7 - 2 3 1 0 : 0 7 F i l e : I : \ P r o j e c t s \ 2 3 \ 7 0 \ 1 0 4 8 \ M a p s \ R e p o r t s \ T e c h _ M e m o _ J u l y 2 0 1 5 \ F i g 0 2 - 1 0 A t l a s 1 4 1 0 0 - Y e a r 3 0 - d a y R a i n f a l l D e p t h . m x d U s e r : m b s 2 1.5 0 1.5Miles Figure 2 -10 ATLAS 14 100 -YEAR 30 -DAY RA I NFALL DEPTH PCSWMM Modeling Technical Memo Prior Lake-Spring Lake Watershed District Scott County, MinnesotaSubwatershed Boundary Public Water I nventory Basin Watershed Management Districts Municipal Boundary HUNDRETH TENTH WHOLE Atlas 14 100 -Year 30 -Day Grid High : 14 .60 Low : 14 .42 303Kilometers DRAFT To: Prior Lake-Spring Lake Watershed District Managers From: Greg Fransen and Greg Wilson, Barr Engineering Subject: Prior Lake Watershed Model Development and Calibration Date: November 23, 2015 Page: 27 P:\Mpls\23 MN\70\23701048 Prior Lake Flood Damage Reduct\WorkFiles\Tech Memos\Model_Development_TechMemo_v03.docx 3.0 Model Calibration 3.1 Monitoring Data PLSLWD staff members collect stream stage and flow data for several sites within the Prior Lake watershed. Continuous (15-minute interval) stage monitoring data were collected at five sites during the 2014 monitoring season – two major tributaries and one minor tributary to Spring Lake, along with the Spring Lake and Prior Lake outlets (Figure 3-1). The PLSLWD used rating curves to convert stage measurements to flow rated. In addition, the Shakopee Mdewakanton Sioux Community (SMSC) provided Arctic Lake outlet synoptic flow monitoring data which Barr used to generally determine whether the Arctic Lake portion of the watershed was contributing flow during the calibration time period. Most of the available monitoring data covered the time period from March 28th through October, 2014, with some synoptic measurements prior to March 28, 2014. Table 3-1 summarizes drainage characteristics for each of the five PLSLWD monitoring sites evaluated for use in the watershed model calibration. The combined drainage area of the Buck Lake and County Ditch 13 monitoring sites accounts for 76 percent of the Spring Lake watershed and 52 percent of the Prior Lake watershed. Table 3-1 Monitoring Site—Site # Watershed Area (acres)1 Percent of Spring Lake Watershed Area Percent of Prior Lake Watershed Area Marshall Road Crossing— Site 19 401 3% 2% Buck Lake Outlet—Site 14 4,036 32% 21% County Ditch 13—Site 7 5,526 44% 29% Spring Lake Outlet—Site 21 12,703 100% 66% Prior Lake Outlet 19,239 -- 100% 1 Watershed areas include potentially landlocked or non-contributing areas. D RAFT ") ") ") ") ") Lo w e r P r i o r L a k e Sp r i n g L a k e Fi s h L a k e Su t t o n L a k e Up p e r P r i o r L a k e Ri c e L a k e Bu c k L a k e Sw a m p L a k e Pi k e L a k e Cr y s t a l L a k e Ca t e ' s o r Hi d d e n L a k e Li t t l e P r i o r L a k e Pr i o r L a k e O u t l e t Co u n t y D i t c h 13 -- S i t e 7 Bu c k L a k e O u t l e t - - S i t e 14 Sp r i n g L a k e O u t l e t - S i t e 21 Ma r s h a l l R o a d C r o s s i n g - - S i t e 19 Pr i o r L a k e Sa v a g e Sh a k o p e e !;N B a r r F o o t e r : A r c G I S 1 0 . 3 , 2 0 1 5 - 1 1 - 2 3 1 4 : 5 3 F i l e : I : \ P r o j e c t s \ 2 3 \ 7 0 \ 1 0 4 8 \ M a p s \ R e p o r t s \ T e c h _ M e m o _ J u l y 2 0 1 5 \ F i g 0 3 - 0 1 C a l i b r a t i o n F l o w M o n i t o r i n g S t a t i o n s . m x d U s e r : g d f 4,500 0 4,500Feet Figure 3 .1 CAL I BRAT I ON FLOW MON I TOR I NG STAT I ONS PCSWMM Modeling Technical Memo Prior Lake-Spring Lake Watershed District Scott County, Minnesota")Monitoring Station Subwatershed Boundary Public Water I nventory Basin Prior Lake-Spring Lake Watershed District Municipal Boundary 1,500 0 1,500Meters DRAFT To: Prior Lake-Spring Lake Watershed District Managers From: Greg Fransen and Greg Wilson, Barr Engineering Subject: Prior Lake Watershed Model Development and Calibration Date: November 23, 2015 Page: 29 P:\Mpls\23 MN\70\23701048 Prior Lake Flood Damage Reduct\WorkFiles\Tech Memos\Model_Development_TechMemo_v03.docx Figure 3-2 shows the measured hydrograph data for the five watershed monitoring sites for the period between the end of April and July, 2014. More than seven inches of rainfall fell across most of the watershed between June 15th and the 20th, which contributed to the peak discharge rates observed at all five monitoring sites. While all three of the tributary stations experienced peak flow on June 19 th, the flow monitoring results show that the tributary stations experienced varying levels of flashiness and relative magnitudes of peak discharge. All of the storm flow at Site 19 discharged within two days, while flow at Sites 7 and 14 returned to pre-storm levels within six days after peak discharge. For the Spring Lake outlet, peak discharge occurred on June 22nd – three days after the peak discharge rates occurred at the three tributary monitoring stations. Flow out of the Spring Lake outlet returned to pre-storm levels within 14 days of the peak discharge. The peak Prior Lake level and outlet discharge rate occurred on June 30th and Prior Lake did not return to pre-storm levels for more than 40 days. Barr used data from all the monitoring sites except for the Marshall Road Crossing (Site 19). The watershed contributing to Site 19 contains a commercial area with stormwater ponds and associated stormwater piping, and it was beyond the scope of this project to include a detailed flow network in the PCSWMM model. DRAFT To: Prior Lake-Spring Lake Watershed District Managers From: Greg Fransen and Greg Wilson, Barr Engineering Subject: Prior Lake Watershed Model Development and Calibration Date: November 23, 2015 Page: 30 P:\Mpls\23 MN\70\23701048 Prior Lake Flood Damage Reduct\WorkFiles\Tech Memos\Model_Development_TechMemo_v03.docx Figure 3-2 Calibration Station Flow Data for 2014 3.2 Calibration Procedure Barr calibrated the model using historic rainfall between the dates of April 1, 2014 and July 25, 2014, as described above. The model was calibrated to one monitoring site at a time, moving in an upstream to downstream direction. As each site was calibrated, the calibrated parameters were incorporated into the model before calibrating at the next downstream site. The focus of this models calibration was to match the volume of water that reached Prior Lake before and during the extended 2014 flooding event, and the peak Prior Lake elevation. Barr began by calibrating the watersheds lying upstream of the County Ditch 13 (CD13) and Buck Lake monitoring sites. These watersheds have been highly ditched and drained for agriculture. The initial runs for CD13 were performed with groundwater flow turned off for every subwatershed. This resulted in much lower discharge at the calibration station than was seen in the monitored data. The groundwater feature was then turned on for all of the subwatersheds, and groundwater flow parameters were calculated for DRAFT To: Prior Lake-Spring Lake Watershed District Managers From: Greg Fransen and Greg Wilson, Barr Engineering Subject: Prior Lake Watershed Model Development and Calibration Date: November 23, 2015 Page: 31 P:\Mpls\23 MN\70\23701048 Prior Lake Flood Damage Reduct\WorkFiles\Tech Memos\Model_Development_TechMemo_v03.docx each subwatershed as described above. The initial groundwater surface elevation and the initial soil moisture conditions were first calibrated to bring the early hydrographs into agreement. The deep percolation rate was then calibrated to bring the recession limbs of hydrographs following storm events into agreement with the monitoring data. Figure 3-3 compares the uncalibrated and the calibrated hydrographs to the monitoring data for County Ditch 13. Figure 3-3 County Ditch 13 Calibration The initial calibration runs for the Buck Lake watershed were carried out using the initial groundwater surface elevation, soil moisture conditions, and deep percolation rate that were developed during the CD13 calibration. Figure 3-4 compares the uncalibrated and the calibrated hydrographs to the monitoring data for Buck Lake. DRAFT To: Prior Lake-Spring Lake Watershed District Managers From: Greg Fransen and Greg Wilson, Barr Engineering Subject: Prior Lake Watershed Model Development and Calibration Date: November 23, 2015 Page: 32 P:\Mpls\23 MN\70\23701048 Prior Lake Flood Damage Reduct\WorkFiles\Tech Memos\Model_Development_TechMemo_v03.docx Figure 3-4 Buck Lake Outlet Calibration Barr then calibrated to the Spring Lake site, which lies at a sufficiently high elevation above Prior Lake that the Prior Lake elevation has no impact on discharge from Spring Lake. The first calibration runs again showed insufficient volume with groundwater turned off for all subwatersheds. Groundwater was then turned on for all the nodes upstream of Spring Lake not already included in the Buck Lake or County Ditch 13 watersheds. The initial groundwater surface elevation, initial soil moisture, and deep percolation rate were set at the County Ditch 13 watershed calibrated values, and adjusted as needed to bring the Spring Lake hydrograph into agreement. Figure 3-5 compares the uncalibrated and the calibrated hydrographs to the monitoring data for Spring Lake. DRAFT To: Prior Lake-Spring Lake Watershed District Managers From: Greg Fransen and Greg Wilson, Barr Engineering Subject: Prior Lake Watershed Model Development and Calibration Date: November 23, 2015 Page: 33 P:\Mpls\23 MN\70\23701048 Prior Lake Flood Damage Reduct\WorkFiles\Tech Memos\Model_Development_TechMemo_v03.docx Figure 3-5 Spring Lake Calibration Last, Barr calibrated the watersheds contributing directly to Upper Prior Lake and Lower Prior Lake. Groundwater was turned on only for the rural portions of the Crystal Lake and Rice Lake subwatersheds, and the initial surface elevation, initial soil moisture, and deep percolation rate were adjusted. The resulting model generally matched the Prior Lake elevation, but over-predicted the monitored peak elevation that occurred on June 30, and predicted a slower recession rate than the shown by the monitoring data. Sensitivity tests of the Prior Lake outlet discharge curve and the evaporation rate showed that underestimates of the evaporation rate or the discharge rate at the Prior Lake Outlet could not explain the difference in recession rate. Examination of the underlying geology of Lower Prior Lake suggested that the higher observed recession rate could be explained by seepage to groundwater at high water elevations. Lower Prior Lake lies at the upper reaches of an outwash aquifer which connects to the Minnesota River, as shown in Figure 3-6. Regional groundwater-surface water interaction maps also show Lower Prior Lake as a water body that is connected to groundwater (Figure 3-7). D RAFT Sh a k o p e e Sa v a g e La k e v i l l e Bu r n s v i l l e Pr i o r L a k e Ch a s k a Ca r v e r Jo r d a n Bl o o m i n g t o n Ch a n h a s s e n !;N B a r r F o o t e r : A r c G I S 1 0 . 3 , 2 0 1 5 - 1 1 - 2 3 1 9 : 3 3 F i l e : I : \ P r o j e c t s \ 2 3 \ 7 0 \ 1 0 4 8 \ M a p s \ R e p o r t s \ T e c h _ M e m o _ J u l y 2 0 1 5 \ F i g 0 3 - 6 R e g i o n a l A q u i f e r P r o p e r t i e s . m x d U s e r : g d f 1.5 0 1.5Miles Figure 3 -6 REG I ONAL AQU I FER PROPERT I ES PCSWMM Modeling Technical Memo Prior Lake-Spring Lake Watershed District Scott County, MinnesotaSubwatershed Boundary Public Water I nventory Basin Prior Lake-Spring Lake Watershed District Municipal Boundary Regional Aquifer Properties Non-Acquifer (Till, Loess, & Peat)--Less than 1 gallon per minute Outwash (Sand & Gravel)--100 -500 gallons per minute Alluvium (Sand, Silt, & Gravel)--Greater than 500 gallons per minute 303Kilometers D RAFT Lo w e r P r i o r L a k e Sp r i n g L a k e Fi s h L a k e Su t t o n L a k e Up p e r P r i o r L a k e Ri c e L a k e Bu c k L a k e Sw a m p L a k e Pi k e L a k e Cr y s t a l L a k e Ca t e ' s o r Hi d d e n L a k e Li t t l e P r i o r L a k e Pr i o r L a k e Sa v a g e Sh a k o p e e !;N B a r r F o o t e r : A r c G I S 1 0 . 3 , 2 0 1 5 - 1 1 - 2 3 1 9 : 3 5 F i l e : I : \ P r o j e c t s \ 2 3 \ 7 0 \ 1 0 4 8 \ M a p s \ R e p o r t s \ T e c h _ M e m o _ J u l y 2 0 1 5 \ F i g 0 3 - 7 R e g i o n a l G r o u n d w a t e r - S u r f a c e W a t e r C o n n e c t i v i t y . m x d U s e r : g d f 4,500 0 4,500Feet Figure 3 -7 REG I ONAL GROUNDWATER-SURFACE WATER CONNECT I V I TY PCSWMM Modeling Technical Memo Prior Lake-Spring Lake Watershed District Scott County, MinnesotaSubwatershed Boundary Prior Lake-Spring Lake Watershed District Municipal Boundary Basin Groundwater Connectivity Connected I ndeterminate Disconnected 1,500 0 1,500Meters DRAFT To: Prior Lake-Spring Lake Watershed District Managers From: Greg Fransen and Greg Wilson, Barr Engineering Subject: Prior Lake Watershed Model Development and Calibration Date: November 23, 2015 Page: 36 P:\Mpls\23 MN\70\23701048 Prior Lake Flood Damage Reduct\WorkFiles\Tech Memos\Model_Development_TechMemo_v03.docx Barr represented seepage to groundwater with a user-defined rating curve, calibrated to match the peak elevation of Prior Lake and the recession limb of that peak. Figure 3-8 compares the uncalibrated, calibrated (without seepage) and calibrated (with seepage) hydrographs to the Prior Lake monitoring data. Figure 3-8 Prior Lake Calibration DRAFT To: Prior Lake-Spring Lake Watershed District Managers From: Greg Fransen and Greg Wilson, Barr Engineering Subject: Prior Lake Watershed Model Development and Calibration Date: November 23, 2015 Page: 37 P:\Mpls\23 MN\70\23701048 Prior Lake Flood Damage Reduct\WorkFiles\Tech Memos\Model_Development_TechMemo_v03.docx 4.0 Design Event Simulations 4.1 100-year Critical Event Selection Barr used the calibrated model to determine the critical 100-year event, i.e. the 100-year event that resulted in the highest lake elevation for the 2014 existing conditions. Barr evaluated rainfall events ranging from the 100-year, 10-day event through the 100-year, 60-day event. The 30-day event was selected as the critical event for Prior Lake. Rainfall depths and the resulting peak lake elevations for Spring Lake and Prior Lake are shown in Table 4-1. Table 4-1 Event Duration (days) Rainfall Depth (in) Spring Lake Peak Elevation Prior Lake Peak Elevation 10 10.3 914.2 906.3 30 14.6 914.5 907.1 45 17.6 914.7 907.1 60 20.5 914.7 907.0 4.2 30-day Events Barr used the calibrated model to evaluate the 30-day event for several return recurrences in addition to the 100-year event, including the 2-, 10-, 25-, 50-, and 500-year events. Table 4-2 shows the peak elevations of Spring Lake and Prior Lake for each of the 30-day events. Table 4-2 Event Return Recurrence Rainfall Depth (in) Spring Lake Peak Elevation (ft) Prior Lake Peak Elevation (ft) 2-year 8.31 912.4 903.6 10-year 10.8 913.3 904.8 25-year 12.3 913.7 905.6 50-year 13.5 914.1 906.3 100-year 14.6 914.5 907.1 500-year 17.1 915.3 908.4 Technical Memorandum To: Prior Lake-Spring Lake Watershed District Managers From: Greg Fransen and Greg Wilson, Barr Engineering Subject: Analysis of Flood Damage Reduction Measures for Prior Lake Date: November 25, 2015 Project:23701048.00 1.0 Introduction The purpose of this memorandum is to document the methodology used by Barr Engineering (Barr) to evaluate potential flood reduction measures for Prior Lake and the results of that analysis. Barr used a hydrologic and hydraulic model of the Prior Lake watershed calibrated for the 2014 Prior Lake flooding event to simulate the effects of the 100-year, 30-day Atlas 14 rainfall event on the peak water surface elevations of Prior Lake and Spring Lake. Barr, working with the Technical Advisory Committee (TAC) then identified potential flood reduction measures, modeled their simulated impacts on the peak elevations of Prior Lake and Spring Lake, and developed planning level cost estimates to implement the potential projects. 2.0 Analysis of the Prior Lake 2014 Flooding Event PLSLWD staff members collected continuous (15-minute interval) stage monitoring data at five sites during the 2014 monitoring season – two major tributaries and one minor tributary to Spring Lake, along with Spring Lake and Prior Lake (Figure 2-1). PLSLWD staff used rating curves to convert stage measurements to discharge at each site. In addition, the Shakopee Mdewakanton Sioux Community (SMSC) provided Arctic Lake outlet synoptic flow monitoring data which Barr used to generally determine whether the Arctic Lake portion of the watershed was contributing flow during the calibration time period. Most of the available monitoring data covered the time period from March 28 th through October, 2014, with some synoptic measurements prior to March 28, 2014. Table 2-1 summarizes drainage characteristics for each of the five PLSLWD monitoring stations. The combined drainage area of the Buck Lake and County Ditch 13 monitoring sites accounts for 76 percent of the Spring Lake watershed and 52 percent of the Prior Lake watershed. DRAFT ") ") ") ") ") Lo w e r P r i o r L a k e Sp r i n g L a k e Fi s h L a k e Su t t o n L a k e Up p e r P r i o r L a k e Ri c e L a k e Bu c k L a k e Sw a m p L a k e Pi k e L a k e Cr y s t a l L a k e Ca t e ' s o r Hi d d e n L a k e Li t t l e P r i o r L a k e Pr i o r L a k e O u t l e t Co u n t y D i t c h 13 -- S i t e 7 Bu c k L a k e O u t l e t - - S i t e 14 Sp r i n g L a k e O u t l e t - S i t e 21 Ma r s h a l l R o a d C r o s s i n g - - S i t e 19 Pr i o r L a k e Sa v a g e Sh a k o p e e !;N B a r r F o o t e r : A r c G I S 1 0 . 3 , 2 0 1 5 - 1 1 - 2 5 1 9 : 3 4 F i l e : I : \ P r o j e c t s \ 2 3 \ 7 0 \ 1 0 4 8 \ M a p s \ R e p o r t s \ T e c h _ M e m o _ J u l y 2 0 1 5 \ F i g 0 2 - 0 1 P L S L W D F l o w M o n i t o r i n g S t a t i o n s . m x d U s e r : g d f 4,500 0 4,500Feet Figure 2 -1 PLSLWD FLOW MON I TOR I NG STAT I ONS PCSWMM Modeling Technical Memo Prior Lake-Spring Lake Watershed District Scott County, Minnesota")Monitoring Station Subwatershed Boundary Public Water I nventory Basin Prior Lake-Spring Lake Watershed District Municipal Boundary 1,500 0 1,500Meters DRAFT To: Prior Lake-Spring Lake Watershed District Managers From: Greg Fransen and Greg Wilson, Barr Engineering Subject: Analysis of Flood Damage Reduction Measures for Prior Lake Date: November 25, 2015 Page: 3 P:\Mpls\23 MN\70\23701048 Prior Lake Flood Damage Reduct\WorkFiles\Tech Memos\FloodReductionScenario_TechMemo_v01.docx Table 2-1 Monitoring Site—Site # Watershed Area (acres)1 Percent of Spring Lake Watershed Area Percent of Prior Lake Watershed Area Marschall Road Crossing— Site 19 401 3% 2% Buck Lake Outlet—Site 14 4,036 32% 21% County Ditch 13—Site 7 5,526 44% 29% Spring Lake Outlet—Site 21 12,703 100% 66% Prior Lake Outlet 19,239 -- 100% 1 Watershed areas include potentially landlocked or non-contributing areas. Figure 2-2 shows the measured hydrograph data for the five watershed monitoring sites for the period between the end of April and July, 2014. More than seven inches of rainfall fell across most of the watershed between June 15th and the 20th, which contributed to the peak discharge rates observed at all five monitoring sites. While all three of the tributary stations experienced peak flow on June 19th, the flow monitoring results show that the tributary stations experienced varying levels of flashiness and relative magnitudes of peak discharge. All of the storm flow at Site 19 discharged within two days, while flow at Sites 7 and 14 returned to pre-storm levels within six days after peak discharge. For the Spring Lake outlet, peak discharge occurred on June 22nd – three days after the peak discharge rates occurred at the three tributary monitoring stations. Flow out of the Spring Lake outlet returned to pre-storm levels within 14 days of the peak discharge. The peak Prior Lake level and outlet discharge rate occurred on June 30 th and Prior Lake did not return to pre-storm levels for more than 40 days. It is important to note that flow out of the Prior Lake outlet did not begin until April 29, 2014 because the beginning lake elevation was 900.10 feet MSL on March 28th, which would have required approximately 3,460 acre-feet of inflow to raise the Prior Lake elevation to the control elevation of 902.5 feet MSL. In addition, lake level and outflow estimates from the Spring Lake outlet do not begin until April 29 th, when the estimated outflow rate already exceeded 50 cfs. DRAFT To: Prior Lake-Spring Lake Watershed District Managers From: Greg Fransen and Greg Wilson, Barr Engineering Subject: Analysis of Flood Damage Reduction Measures for Prior Lake Date: November 25, 2015 Page: 4 P:\Mpls\23 MN\70\23701048 Prior Lake Flood Damage Reduct\WorkFiles\Tech Memos\FloodReductionScenario_TechMemo_v01.docx Figure 2-2 Observed Flow Data for 2014 Figure 2-3 shows how the combined hydrograph of the three monitored tributaries compares to the flow discharging from the Spring Lake and Prior Lake outlets. This figure allows for a more direct comparison of the magnitude and timing of the Spring Lake inflow hydrographs to the resulting discharges from each of the lake outlets. The peak Spring Lake elevation occurred three days after the peak discharge rates occurred in the upper watershed, while Prior Lake did not reach its peak flood level until 11 days after the peak discharge occurred in the upper portion of the Spring Lake watershed. DRAFT To: Prior Lake-Spring Lake Watershed District Managers From: Greg Fransen and Greg Wilson, Barr Engineering Subject: Analysis of Flood Damage Reduction Measures for Prior Lake Date: November 25, 2015 Page: 5 P:\Mpls\23 MN\70\23701048 Prior Lake Flood Damage Reduct\WorkFiles\Tech Memos\FloodReductionScenario_TechMemo_v01.docx Figure 2-3 Table 2-2 summarizes the measured flow volume through September 10th, expressed both in acre-feet and as the yield (volume divided watershed area) in inches, as well as two expressions of the peak discharge rates at the five monitoring stations (with the last column normalized to watershed area). DRAFT To: Prior Lake-Spring Lake Watershed District Managers From: Greg Fransen and Greg Wilson, Barr Engineering Subject: Analysis of Flood Damage Reduction Measures for Prior Lake Date: November 25, 2015 Page: 6 P:\Mpls\23 MN\70\23701048 Prior Lake Flood Damage Reduct\WorkFiles\Tech Memos\FloodReductionScenario_TechMemo_v01.docx Table 2-2 Monitoring Site—Site # Measured Flow Volume—thru 9/10/14 (acre-feet) Watershed Yield (inches) Peak Discharge Rate (cfs) Area-Normalized Peak Discharge Rate (cfs/acre) Marschall Road Crossing—Site 19 468 13.5 62 0.155 Buck Lake Outlet—Site 14 3,732 10.8 71 0.018 County Ditch 13—Site 7 8,259 17.6 207 0.038 Combined Upper Watershed— Sites 7, 14 & 19 12,458 14.6 318 0.032 Spring Lake Outlet—Site 21 12,0211 11.4 222 0.018 Prior Lake Outlet 15,4852 9.7 64 0.003 1 Does not include unmonitored volume associated with the Spring Lake outlet discharge prior to April 29, 2014. 2 Includes water volume associated with lake level rise in advance of discharge from the Prior Lake outlet. The following conclusions can be drawn from the 2014 monitoring data: The upper Spring Lake watershed, which represents 52% of the Prior Lake watershed, contributed more than 80% of the flow volume that discharged from the Prior Lake outlet in 2014 The County Ditch 13 watershed, which represents 29% of the Prior Lake watershed, contributed more than 53% of the flow volume that discharged from the Prior Lake outlet in 2014 The Buck Lake watershed, which represents 21% of the Prior Lake watershed, contributed 24% of the flow volume that discharged from the Prior Lake outlet in 2014 The Marschall Road Crossing watershed, which represents 2% of the Prior Lake watershed, contributed 3% of the flow volume that discharged from the Prior Lake outlet in 2014 Watershed yield from the County Ditch 13 watershed was significantly higher (more than 30% higher) than any of the other watershed monitoring stations Watershed yield from the Buck Lake tributary was significantly lower than the other upper Spring Lake watershed tributaries, but higher than the areas draining directly to Prior Lake The available lake and wetland storage in the Buck Lake Outlet watershed results in less than half the normalized peak discharge rate computed for the County Ditch 13 watershed The normalized peak discharge rate for the Buck Lake tributary was approximately equal to the Spring Lake outlet, which was six times higher than the normalized peak discharge rate for the Prior Lake outlet in 2014 DRAFT To: Prior Lake-Spring Lake Watershed District Managers From: Greg Fransen and Greg Wilson, Barr Engineering Subject: Analysis of Flood Damage Reduction Measures for Prior Lake Date: November 25, 2015 Page: 7 P:\Mpls\23 MN\70\23701048 Prior Lake Flood Damage Reduct\WorkFiles\Tech Memos\FloodReductionScenario_TechMemo_v01.docx Barr’s analysis of the 2014 monitoring data indicated that enhanced upper watershed storage – particularly in the County Ditch 13 watershed – and better control of peak discharge from the Spring Lake outlet have the highest potential for reducing peak elevations in Prior Lake. 3.0 Flood Reduction Measures Barr modeled a number of potential flood-reduction measures. Each measure was evaluated separately (i.e. as if it was the only measure that was implemented) and then all of the potential flood-reduction measures were implemented together (i.e. as if they were all implemented). Those measures that were judged to be reasonable and to have a measurable impact on the peak flood elevation of Prior Lake are described in more detail below. 3.1 Upland Detention Storage Areas Working with the Technical Advisory Committee (TAC), Barr identified a number of potential upland detention storage areas. The sites selected for further evaluation included locations where modeling showed temporary ponding already occurred under existing conditions, locations where the existing topography allowed for significant increased storage by adding limited additional infrastructure, and other locations where local interest had previously been shown. The peak 100-year Prior Lake elevation determined by this study occurred approximately 14 days after the peak rainfall intensity of the 100-year, 30-day Atlas 14 rainfall event. To have a noticeable effect on the peak Prior Lake elevation, detention storage areas would need to detain water until after Prior Lake had reached its peak. Barr performed more-detailed modeling on nine sites that met this criterion. Barr modeled each area with a restrictive outlet that would decrease its discharge rate and assumed that berms would be constructed as needed to increase the area’s storage volume in order to achieve the required detention time. Table 3-1 summarizes the existing and proposed outlet characteristics for each of the upland detention storage areas. Appendix A shows planning-level cost estimates for each site. DRAFT To: Prior Lake-Spring Lake Watershed District Managers From: Greg Fransen and Greg Wilson, Barr Engineering Subject: Analysis of Flood Damage Reduction Measures for Prior Lake Date: November 25, 2015 Page: 8 P:\Mpls\23 MN\70\23701048 Prior Lake Flood Damage Reduct\WorkFiles\Tech Memos\FloodReductionScenario_TechMemo_v01.docx Table 3-1 Upland Detention Storage Summary Detention Site Name Existing Outlet Proposed Restrictive Outlet and Overflow Structure Proposed Overflow Elevation (NGVD29) Proposed Embankment Length (ft) S-BL-001 (Buck Lake) Open channel 2-ft diameter orifice 6 x 6 ft box 921.8 110 S-BL-020 Open channel 0.5-ft diameter orifice 6 x 6 ft box 959.0 165 S-LPL-048 (Little Prior Lake) 1.5-ft diameter storm sewer pipe with skimmer (gate open) 1.5-ft diameter storm sewer pipe with skimmer (gate closed) 925.2 N/A S-SPL-046 30-ft weir 2-ft diameter orifice 30-ft weir 922.0 650 S-SPL-054 Open channel 0.3-ft diameter orifice 6 x 6 ft box 934.5 430 S-SPL-078 4-ft diameter culvert 0.5-ft diameter orifice 6 x 6 ft box 943.5 219 S-SPL-080 2-ft diameter culvert (estimated) 0.5-ft diameter orifice 6 x 6 ft box 951.5 500 S-SPL-094 5-ft diameter culvert 2-ft diameter orifice 6 x 6 ft box 936.5 N/A S-SUL-001 (Sutton Lake) 4-ft diameter culvert 1-ft diameter orifice 6 x 6 ft box 945.3 N/A 3.2 Spring Lake Outlet Modification The existing Spring Lake Outlet structure is a concrete sill with a bottom width of 16 feet and invert elevation of 909.9 ft msl. In low flow conditions, a sand bar has been observed to form at a higher elevation within Spring Lake, near the structure (). In high flow conditions, it is likely that the sand bar is washed away and the sill becomes the lake elevation control. DRAFT To: Prior Lake-Spring Lake Watershed District Managers From: Greg Fransen and Greg Wilson, Barr Engineering Subject: Analysis of Flood Damage Reduction Measures for Prior Lake Date: November 25, 2015 Page: 9 P:\Mpls\23 MN\70\23701048 Prior Lake Flood Damage Reduct\WorkFiles\Tech Memos\FloodReductionScenario_TechMemo_v01.docx Figure 3-1 Spring Lake Outlet During Low Flow Conditions During extreme Prior Lake flooding events, temporary sandbagging at the Spring Lake outlet has been implemented to increase the available storage within Spring Lake. Barr evaluated a permanent embankment with a weir overflow and a 24-inch diameter outlet pipe that would maintain the Spring Lake normal water elevation of 909.9 during typical flows while allowing for additional storage during high flows. 3.3 Arctic Lake Diversion Arctic Lake lies to the north of Spring Lake and west of Upper Prior Lake. Because Arctic Lake has a relatively small watershed of approximately 560 acres and is only a short distance upstream of Prior Lake, increasing the available storage within the Arctic Lake without increasing its drainage area would only have a small impact on the Prior Lake peak elevation. Therefore, Barr modeled a 48-inch diameter storm sewer pipe system that would convey water from Spring Lake to Arctic Lake during high flow periods. DRAFT To: Prior Lake-Spring Lake Watershed District Managers From: Greg Fransen and Greg Wilson, Barr Engineering Subject: Analysis of Flood Damage Reduction Measures for Prior Lake Date: November 25, 2015 Page: 10 P:\Mpls\23 MN\70\23701048 Prior Lake Flood Damage Reduct\WorkFiles\Tech Memos\FloodReductionScenario_TechMemo_v01.docx FIGURE X shows the potential 5,400-foot route evaluated by Barr. The Arctic Lake diversion pipe would require raising of the elevation of Spring Lake using the Spring Lake Outlet modification as described above to create sufficient head to divert flow through the pipe. 3.4 Prior Lake Outlet Modification The Prior Lake outlet structure is limited to a maximum flow of approximately 65 cfs through its design and through legal agreements with downstream communities. A key factor in the 2014 flooding caused by Prior Lake was the occurrence of several consecutive rainstorms separated by intervals that were too short for Prior Lake to return to its normal water elevation before the next storm. Barr modeled a modified outlet rating curve with a peak discharge rate of 85 cfs to evaluate the impact of a modified outlet on the peak Prior Lake water elevation. To achieve the desired discharge rate, either the existing outlet pipe system would need to be replaced with a larger pipe or a second pipe parallel to the existing pipe would need to be constructed. Barr performed a planning level cost estimate for a second pipe system of 24-inch diameter pipe that would be constructed with a separate inlet at the lake and a gate valve that could be left closed during normal flow conditions and opened during flood flow conditions. The pipe system would be approximately 2,600 feet in length. 4.0 Results and Conclusions Barr evaluated each potential flood damage reduction measure by modeling it separately using the calibrated Prior Lake model and the 100-year, 30-day Atlas 14 rainfall event. Barr also evaluated all of the potential upland detention storage areas together, and all of the flood damage reduction measures together. The results of the simulations along with the associated planning level cost estimates are shown in Table 4-1. Figures showing the location of the detention areas and comparing the change in inundation extents from existing to proposed conditions are shown in Appendix A. DRAFT To: Prior Lake-Spring Lake Watershed District Managers From: Greg Fransen and Greg Wilson, Barr Engineering Subject: Analysis of Flood Damage Reduction Measures for Prior Lake Date: November 25, 2015 Page: 11 P:\Mpls\23 MN\70\23701048 Prior Lake Flood Damage Reduct\WorkFiles\Tech Memos\FloodReductionScenario_TechMemo_v01.docx Table 4-1 Flood Damage Reduction Measure Summary Name Type of Flood Reduction Measure Spring Lake Peak Elevation Change1 (ft) Prior Lake Peak Elevation Change1 (ft) Estimated Implementation Cost2,3,4 Arctic Lake Diversion Outlet Modification, Diversion, and Detention Storage + 0.8 - 1.0 $ 7,400,0005,6 Spring Lake Outlet Modification Outlet Modification and Detention Storage + 0.9 - 0.8 $ 4,140,0006 Prior Lake Outlet Modification Outlet Modification No Change - 0.8 $ 2,970,000 S-BL-001 (Buck Lake) Upland Detention Storage - 0.4 - 0.3 $ 630,000 S-BL-020 Upland Detention Storage - 0.1 - 0.1 $ 250,000 S-LPL-048 (Little Prior Lake) Gated Detention Storage (Existing Infrastructure) < - 0.1 < - 0.1 N/A S-SPL-046 Upland Detention Storage - 0.2 - 0.1 $ 840,000 S-SPL-054 Upland Detention Storage < - 0.1 < - 0.1 $ 380,000 S-SPL-078 Upland Detention Storage - 0.1 - 0.1 $ 300,000 S-SPL-080 Upland Detention Storage - 0.1 - 0.1 $ 260,000 S-SPL-094 Upland Detention Storage - 0.1 - 0.5 $ 130,000 S-SUL-001 (Sutton Lake) Upland Detention Storage < - 0.1 - 0.3 $ 70,000 All Nine Upland Detention Storage Sites Upland Detention Storage - 1.2 - 1.3 $ 2,860,000 All Reduction Measures N/A - 0.1 - 2.7 $ 17,850,000 1) Rounded to the nearest 0.1 feet. 2) Planning level cost estimate 3) 2015 Dollars 4) Rounded to nearest $10,000 5) Includes cost for Spring Lake Outlet Modification 6) Assumes full purchase value for any additional inundated area DRAFT To: Prior Lake-Spring Lake Watershed District Managers From: Greg Fransen and Greg Wilson, Barr Engineering Subject: Analysis of Flood Damage Reduction Measures for Prior Lake Date: November 25, 2015 Page: 12 P:\Mpls\23 MN\70\23701048 Prior Lake Flood Damage Reduct\WorkFiles\Tech Memos\FloodReductionScenario_TechMemo_v01.docx Appendix A Detention Storage Area Maps DRAFT S- A L - 00 1 (A r c t i c L a k e D i v e r s i o n ) Ex i s t i n g P e a k E l e v a t i o n : 91 1 .8 F t Pr o p o s e d P e a k E l e v a t i o n : 91 4 .9 F t Es t . S t o r a g e I nc r e a s e : 28 7 A c - F t Sp r i n g L a k e To w n s h i p Sp r i n g L a k e To w n s h i p Pr i o r La k e Grainwood Cir 15 4 t h S t Lords St Mystic Lake Dr G l y n w a t e r T r E a g l e C r e e k C r M y s t i c L a k e B l v d Nort hwood R d W i l d s P k w y W i l d s P k w y W i l d s P k w y W i l d s P k w y 16 5 t h S t I nguad o n a Beach Cir SW Grous e C i r Island View Rd E au Claire Cir Bo b c a t C r W a l n u t A v e B o b c a t T r l Cr ysta l C i r Dakota Tr W Stemmer Ridge R d Vi e w c r e s t Ci r Wo o d D u c k D r Lake Bear Cr W o o d T r Calmut Ave G r e e n H e i g h t s T r P a l o m i n o S t Visionar y Height s Cir NW W e s t b u r y A v e Pa rk Ave S c h r o e d e r Cir B r i o r w o o d L a Or i o n Ci r N W Calmut Ave Five Hawks Ave Wi l d s P k w y Black Oak Rd G r a i n w o o d T r F o x t a i l T r I s l a n d C i r P e r s h i n g S t Fr e m o n t C i r N a u t i c a C i r L ake Bluff Cir D e w i t t e A v e Je f f e r s C t Be l m o n t A v e K n o l l r i d g e C t N W Arctic Cir S i m p k i n s A v e Ea g l e Ci r W i l l o w B e a c h T r Ma r s h S t P a r k A v e S E Cj Cir SE F o x T a i l C t Wild Turkey C t B a s s w o o d C i r Val e Cir M o n r o e A v e B ay A ve Ke n t St Cen ter Rd S p r i n g C i r Fr e m o n t S t F o x T a i l R d Wil d Ho u rs e Pa s s L akeside Ave Hi gh lan d A v e B e r e n s C t Lake H a v e n C t Cr o s s St D r a k e A v e WagonBridgeCir Eagle s Rdg Ke s t r e l St R o o s e v e l t S t Pa r k P l Pl N W Dakot a T r N W E l m A v e 1 54 t h S t S i m p k i n s C i r My s t i c La k e Bl v d N W Eagle Creek Ave D a k o t a T r E Butte r n u t Cir B a n d e l P a s s S h o r e l i n e L a Colorado St SE H aw k R id g e Ct N W S a g e W a y B r o o k s i d e L a Jeffe r s P a s s Shady Cove Pt L i n d e n C i r Da k o t a T r S B a y K n o l l s D r T a h i n k a C t Ke n t S t 17 0 t h S t S W O r i o n R d F r e m o n t A v e T o d d R d W o o d l a n d C i r S a n t e e T r W o o d D u c k T r Eagle Cree k Ave W i l l o w L a R e g a l P a s s D a k o t a T r N F a i r w a y H e i g h t s T r B r e n t w o o d P a s s Big H o r n P a s s F l a n d r e a u T r Wi l d H o r s e C i r No r w o o d R d W o z a n i O c a n k u N W T a h i n k a P l S k y l i n e A v e Cen t e r R d S W 16 5 t h S t E Eau Claire Tr W i l l o w B e a c h S t R a s p b e r r y R i d g e R d 170t h S t W i l l o w w o o d R d Bo b c a t T r W i l d e r n e s s R d K n o l l r i d g e D r H a w k R i d g e R d N W S y c a m o r e T r S W T r a i l o f D r e a m s N W I s l a n d V i e w C i r S h o r e l i n e B l v d How a r d L ak e Rd ¨©81 4 5 6 7 83 4 5 6 7 83 4 5 6 7 21 4 5 6 7 23456721 4 5 6 7 82 4 5 6 7 12 13 SP L - 64 Crystal_Lake Cr y s t a l _ L a k e AL - 03 CL - 03 UP L - 20 UPL-12 UP L - 17 UP L - 18 UP L - 19 Lo w e r _ P r i o r _ L a k e UP L - 11UP L - 02 UP L - 25 Up p e r _ P r i o r _ L a k e UP L - 24 UP L - 22 UP L - 21 Sp r i n g _ L a k e Sp r i n g _ L a k e SP L - 55 UP L - 27 UP L - 26 UP L - 23 UPL-16UPL-04 UP L - 03 AL - 02 Ar c t i c _ L a k e AL - 04 PR I O R LA K E - S P R I N G LA K E SC O T T Page 5 Pa g e 1 Un n a m e d 70 0 0 8 5 0 0 P Un n a m e d 70 0 0 5 5 0 0 W Ho w a r d 70 0 0 7 3 0 0 P Sp r i n g 70 0 0 5 4 0 0 P Up p e r Pr i o r 70 0 0 7 2 00 P Un n a m e d 70 0 1 85 0 0 P Un n a m e d 70 0 1 67 0 0 W !;N B a r r F o o t e r : A r c G I S 1 0 . 3 , 2 0 1 5 - 0 9 - 1 1 0 9 : 1 0 F i l e : I : \ P r o j e c t s \ 2 3 \ 7 0 \ 1 0 4 8 \ M a p s \ R e p o r t s \ F D R S _ S u b m i t t a l _ S e p t 2 0 1 5 \ U p l a n d S t o r a g e R e v i e w M a p b o o k . m x d U s e r : m b s 2 500 0 5 0 0 1 , 0 0 0 Feet Figure A-1 S-AL-001 (Arctic Lake Diverson)UPLAND DETENT I ON STORAGE EVALUAT I ON Prior Lake Flood Damage Reduction Scott County, MinnesotaPotential Diversion Alignment Flow Direction Arrow Existing I nundation Extents Proposed I nundation Extents Public Water I nventory Basin Storage Area Watershed (Labeled with Watershed Name and Estimated Storage Volumes)SWMM Watershed Parcel Boundary Civil Township Watershed Management Districts Municipal Boundary 4 31 856279111210Prior LakeShakopeeSavage I ndex Map DRAFT S- B L - 00 1 Ex i s t i n g P e a k E l e v a t i o n : 92 0 .1 F t Pr o p o s e d M a x . E l e v a t i o n : 92 2 .5 F t Es t . S t o r a g e I nc r e a s e : 33 0 .5 A c - F t Sp r i n g L a k e To w n s h i p Fairlawn AveCountrySquiresCirCountrySquiresCir 18 0 t h S t E B e n c h m o r n D r V e r g u s A v e 18 9 t h S t E P a n d o r a B l v d 18 5 th S t E B u c k L a k e C i r ¨©81 SP L - 10 BL-22 BL-21 SP L - 12 SP L - 04 BL-37 Bu c k _ L a k e SP L - 07 BL-19 BL - 20 BL - 18 BL - 17 BL-36 SP L - 05 SP L - 08 SP L - 06 BL-39 SP L - 68 SP L - 69 SP L - 70 PR I O R LA K E - S P R I N G LA K E Pa g e 6 Pa g e 2 Pa g e 4 Pa g e 3 Bu c k 70 0 0 6 5 0 0 W Un n a m e d 70 0 2 06 0 0 W !;N B a r r F o o t e r : A r c G I S 1 0 . 3 , 2 0 1 5 - 1 1 - 2 5 1 8 : 0 0 F i l e : I : \ P r o j e c t s \ 2 3 \ 7 0 \ 1 0 4 8 \ M a p s \ R e p o r t s \ T e c h _ M e m o _ N o v _ 2 0 1 5 \ U p l a n d S t o r a g e A p p e n d i x . m x d U s e r : g d f 500 0 500Feet Figure A-2 S-BL-001 (Buck Lake)UPLAND DETENT I ON STORAGE EVALUAT I ON Prior Lake Flood Damage Reduction Scott County, MinnesotaFlow Direction Arrow Existing I nundation Extents Proposed I nundation Extents Public Water I nventory Basin Storage Area Watershed (Labeled with Watershed Name and Estimated Storage Volumes)SWMM Watershed Parcel Boundary Civil Township Watershed Management Districts Municipal Boundary 4 31 856279111210Prior LakeShakopeeSavage I ndex Map DRAFT S- B L - 00 1 Ex i s t i n g P e a k E l e v a t i o n : 92 0 .1 F t Pr o p o s e d M a x . E l e v a t i o n : 92 2 .5 F t Es t . S t o r a g e I nc r e a s e : 33 0 .5 A c - F t Sp r i n g L a k e To w n s h i p L i m e r i c k St 19 5 t h S t E Erin Ave Hill s of Claire A v e 18 9 t h S t E O e l k e D r L a n c e r C i r V e r g u s A v e 19 0 t h S t E SP L - 10 SP L - 13 SP L - 12 BL - 03 BL - 10 BL - 04 BL - 08 BL - 11 Bu c k _ L a k e BL - 12 BL - 09 BL-06 BL - 06 SP L - 07 BL - 02 SP L - 24 SP L - 08 SP L - 06 BL-41 PR I O R LA K E - S P R I N G LA K E Pa g e 2 Pa g e 4 Pa g e 7 Pa g e 3 Bu c k 70 0 0 6 5 0 0 W !;N B a r r F o o t e r : A r c G I S 1 0 . 3 , 2 0 1 5 - 1 1 - 2 5 1 8 : 0 0 F i l e : I : \ P r o j e c t s \ 2 3 \ 7 0 \ 1 0 4 8 \ M a p s \ R e p o r t s \ T e c h _ M e m o _ N o v _ 2 0 1 5 \ U p l a n d S t o r a g e A p p e n d i x . m x d U s e r : g d f 500 0 500Feet Figure A-3 S-BL-001 (Buck Lake)UPLAND DETENT I ON STORAGE EVALUAT I ON Prior Lake Flood Damage Reduction Scott County, MinnesotaFlow Direction Arrow Existing I nundation Extents Proposed I nundation Extents Public Water I nventory Basin Storage Area Watershed (Labeled with Watershed Name and Estimated Storage Volumes)SWMM Watershed Parcel Boundary Civil Township Watershed Management Districts Municipal Boundary 4 31 856279111210Prior LakeShakopeeSavage I ndex Map DRAFT S- B L - 02 0 Ex i s t i n g P e a k E l e v a t i o n : 95 9 .2 F t Pr o p o s e d M a x . E l e v a t i o n : 96 0 .3 F t Es t . S t o r a g e I nc r e a s e : 26 .3 A c - F t Sp r i n g L a k e To w n s h i p F a i r l a w n A v e R i c e R d 18 2 n d S t E Y o r k s h i r e A v e B u c k L a k e C i r C o u n t r y S q u i r e s C i r ¨©81 RL - 15 RL-16 RL-17 RL-17 BL - 22 BL - 21 BL - 23 BL - 34 SP L - 04 BL - 37 Bu c k _ L a k e RL - 20 BL - 19 BL - 18 BL - 17 BL - 36 BL - 24 BL - 39 PR I O R LA K E - S P R I N G LA K E Pa g e 2 Pa g e 4 Pa g e 3 Bu c k 70 0 0 6 5 0 0 W Un n a m e d 70 0 1 86 0 0 W Un n a m e d 70 0 1 9 2 00 W Un n a m e d 70 0 1 93 0 0 W !;N B a r r F o o t e r : A r c G I S 1 0 . 3 , 2 0 1 5 - 1 1 - 2 5 1 8 : 0 0 F i l e : I : \ P r o j e c t s \ 2 3 \ 7 0 \ 1 0 4 8 \ M a p s \ R e p o r t s \ T e c h _ M e m o _ N o v _ 2 0 1 5 \ U p l a n d S t o r a g e A p p e n d i x . m x d U s e r : g d f 500 0 500Feet Figure A-4 S-BL-020 UPLAND DETENT I ON STORAGE EVALUAT I ON Prior Lake Flood Damage Reduction Scott County, MinnesotaFlow Direction Arrow Existing I nundation Extents Proposed I nundation Extents Public Water I nventory Basin Storage Area Watershed (Labeled with Watershed Name and Estimated Storage Volumes)SWMM Watershed Parcel Boundary Civil Township Watershed Management Districts Municipal Boundary 4 31 856279111210Prior LakeShakopeeSavage I ndex Map DRAFT S- L P L - 04 8 Ex i s t i n g P e a k E l e v a t i o n : 92 3 F t Pr o p o s e d M a x . E l e v a t i o n : 92 4 .5 F t Es t . S t o r a g e I nc r e a s e : 24 .9 A c - F t Pr i o r La k e R i d g e m o n t A v e Cr o s s a n d r a S t S E Bounty St C r e d i t R i v e r R d S E Bro oks C i r 160 t h St S E Five Hawks Ave Ea g l e Cr e ek Av e Ba s s S t C at e s S t H idden Oaks Tr M a i n S t A r c a d i a A v e S u n f i s h T r E D a k ota S t Pl e a s a n t S t R e d O a k s R d Con d o n s S t C a m b r i d g e C i r Robinson Cir Calmut A v e L a k e v i e w C i r E r i e A v e Da k o t a S t M a i n A v e Five Hawks Ave SE Su m m e r S t R u t l e d g e S t Q u i n c y S t Cj Ci r S E E v a n s t o n A v e D u l u t h A v e B i r c h A v e Oa k S t W e s t A v e L a k e s i d e A v e Itasca Ave SE J o r d a n A v e A r c a d i a A v e Itasca Ave Wagon Bridge Cir F r a n k l i n T r A l b a n y A v e Ho p e S t Cred i t River Rd S E W a l k e r A v e Mi n n e s o t a S t Eag l e Cree k Ave Lemley Cir Ea g l e C r e e k A v e Eau Cla i r e T r M a n d a n A v e S E 16 0 t h S t S E We s t w o o d D r S E Mitchell C i r Cambr i d ge Ci r Co l o r a d o S t S E Ga t e w a y S t M e m o r i a l T r G r a i n w o o d C i r Co l o r a d o St S E W D a k o t a S t Cre d i t R i v e r R d S E F r a n k l i n C i r Pl e a s a n t S t Sain t P a u l Ave Pr i o r V i e w S t B l u f f H e i g h t s T r Kop Pkwy 4 5 6 7 21 4 5 6 7 44 4 5 6 7 21 4 5 6 7 21 13 13 13 UP L - 09 UP L - 10 UP L - 06 LP L - 03 LP L - 05 UP L - 17 Lo w e r _ P r i o r _ L a k e LP L - 06 UP L - 07 UP L - 11 UP L - 02 UP L - 05 UP L - 08 Up p e r _ P r i o r _ L a k e Up p e r _ P r i o r _ L a k e LP L - 02 UP L - 16 UP L - 04 UP L - 03 LP L - 04 UP L - 28 PR I O R LA K E - S P R I N G LA K E SC O T T Pa g e 5 Un n a m e d (W e s t P o r t i o n ) 70 0 1 8 1 0 1 W Lo w e r P r i o r 70 0 0 2 60 0 P Li t t l e Pr i o r 70 0 1 69 0 0 P Up p e r Pr i o r 70 0 0 7 2 00 P Un n a m e d 70 0 1 80 0 0 W !;N B a r r F o o t e r : A r c G I S 1 0 . 3 , 2 0 1 5 - 1 1 - 2 5 1 8 : 0 0 F i l e : I : \ P r o j e c t s \ 2 3 \ 7 0 \ 1 0 4 8 \ M a p s \ R e p o r t s \ T e c h _ M e m o _ N o v _ 2 0 1 5 \ U p l a n d S t o r a g e A p p e n d i x . m x d U s e r : g d f 500 0 500Feet Figure A-5 S-LPL-048 (Little Prior Lake)UPLAND DETENT I ON STORAGE EVALUAT I ON Prior Lake Flood Damage Reduction Scott County, MinnesotaFlow Direction Arrow Existing I nundation Extents Proposed I nundation Extents Public Water I nventory Basin Storage Area Watershed (Labeled with Watershed Name and Estimated Storage Volumes)SWMM Watershed Parcel Boundary Civil Township Watershed Management Districts Municipal Boundary 4 31 856279111210Prior LakeShakopeeSavage I ndex Map DRAFT S- S P L - 04 6 Ex i s t i n g P e a k E l e v a t i o n : 91 6 F t Pr o p o s e d M a x . E l e v a t i o n : 92 3 .9 F t Es t . S t o r a g e I nc r e a s e : 33 0 A c - F t Sp r i n g L a k e To w n s h i p La n g f o r d Bl v d Langford Ave 18 6 t h S t E L a n g f o r d A v e La n g f o r d B l v d Halifax La S p r i n g L a k e C i r C o u n t r y T r W L a n g f o r d A v e456717 28 2 13 SP L - 61 SP L - 09 SP L - 10 SP L - 16 SP L - 18 SPL-12 SP L - 45 SP L - 67 SP L - 63 SP L - 62 Sp r i n g _ L a k e SP L - 17 SP L - 20 SP L - 19 SP L - 11 SPL-69 PR I O R LA K E - S P R I N G LA K E Pa g e 6 Page 2 Sp r i n g 70 0 0 5 4 0 0 P !;N B a r r F o o t e r : A r c G I S 1 0 . 3 , 2 0 1 5 - 1 1 - 2 5 1 8 : 0 0 F i l e : I : \ P r o j e c t s \ 2 3 \ 7 0 \ 1 0 4 8 \ M a p s \ R e p o r t s \ T e c h _ M e m o _ N o v _ 2 0 1 5 \ U p l a n d S t o r a g e A p p e n d i x . m x d U s e r : g d f 500 0 500Feet Figure A-6 S-SPL-046 UPLAND DETENT I ON STORAGE EVALUAT I ON Prior Lake Flood Damage Reduction Scott County, MinnesotaFlow Direction Arrow Existing I nundation Extents Proposed I nundation Extents Public Water I nventory Basin Storage Area Watershed (Labeled with Watershed Name and Estimated Storage Volumes)SWMM Watershed Parcel Boundary Civil Township Watershed Management Districts Municipal Boundary 4 31 856279111210Prior LakeShakopeeSavage I ndex Map DRAFT S- S P L - 05 4 Ex i s t i n g P e a k E l e v a t i o n : 93 1 .3 F t Pr o p o s e d M a x . E l e v a t i o n : 93 2 .5 F t Es t . S t o r a g e I nc r e a s e : 29 .8 A c - F t Sp r i n g L a k e To w n s h i p W e l l s L a L a n g f o r d A v e B u t t e r f l y L a 13 SP L - 13 SP L - 15 SP L - 53 SP L - 21 BL - 09 SP L - 40 SP L - 39 SP L - 39 SP L - 67 SP L - 23 SP L - 22 SP L - 14 SP L - 44 SP L - 26 SP L - 27 SP L - 24 SP L - 25 SP L - 73 PR I O R LA K E - S P R I N G LA K E Pa g e 9 Pa g e 7 Pa g e 3 Pa g e 1 0 !;N B a r r F o o t e r : A r c G I S 1 0 . 3 , 2 0 1 5 - 1 1 - 2 5 1 8 : 0 0 F i l e : I : \ P r o j e c t s \ 2 3 \ 7 0 \ 1 0 4 8 \ M a p s \ R e p o r t s \ T e c h _ M e m o _ N o v _ 2 0 1 5 \ U p l a n d S t o r a g e A p p e n d i x . m x d U s e r : g d f 500 0 500Feet Figure A-7 S-SPL-054 UPLAND DETENT I ON STORAGE EVALUAT I ON Prior Lake Flood Damage Reduction Scott County, MinnesotaFlow Direction Arrow Existing I nundation Extents Proposed I nundation Extents Public Water I nventory Basin Storage Area Watershed (Labeled with Watershed Name and Estimated Storage Volumes)SWMM Watershed Parcel Boundary Civil Township Watershed Management Districts Municipal Boundary 4 31 856279111210Prior LakeShakopeeSavage I ndex Map DRAFT S- S P L - 08 0 Ex i s t i n g P e a k E l e v a t i o n : 94 8 .6 F t Pr o p o s e d M a x . E l e v a t i o n : 94 9 .5 F t Es t . S t o r a g e I nc r e a s e : 29 A c - F t S- S P L - 07 8 Ex i s t i n g P e a k E l e v a t i o n : 94 3 .8 F t Pr o p o s e d M a x . E l e v a t i o n : 94 5 .3 F t Es t . S t o r a g e I nc r e a s e : 69 .6 A c - F t Sp r i n g L a k e To w n s h i p B e n t l y C i r M e a d o w W o o d C t L a n g f o r d A v e Langford W a y 21 5 t h S t P a r k f i e l d A v e Vergus Ave 13 BL - 16 SP L - 36 SP L - 31 SP L - 34 SP L - 33 SP L - 32 SP L - 38 SP L - 30 SP L - 35 SP L - 74 PR I O R LA K E - S P R I N G LA K E SC O T T Pa g e 8 Cynthia 70005 2 00 P!;N B a r r F o o t e r : A r c G I S 1 0 . 3 , 2 0 1 5 - 1 1 - 2 5 1 8 : 0 0 F i l e : I : \ P r o j e c t s \ 2 3 \ 7 0 \ 1 0 4 8 \ M a p s \ R e p o r t s \ T e c h _ M e m o _ N o v _ 2 0 1 5 \ U p l a n d S t o r a g e A p p e n d i x . m x d U s e r : g d f 500 0 500Feet Figure A-8 S-SPL-078 & S-SPL-080 UPLAND DETENT I ON STORAGE EVALUAT I ON Prior Lake Flood Damage Reduction Scott County, MinnesotaFlow Direction Arrow Existing I nundation Extents Proposed I nundation Extents Public Water I nventory Basin Storage Area Watershed (Labeled with Watershed Name and Estimated Storage Volumes)SWMM Watershed Parcel Boundary Civil Township Watershed Management Districts Municipal Boundary 4 31 856279111210Prior LakeShakopeeSavage I ndex Map DRAFT S- S P L - 09 4 Ex i s t i n g P e a k E l e v a t i o n : 93 5 .8 F t Pr o p o s e d M a x . E l e v a t i o n : 93 4 .9 F t Es t . S t o r a g e I nc r e a s e : - 18 0 .5 A c - F t Sa n d C r e e k To w n s h i p Sp r i n g L a k e To w n s h i p N Sutton Lake Blvd X e o n A v e¨©79 4 5 6 7 10 SP L - 47 SP L - 49 SP L - 45 SP L - 45 SP L - 53 SU L - 02 SU L - 02 SP L - 41 SP L - 40 SP L - 42 Su t t o n _ L a k e SPL-44SPL-44 SP L - 43 SP L - 52 SU L - 10 PR I O R LA K E - S P R I N G LA K E Pa g e 9 Page 7 Pa g e 1 0 Pa g e 11 Su t t o n 70 0 0 9 4 0 0 P !;N B a r r F o o t e r : A r c G I S 1 0 . 3 , 2 0 1 5 - 1 1 - 2 5 1 8 : 0 0 F i l e : I : \ P r o j e c t s \ 2 3 \ 7 0 \ 1 0 4 8 \ M a p s \ R e p o r t s \ T e c h _ M e m o _ N o v _ 2 0 1 5 \ U p l a n d S t o r a g e A p p e n d i x . m x d U s e r : g d f 500 0 500Feet Figure A-9 S-SPL-094 UPLAND DETENT I ON STORAGE EVALUAT I ON Prior Lake Flood Damage Reduction Scott County, MinnesotaFlow Direction Arrow Existing I nundation Extents Proposed I nundation Extents Public Water I nventory Basin Storage Area Watershed (Labeled with Watershed Name and Estimated Storage Volumes)SWMM Watershed Parcel Boundary Civil Township Watershed Management Districts Municipal Boundary 4 31 856279111210Prior LakeShakopeeSavage I ndex Map DRAFT S- S P L - 09 4 Ex i s t i n g P e a k E l e v a t i o n : 93 5 .8 F t Pr o p o s e d M a x . E l e v a t i o n : 93 4 .9 F t Es t . S t o r a g e I nc r e a s e : - 18 0 .5 A c - F t Sa n d C r e e k To w n s h i p Sp r i n g L a k e To w n s h i p X e o n A v e 20 5 t h S t E X e o n A v e N S u t t o n L a k e B l v d ¨©79 4 5 6 7 10 SU L - 02 SP L - 41 SP L - 40 SP L - 42 SU L - 07 Su t t o n _ L a k e SP L - 38SPL-44 SP L - 28 SU L - 06 SPL-37 SU L - 10 PR I O R LA K E - S P R I N G LA K E Pa g e 9 Page 7 Pa g e 1 0 Pa g e 11 Pa g e 12 Su t t o n 70 0 0 9 4 0 0 P !;N B a r r F o o t e r : A r c G I S 1 0 . 3 , 2 0 1 5 - 1 1 - 2 5 1 8 : 0 0 F i l e : I : \ P r o j e c t s \ 2 3 \ 7 0 \ 1 0 4 8 \ M a p s \ R e p o r t s \ T e c h _ M e m o _ N o v _ 2 0 1 5 \ U p l a n d S t o r a g e A p p e n d i x . m x d U s e r : g d f 500 0 500Feet Figure A-10 S-SPL-094 UPLAND DETENT I ON STORAGE EVALUAT I ON Prior Lake Flood Damage Reduction Scott County, MinnesotaFlow Direction Arrow Existing I nundation Extents Proposed I nundation Extents Public Water I nventory Basin Storage Area Watershed (Labeled with Watershed Name and Estimated Storage Volumes)SWMM Watershed Parcel Boundary Civil Township Watershed Management Districts Municipal Boundary 4 31 856279111210Prior LakeShakopeeSavage I ndex Map DRAFT S- S P L - 09 4 Ex i s t i n g P e a k E l e v a t i o n : 93 5 .8 F t Pr o p o s e d M a x . E l e v a t i o n : 93 4 .9 F t Es t . S t o r a g e I nc r e a s e : - 18 0 .5 A c - F t S- S U L - 00 1 Ex i s t i n g P e a k E l e v a t i o n : 94 2 .5 F t Pr o p o s e d M a x . E l e v a t i o n : 94 2 .7 F t Es t . S t o r a g e I nc r e a s e : 10 1 .4 A c - F t Sa n d C r e e k To w n s h i p R e d w i n g A v e S Sut t o n L a k e B l v d N Sutto n L a k e B l v d 4 5 6 7 10 SU L - 02 SU L - 02 SU L - 04 SPL-42 SP L - 42 Su t t o n _ L a k e SU L - 03 SU L - 06 SU L - 08 SU L - 09 SU L - 10 SU L - 11 PR I O R LA K E - S P R I N G LA K E Pa g e 9 Pa g e 1 0 Pa g e 11 Pa g e 12 Su t t o n 70 0 0 9 4 0 0 P !;N B a r r F o o t e r : A r c G I S 1 0 . 3 , 2 0 1 5 - 1 1 - 2 5 1 8 : 0 0 F i l e : I : \ P r o j e c t s \ 2 3 \ 7 0 \ 1 0 4 8 \ M a p s \ R e p o r t s \ T e c h _ M e m o _ N o v _ 2 0 1 5 \ U p l a n d S t o r a g e A p p e n d i x . m x d U s e r : g d f 500 0 500Feet Figure A-11 S-SUL-001 (Sutton Lake)UPLAND DETENT I ON STORAGE EVALUAT I ON Prior Lake Flood Damage Reduction Scott County, MinnesotaFlow Direction Arrow Existing I nundation Extents Proposed I nundation Extents Public Water I nventory Basin Storage Area Watershed (Labeled with Watershed Name and Estimated Storage Volumes)SWMM Watershed Parcel Boundary Civil Township Watershed Management Districts Municipal Boundary 4 31 856279111210Prior LakeShakopeeSavage I ndex Map DRAFT S- S U L - 00 1 Ex i s t i n g P e a k E l e v a t i o n : 94 2 .5 F t Pr o p o s e d M a x . E l e v a t i o n : 94 2 .7 F t Es t . S t o r a g e I nc r e a s e : 10 1 .4 A c - F t Sa n d C r e e k To w n s h i p Sp r i n g L a k e To w n s h i p 205th St E N S u t t o n L a k e B l v d S Sut t o n L a k e B l v d Xeon Ave¨©79 4 5 6 7 10 SPL-40 SP L - 42 SUL-07 Su t t o n _ L a k e SPL-28 SU L - 06 PR I O R LA K E - S P R I N G LA K E SCOTT SC O T T Pa g e 1 0 Pa g e 11 Pa g e 12 Su t t o n 70 0 0 9 4 0 0 P !;N B a r r F o o t e r : A r c G I S 1 0 . 3 , 2 0 1 5 - 1 1 - 2 5 1 8 : 0 0 F i l e : I : \ P r o j e c t s \ 2 3 \ 7 0 \ 1 0 4 8 \ M a p s \ R e p o r t s \ T e c h _ M e m o _ N o v _ 2 0 1 5 \ U p l a n d S t o r a g e A p p e n d i x . m x d U s e r : g d f 500 0 500Feet Figure A-12 S-SUL-001 (Sutton Lake)UPLAND DETENT I ON STORAGE EVALUAT I ON Prior Lake Flood Damage Reduction Scott County, MinnesotaFlow Direction Arrow Existing I nundation Extents Proposed I nundation Extents Public Water I nventory Basin Storage Area Watershed (Labeled with Watershed Name and Estimated Storage Volumes)SWMM Watershed Parcel Boundary Civil Township Watershed Management Districts Municipal Boundary 4 31 856279111210Prior LakeShakopeeSavage I ndex Map DRAFT Appendix B—TP40/Atlas 14 Precipitation Comparison -MORE- MINNESOTA DEPARTMENT OF TRANSPORTATION Engineering Services Division Technical Memorandum No. 13-08-B-04 May 28, 2013 To:Electronic Distribution Recipients From:Jon M. Chiglo, P.E. Division Director, Engineering Services Subject: Use of Atlas 14 Volume 8 Precipitation Frequency Estimates Expiration This is a new Technical Memorandum and shall remain in effect until May 28, 2018 unless superseded or included in the MnDOT Drainage Manual prior to that date. Implementation The guidelines contained in this Memorandum are effective immediately for trunk highway projects where feasible. Use the Atlas 14 precipitation data for hydraulic design on all trunk highway projects let after June 30, 2014. Local road authorities are encouraged to adopt these or similar guidelines. Introduction The National Oceanic and Atmospheric Administration (NOAA) published new precipitation frequency estimates for the Midwestern States in Atlas 14 Volume 8. This information supersedes Technical Paper (TP)-40 published in1961 and NOAA Technical Memorandum NWS Hydro 35 published in 1977 that are the sources of precipitation frequency data and Intensity-Duration-Frequency (IDF) curves recommended in the Drainage Manual. Purpose This Technical Memorandum updates MnDOT design precipitation criteria to use the precipitation frequency data from NOAA Atlas 14. This replaces the design rainfall data in the Drainage Manual (2000). Guidelines Use Atlas 14 Precipitation Frequency Estimates when using rainfall-runoff models to compute hydrology for the design of hydraulic infrastructure. The data is obtained from NOAA’s Precipitation Frequency Data Server (PFDS) at http://hdsc.nws.noaa.gov/hdsc/pfds/based on the project location. For rainfall-runoff models that use the Natural Resources Conservation Service (NRCS) rainfall distributions, if feasible, use a rainfall distribution based on the Atlas 14 data. Use the NRCS Type II rainfall distribution for NRCS peak flow methodology or for other projects where developing a rainfall distribution is not feasible. Atlas 14 precipitation data should be used immediately for trunk highway projects using rainfall-runoff models provided its application does not jeopardize letting dates of projects already in the design phase. Use the Atlas 14 precipitation data for the hydraulic design of all trunk highway projects let after June 30, 2014. Where use of Atlas 14 is not feasible, evaluate the impacts of using Atlas 14 and document the justification for using the criteria from the Drainage Manual (2000). Notify the State Hydraulics Engineer about projects designed with rainfall-runoff models let after June 30, 2014 that are not designed with Atlas 14 precipitation data. Technical Memorandum No. 13-08-B-04 Use of Atlas 14 Volume 8 Precipitation Frequency Estimates May 28, 2013 Page 2 -END- Questions Any questions regarding the technical provisions of this memorandum can be addressed to the following: Andrea Hendrickson, State Hydraulics Engineer, at (651) 366-4466 Any questions regarding publication of this Technical Memorandum should be referred to the Design Standards Unit, DesignStandards.DOT@state.mn.us. A link to all active and historical Technical Memoranda can be found at http://techmemos.dot.state.mn.us/techmemo.aspx. To add, remove, or change your name on the Technical Memoranda mailing list, please visit the web page http://techmemos.dot.state.mn.us/subscribe.aspx re s o u r c e f u l . n a t u r a l l y . 1. T e c h n i c a l P a p e r 4 0 ( T P - 4 0 ) • ke y d o c u m e n t f o r h y d r o l o g i s t s a n d wa t e r p l a n n e r s , c r e a t e d b y N O A A i n 1 9 6 1 • de v e l o p e d u s i n g a v a i l a b l e r a i n f a l l i n f o r m a t i o n fr o m f a r f e w e r s t a t i o n s t h a n e x i s t t o d a y • in c l u d e d t h e “ d u s t - b o w l ” y e a r s o f t h e 1 9 3 0 ’ s • gi v e s r a i n f a l l d a t a f o r e n t i r e c o u n t r y – ra i n f a l l f r e q u e n c y o r r e c u r r e n c e i n t e r v a l s : 1- , 2 - , 5 - , 1 0 - , 2 5 - , 5 0 - y e a r , a n d 1 0 0 - y e a r e v e n t s – ra i n f a l l d u r a t i o n s : 30 - m , 1 - h , 2 - h , 3 - h , 6 - h , 1 2 - h , 2 4 - h , 2 - d , a n d 4 - d e v e n t s re s o u r c e f u l . n a t u r a l l y . 2. A t l a s 1 4 ( t h e n e w T P - 4 0 ) 11 s t a t e s ( d a r k b l u e ) p o o l e d f u n d s t o u p d a t e So u r c e : N O A A , p e e r t e c h n i c a l r e v i e w d o c u m e n t re s o u r c e f u l . n a t u r a l l y . 2. A t l a s 1 4 ( t h e n e w T P - 4 0 ) TP - 4 0 M i n n e s o t a d a i l y s t a t i o n s A t l a s 1 4 M i n n e s o t a d a i l y s t a t i o n s re s o u r c e f u l . n a t u r a l l y . 2. A t l a s 1 4 ( t h e n e w T P - 4 0 ) TP - 4 0 M i n n e s o t a s u b - d a i l y s t a t i o n s A t l a s 1 4 M i n n e s o t a s u b - d a i l y s t a t i o n s re s o u r c e f u l . n a t u r a l l y . 2. A t l a s 1 4 ( t h e n e w T P - 4 0 ) • av e r a g e r e c o r d l e n g t h n o w o v e r 5 0 y e a r s – m o r e t h a n d o u b l e t h e r e c o r d u s e d i n o r i g i n a l s t u d i e s – o l d e s t M i n n e s o t a d a t a s e t f r o m 1 8 3 6 ( F t . S n e l l i n g / Mi n n e a p o l i s S t . P a u l A i r p o r t ) • cu r r e n t l y u s e d b y s t a t e a n d f e d e r a l a g e n c i e s f o r mo d e l i n g a n d h y d r a u l i c d e s i g n – h a s b e c o m e a c c e p t e d r e s o u r c e f o r o t h e r l o c a l go v e r n m e n t a l u n i t s t h r o u g h o u t M i n n e s o t a re s o u r c e f u l . n a t u r a l l y . 3. T P - 4 0 / A t l a s 1 4 c o m p a r i s o n s • ma n y s i g n i f i c a n t i n c r e a s e s : e . g . , M i n n e a p o l i s , M N - 6. 0 t o 7 . 5 i n c h e s ( + 2 5 % ) • so m e d e c r e a s e s f o r c e r t a i n s t o r m s c e n t r a l Mi n n e s o t a • de g r e e o f c h a n g e t e n d s t o i n c r e a s e a s s t o r m fr e q u e n c y d e c r e a s e s MS P I n t e r n a t i o n a l A P Fr e q u e n c y % C h a n g e 20 5- 3 10 0 50 2 1 10 0 2 5 re s o u r c e f u l . n a t u r a l l y . St . C l o u d (F r e q u e n c y ) ( % C h a n g e ) 24 50 10 - 2 50 8 10 0 1 0 Mi n n e a p o l i s / S t . P a u l (F r e q u e n c y ) ( % C h a n g e ) 20 5- 3 10 0 50 2 1 10 0 2 5 re s o u r c e f u l . n a t u r a l l y . 3. A t l a s 1 4 / T P 4 0 co m p a r i s o n • Tw i n C i t i e s Me t r o a r e a – 1 0 0 - y e a r , 24 - h o u r ev e n t