Lake Minnetonka Direct Infiltration. (Catchments NA-LM, GB-LM, SLL-LM) Infiltration & Pollutant Reduction Study GB-LM SLL-LM. May 25, 2011 DRAFT

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1 Long Lake Independence Minnetrista Plymouth Orono Wayzata NA-LM Woodland GB-LM Minnetonka-Lower Lake Mound Spring Park Minnetonka Beach Deephaven Tonka Bay Shorewood Minnetonka SLL-LM Excelsior Lake Minnetonka Direct Infiltration (Catchments NA-LM, GB-LM, SLL-LM) Infiltration & Pollutant Study May 25, 2011 DRAFT Prepared by: Emmons & Olivier Resources, Inc. for the Minnehaha Creek Watershed District Minnehaha Creek Watershed District Lake Minnetonka Direct Infiltration North Arm (NA-LM) Location Map

2 LAKE MINNETONKA DIRECT INFILTRATION (CATCHMENTS NA-LM, GB-LM, SLL-LM) INFILTRATION AND POLLUTANT REDUCTION STUDY Watershed Background Lake Minnetonka is divided into 26 direct drainage subwatersheds encompassing approximately 23,330 acres. Three of these subwatersheds were selected by Minnehaha Creek Watershed District (MCWD or District) for evaluation in this study. The subwatersheds North Arm of Lake Minnetonka (NA-LM), Grays Bay of Lake Minnetonka (GB-LM) and South Lower Lake of Lake Minnetonka (SLL-LM) make up the 2,931 acre study area. Much of the study area is dominated by lake and wetland (Table 1). For further background, the 2007 Water Resources Management Plan presented a brief overview of issues and resources within each of these subwatersheds (Table 2). Table 1. Subwatershed Study Area Summary Area (acres) % Total Lake Wetland Upland Upland GB-LM NA-LM SLL-LM 1,547 1, Table 2. Additional Subwatershed Characteristics Flooding Contain High Quality Wetlands Dominant Infiltration Potential Contains Very High Aquifer Sensitivity Contain Key Conservation Areas GB-LM No Yes medium Yes No NA-LM No Yes medium Yes Yes SLL-LM No No medium Yes Yes NA-LM is on the north side of Lake Minnetonka and located entirely within the City of Orono (Figure 1). NA-LM is composed primarily of single family residential development largely consisting of older lake shore lots. Lakeview golf course is located near the northwestern portion of the watershed divide with only a small portion of the golf course flowing into NA-LM. Public property is limited to a few isolated outlots within residential areas and the two public accesses off of Park Lane and North Shore Drive. NA-LM does not meet its water quality goals established in the 2007 Water Resources Management Plan of 30 μg/l of total phosphorus (TP). The current in-lake phosphorus concentration is on average 31μg/L, and is predicted to increase by Minnehaha Creek Watershed District Page 1

3 GB-LM is on the far eastern side of Lake Minnetonka and located within the cities of Minnetonka and Wayzata (Figure 2). GB-LM is composed primarily of single-family residential development of various age with one newer development (Locust Hills) located just north of Gray s Bay. A small area of commercial/industrial land use (Cargill property) is near the eastern boundary of the watershed. Public property consists primarily of scatted outlots containing wetlands areas, the public access and parking in the southwest corner of Grays Bay and park property adjacent to the Gray s Bay Dam on the eastern side of Gray s Bay. GB-LM does not meet its water quality goals established in the 2007 Water Resources Management Plan of 20 μg/l. The current in-lake phosphorus concentration is on average 21μg/L, and is predicted to increase by SLL-LM is on the south side of Lake Minnetonka and located within the cities of Excelsior, Greenwood, and Tonka Bay (Figure 3). SLL-LM has a wide variety of land uses. On the north side of the watershed is Big Island which consist of park land/openspace and water-access lake shore lots. The southern portion of the watershed is downtown Excelsior which contains commercial, industrial, high density residential and single-family residential landuses along with park land located along the lake shore. The northeast side of the watershed is primarily singlefamily and lake lot residential. The northwest side of the watershed contains commercial, single family residential, and a small community park/beach. SLL-LM does meet its water quality goals established in the 2007 Water Resources Management Plan of 20 μg/l. Purpose The two main objectives of this study are to: 1. Identify BMP opportunities in Subwatersheds NA-LM, GB-LM and SLL-LM to reduce the annual volume and phosphorus load to Lake Minnetonka. 2. Prioritization of BMP opportunities for further consideration and potential implementation by MCWD and/or project partners. Minnehaha Creek Watershed District Page 2

4 Figure 1. Subwatershed NA-LM Area Minnehaha Creek Watershed District Page 3

5 Figure 2. Subwatershed GB-LM Area Minnehaha Creek Watershed District Page 4

6 Figure 3. Subwatershed SLL-LM Area Minnehaha Creek Watershed District Page 5

7 Watershed Loading Summary Pollutant loading in Subwatersheds NA-LM, GB-LM and SLL-LM was previously calculated by MCWD in the Hydrologic and Hydraulic and Pollutant Loading Study (HHPLS) in Table 3 shows loading values for the subwatershed from the HHPLS. While loadings are expected to increase from existing to 2020 landuse as a result of additional development, the 2020 HHPLS loadings reported are likely overestimated given that MCWD has since adopted a 1-inch infiltration standard that will mitigate development impacts. Numeric reduction goals have not been indentified for this project; however the direct drainage area bays of Lake Minnetonka associated with the project subwatersheds are not currently meeting water quality goals as established in the 2007 Water Resources Management Plan. s in volumes and TP will assist in meeting water quality goals. Table 3. Loading in NA-LM, GB-LM and SLL-LM Subwatersheds Scenario Description (lbs/yr) (ac-ft) Source Existing Landuse NA-LM HHPLS GB-LM HHPLS SLL-LM HHPLS NA-LM HHPLS 2020 Landuse GB-LM HHPLS SLL-LM HHPLS BMP IDENTIFICATION AND ASSESSMENT To identify BMP opportunities in NA-LM, GB-LM and SLL-LM; a base map utilizing land-use, parcels, land-cover, soils, topography and aerial photography was created. Hard copy storm sewer data was also obtained from several cities to assist in identifying drainage routes and potential BMP locations. After review of all available data, comprehensive field work was completed within each subwatershed to identify potential BMP opportunities. Upon completion of the field work, BMP locations and types were entered into a GIS database. Where applicable, catchments were delineated for the identified BMPs and water quality modeling was performed to assess potential runoff volume and water quality loading reductions. Detailed BMP locations maps, descriptions, and cost/benefit are contained the subsequent Subwatershed BMP Opportunities Sections. Minnehaha Creek Watershed District Page 6

8 To assess the pollutant removal potential of BMP opportunities identified, a PLOAD model was created to estimate loading from each catchment area. PLOAD uses event mean concentrations (EMCs) to calculate average annual phosphorus loading based on landuse. This model was used to calculate the annual loadings to each of the BMPs. This model was calibrated to the regional average runoff depth of 4.7 to be consistent with the loadings reported in the original HHPLS (Table 4). The effect of this is that the unit area loading rates for each landuse is decreased from uncalibrated PLOAD values. Table 4. Unit Area Loadings by Landuse Calibrated to Original HHPLS Landuse Uncalibrated Total Phosphorus Unit Area Loading (lb/ac/year) Calibrated Total Phosphorus Unit Area Loading (lb/ac/year) Commercial Industrial Single Family Residential Institutional It should be recognized that the use of the calibrated (lower) loading rates affect the BMP opportunity cost benefit analysis and that caution should be used when cross-comparing dollar per pound removals with other studies or reports that utilize uncalibrated values. The cost benefit analysis of this study is best utilized for the comparison and ranking BMP opportunities relative to one another. Runoff volume and phosphorus removal efficiency for each infiltration BMP was calculated based on the BMP storage volume and the runoff from the contributing catchment. A standard raingarden size was estimated at 10-ft wide by 25-ft long with an effective depth (including amended soil void space) of 1-ft over the surface area for a storage volume of 250 cubic feet. In areas where a larger raingarden is possible, the storage volume was increased to fit the site. The runoff volume of the contributing drainage areas was calculated based on dominant landuse and soil type (B) within each catchment using curve number methodology. Comparing the storage volume available and the runoff volume of different rainfall depths, a volume removal efficiency was assigned to each BMP based on Figure 4. The pollutant removal efficiency was set equal to the volume removal efficiency for infiltration BMPs. The pollutant removal for biofiltration was calculated utilizing this method for phosphorus removal but was assigned a volume reduction of zero. Other non-infiltrating BMPs (water quality ponds and wetland enhancements) were assigned phosphorus removals of 40% based on literature values from the Minnesota Stormwater Manual and assigned a volume reduction of zero. Table 5 summarizes the pollutant removal methodology for each type of BMP. Some values were manually adjusted to account for treatment already provided by existing features recommended to be enhanced. Minnehaha Creek Watershed District Page 7

9 Figure 4. Reproduced from Figure 1. Issue paper B of the Minnesota Stormwater Manual 2005 Table 5. Pollutant Removal Methodology Summary BMP Raingarden Wetland Enhancement Swale Enhancement Total Phosphorus Removal Infiltration BMP method Removal Infiltration BMP method Source MN Stormwater Manual 40% 0% MN Stormwater Manual Infiltration BMP method Infiltration BMP method Biofiltration Infiltration BMP method 0% Island Rain Garden Future infiltration Infiltration BMP method Infiltration BMP method Pervious Pavement Infiltration BMP method Water Quality Pond Planter Infiltration BMP method Infiltration BMP method Infiltration BMP method 40% 0% Infiltration BMP method Infiltration BMP method Raingarden Removals Raingarden Removals/No Raingarden Removals MN Stormwater Manual MN Stormwater Manual (high pond removal) Raingarden Removals Minnehaha Creek Watershed District Page 8

10 NA-LM BMP OPPORTUNITIES Raingardens Sixteen raingardens were identified in NA-LM. BMP catchments were approximated based on the 2-foot topographical mapping. In some cases clusters of raingardens were grouped in one catchment due to the inability to delineate micro-scale catchments from the available topographic information. Raingardens opportunities are generally scattered throughout the subwatershed although one concentrated grouping of raingardens was identified. The Loma Linda Avenue neighborhood contains 7 raingarden areas along with a wetland restoration opportunity. Targeted outreach to residents in this neighborhood could afford a cost effective neighborhood scale project. The island raingarden (NA-9) identified in this subwatershed is envisioned during road reconstruction as the entire cul-de-sac would need to be re-sloped to the center. The estimated cost is only for the raingarden and does not include pavement reconstruction. Wetland Enhancements Five wetland enhancements were identified in NA-LM. NA-2 is a wetland located on a public parcel located southeast of Elm Street and Park Lane. The site is a low wooded parcel that has a drainage channel flowing through it. This area could be enhanced via dredging/reshaping, outlet modification and vegetation enhancement. The upstream and downstream swales could also use stabilization and enhancement. NA-4 is a large wetland located along Tonkawa Road with NA-3 and NA-5 hydraulically connected and essentially operating as one basin. Increased stormwater storage and vegetation restoration of this City of Orono classified Manage 3 wetland could be considered. The city s Surface Water System Map indicates that this wetland basin (NA-P4) is landlocked, but also indicates a 12-inch culvert connection to the depressional area northwest of Elm Street and Tonkawa Road. An unidentified hydraulic connection, culvert or groundwater seepage must exist as evident by the significant discharge through the downstream Elm Street roadside channel within catchment NA-2. NA-14 is a wooded wetland area along Loma Linda Avenue that could be retrofitted with increased storage and enhanced vegetation. This could be considered in combination with the Loma Linda Avenue neighborhood raingarden projects. Swale Enhancements NA-18 is a ravine located at the north end of Cherry Place. The ravine is down cut and has erosion problems that are sources of sediment and phosphorus to Lake Minnetonka. Check dams with step down pools would provide erosion protection and create a small amount of depressional storage. NA-19 and NA-20 are existing grassed swales that could be enhanced with storage and native vegetation. Since these catchments discharge to NA-18, reduction in volumes here would help reduce erosion at NA-18. It would be advantageous to address NA-18, NA-19 and NA-20 as a combined project. These areas should be stabilized because they are sources of sediment and phosphorus loading and also to protect infrastructure and improve aesthetics. Minnehaha Creek Watershed District Page 9

11 Water Quality Treatment Pond One potential water quality treatment feature (NA-8) was identified on the Lakeview Golf Course near North Arm Drive. A catchment of the golf course discharges east via a culvert under North Arm Drive to the North Arm of Lake Minnetonka. A small water quality treatment pond or wetland treatment feature could be constructed at the existing low turfed area to provide treatment of the fairway and the green. Minnehaha Creek Watershed District Page 10

12 Table 6. North Arm Catchment Summary and Analysis Description ID Catchment Area (ac) (lbs/yr) (ac-ft/yr) Capital ($) Capital per year Maintenance :TP ($/lb) : ($/ac-ft) Raingarden NA % 55% $8,500 $283 $85 $368 $501 $627 Wetland Enhancement NA % 0% $21,262 $709 $213 $921 $949 None Wetland Enhancement NA % 0% $4,646 $155 $46 $201 $630 None Wetland Enhancement NA % 0% $71,223 $2,374 $712 $3,086 $3,601 None Wetland Enhancement NA % 0% $3,296 $0 $0 $0 $0 None Raingarden NA % 20% $2,875 $96 $29 $125 $685 $857 Raingarden NA % 68% $3,875 $129 $39 $168 $545 $682 Water Quality Pond NA % 0% $8,011 $267 $80 $347 $765 None Island Raingarden NA % 68% $3,375 $113 $34 $146 $854 $1,068 Raingarden NA % 86% $3,375 $113 $34 $146 $1,228 $1,537 Raingarden NA % 74% $3,375 $113 $34 $146 $940 $1,176 Raingarden NA % 10% $3,375 $113 $34 $146 $847 $1,060 Raingarden NA % 20% $2,875 $96 $29 $125 $906 $1,133 Wetland Enhancement NA % 0% $10,318 $344 $34 $378 $473 None Raingarden NA % 95% $18,287 $610 $61 $671 $489 $612 Raingarden NA % 80% $10,375 $346 $35 $380 $530 $663 Raingarden/Infiltration Basin NA % 68% $6,625 $221 $22 $243 $433 $542 Swale Enhancement NA % 20% $20,000 $667 $67 $733 $497 $1,244 Swale Enhancement NA % 68% $10,500 $350 $35 $385 $481 $602 Raingarden NA % 20% $4,750 $158 $16 $174 $625 $782 Raingarden NA % 68% $6,250 $208 $21 $229 $625 $782 Raingarden NA % 80% $9,500 $317 $32 $348 $571 $714 Swale Enhancement NA % 0% $5,625 $188 $19 $206 $516 None Raingarden NA % 90% $9,500 $317 $32 $348 $885 $1,107 Swale Enhancement NA % 0% $3,375 $113 $11 $124 $1,297 None Raingarden NA % 88% $2,875 $96 $10 $105 $1,002 $1,254 Totals $258,042 $8,492 $860 $9,352 $759 $1,576 * analysis assumes an operational life of 30 years for all practices. Maintenance costs are assumed to be 30% of the capital cost for all practices. Minnehaha Creek Watershed District Page 11

13 GB-LM BMP OPPORTUNITIES Raingardens Fifty eight raingarden locations were identified in GB-LM. BMP catchments were approximated based on the 2-foot topographical mapping. In some cases clusters of raingardens were grouped in one catchment due to the inability to delineate micro-scale catchments from the available topographic information. Raingardens opportunities are generally scattered throughout the subwatershed although three concentrated groupings of raingardens were identified. The three neighborhoods that could be considered for a neighborhood raingarden project include: Ice Circle Drive Neighborhood White Pine Drive / Morton Road Neighborhood Grays Bay Boulevard Neighborhood GB-21 identified along Crosby Road could potentially be a larger raingarden/infiltration basin serving a 4.8 acre catchment. This site would be a fairly easy retrofit with runoff already routed directed through this area. GB-24 and GB-25 are located on Cargill properties. GB-24 could be one large or multiple smaller raingardens serving building rooftop runoff. GB-25 could serve runoff from the adjacent parking lot areas before discharge to the ponding area to the south. Biofiltration Biofiltration features were differentiated from raingarden opportunities due to obvious close proximity to the groundwater table. The two biofiltration features identified along Bantas Point Lane would be similar to the above raingardens; however a drain tile outlet would be needed to ensure drawdown and filtration. Although some volume reduction would be achieved via evapotranspiration, this benefit was not estimated in the cost benefit analysis. Buffer GB-23 is located on Cargill properties. The current treatment pond/wetland has turf grass mowed to the waters edge. Incorporation of native vegetation around the pond would enhance filtration and the aesthetics. Swale Enhancements GB-22 and GB-33 are existing grassed swales along County Road 16 (McGinty Road). These swales could be enhanced with earthen check dams and native vegetation to promote infiltration and treatment of stormwater runoff. Minnehaha Creek Watershed District Page 12

14 Table 7. Grays Bay Catchment Summary and Analysis Description ID Catchment Area (ac) (lbs/yr) (ac-ft/yr) Capital ($) Capital per year Maintenance :TP ($/lb) : ($/ac-ft) Raingarden GB % 99% $6,625 $221 $66 $287 $3,067 $3,838 Biofiltration GB % 0% $5,250 $175 $53 $228 $2,686 None Biofiltration GB % 0% $5,250 $175 $53 $228 $1,088 None Raingarden GB % 55% $8,125 $271 $81 $352 $650 $814 Raingarden GB % 96% $2,875 $96 $29 $125 $1,981 $2,479 Raingarden GB % 74% $2,875 $96 $29 $125 $800 $1,001 Raingarden GB % 90% $3,875 $129 $39 $168 $1,854 $2,320 Raingarden GB % 94% $6,750 $225 $68 $293 $1,904 $2,382 Raingarden GB % 80% $4,375 $146 $44 $190 $1,300 $1,626 Raingarden GB % 68% $4,750 $158 $48 $206 $498 $624 Raingarden GB % 86% $3,375 $113 $34 $146 $1,199 $1,500 Raingarden GB % 55% $10,125 $337 $101 $439 $613 $767 Raingarden GB % 98% $3,375 $112 $34 $146 $2,780 $3,478 Raingarden GB % 96% $14,500 $483 $145 $628 $2,641 $3,304 Raingarden GB % 95% $13,875 $463 $139 $601 $1,752 $2,192 Raingarden GB % 74% $5,250 $175 $52 $227 - $794 Raingarden GB % 68% $3,375 $113 $34 $146 $843 $1,054 Raingarden GB % 92% $3,375 $113 $34 $146 $1,700 $2,127 Raingarden GB % 92% $3,375 $113 $34 $146 $1,744 $2,182 Raingarden GB % 92% $3,375 $113 $34 $146 $1,763 $2,206 Raingarden GB % 68% $12,750 $425 $128 $553 $499 $625 Swale Enhancement GB % 88% $8,500 $283 $85 $368 $860 $1,076 Buffer GB % 0% $5,510 $184 $55 $239 $908 None Raingarden GB % 55% $9,500 $317 $95 $412 $558 $425 Raingarden GB % 55% $8,625 $288 $86 $374 $727 $554 Raingarden GB % 90% $6,625 $221 $66 $287 $992 $1,242 Raingarden GB % 80% $4,750 $158 $48 $206 $498 $623 Minnehaha Creek Watershed District Page 13

15 Description ID Catchment Area (ac) (lbs/yr) (ac-ft/yr) Capital ($) Capital per year Maintenance :TP ($/lb) : ($/ac-ft) Raingarden GB % 98% $5,250 $175 $52 $227 $2,176 $2,722 Raingarden GB % 97% $2,875 $96 $29 $125 $2,377 $2,975 Raingarden GB % 96% $2,875 $96 $29 $125 $2,089 $2,614 Raingarden GB % 95% $2,875 $96 $29 $125 $1,815 $2,271 Raingarden GB % 88% $4,750 $158 $47 $206 $925 $1,157 Swale Enhancement GB % 88% $4,750 $158 $48 $206 $1,011 $1,265 Raingarden GB % 84% $9,125 $304 $91 $395 $1,063 $1,331 Raingarden GB % 95% $9,125 $304 $91 $395 $2,041 $2,554 Raingarden GB % 94% $2,875 $96 $29 $125 $1,649 $2,064 Raingarden GB % 20% $3,375 $113 $34 $146 $921 $1,152 Raingarden GB % 86% $3,375 $112 $34 $146 $1,266 $1,583 Raingarden GB % 94% $3,375 $113 $34 $146 $2,085 $2,609 Raingarden GB % 68% $4,750 $158 $48 $206 $499 $624 Raingarden GB % 94% $6,750 $225 $68 $293 $1,999 $2,501 Raingarden GB % 55% $2,875 $96 $29 $125 $675 $844 Raingarden GB % 84% $2,875 $96 $29 $125 $1,003 $1,255 Raingarden GB % 86% $3,375 $113 $34 $146 $1,298 $1,624 Raingarden GB % 99% $3,375 $113 $34 $146 $4,916 $6,151 Raingarden GB % 80% $6,750 $225 $68 $293 $1,033 $1,293 Raingarden GB % 88% $3,375 $113 $34 $146 $1,394 $1,745 Biofiltration GB % 0% $11,500 $383 $115 $498 $1,674 None Raingarden GB % 40% $2,875 $96 $29 $125 $570 $714 Raingarden GB % 98% $2,875 $96 $29 $125 $2,852 $3,569 Totals $ 277,010 $ 9,234 $2,770 $12,004 $1,034 $1,299 Minnehaha Creek Watershed District Page 14

16 SLL-LM BMP OPPORTUNITIES Raingardens Seventy five potential raingarden locations were identified in SLL-LM. BMP catchments were approximated based on the 2-foot topographical mapping. In some cases clusters of raingardens were grouped in one catchment due to the inability to delineate micro-scale catchments from the available topographic information. Raingardens opportunities are generally scattered throughout the watershed although three concentrated groupings of raingardens were identified. The three neighborhoods that could be considered for a neighborhood raingarden project include: Lodge Lane / Woods Court Neighborhood Tonka Bay Road / Lakeview Avenue Neighborhood Linwood Road / Bay view Court Neighborhood SLL-44 includes 3 larger landscaped (primary rock) islands in an existing parking lot serving the conference center across the street. These islands could be retrofitted to provide treatment for portions of the parking area. This project could potentially be combined with the biofiltration feature SLL-43 in the northeast corner of the property as well as erosion issues identified near the south end of the parking lot. SLL-36 on school property is an existing infiltration basin that has poor vegetation cover. This feature could easily be improved with native vegetation cover. Biofiltration Seven biofiltration areas were identified. These features would include a drain tile outlet to ensure drawdown and filtration. Although some volume reduction would be achieved via evapotranspiration, this benefit was not estimated in the cost benefit analysis. SLL-41, SLL-42, SLL-43 are all retrofits to existing features. Increasing storage by excavation and/or outlet modifications along with vegetation enhancement would provide added treatment at these locations. The remaining biofiltration features are new and would need to be retrofitted into the landscape. Tree Trenches Tree trenches were identified in downtown Excelsior. Water Street has storefronts on both sides with sidewalk and essentially no green space. Runoff from this highly impervious area discharges north along Water Street via storm sewer and eventually discharges directly into Lake Minnetonka. Tree trenches provide below ground storage and infiltration and also allow for healthy trees without elimination of the pedestrian hardscape surface. Tree trenches could be retrofitted along this entire corridor to provide treatment for stormwater runoff from the roadway corridor. NA-28 includes tree trenches for Water Street and 3 rd Street. NA-24 includes two tree trench areas near the park located off of Lake Street. Minnehaha Creek Watershed District Page 15

17 Stormwater Planters Stormwater planters were identified within SLL-35 as potential BMPs to treat stormwater runoff from the TCF Bank site located along Water Street. Stormwater planters include structural concrete to define the perimeter of and create the elevation change necessary to create stormwater runoff storage and a planting/filtration bed. Curb cuts or slots are used to direct runoff into the stormwater planter. These work well in tight places where a typical raingarden can not be graded into the landscape. Pervious Pavement SLL-29 and SLL-33 are currently large public parking lots that serve downtown Excelsior. These lots are in fair to poor condition and will likely need repaving in the future. Because parking appears to be at a high premium, removing parking space to implement surface features may be difficult. Both of these parking areas seem well suited for pervious pavement and could significantly reduce runoff and pollutants to Lake Minnetonka. Other alternatives that could be considered would be construction of parking ramps and incorporation of green space or a green rooftop on a new parking ramp. Underground infiltration may also be feasible, but groundwater table and soil condition may preclude this option. SLL-25 is a short stretch of pervious pavement that could also be incorporated on Courtland Street. This section of street is in poor condition and there are limited other opportunities for BMPs along this roadway. The cost estimate assumes that the lower ¼ of the street would be pervious pavement and the upper ¾ would be standard pavement. This site could potentially be a demonstration project. SLL-54 is located at the Old Log Theater. The existing pavement is in poor condition and there are significant opportunities for infiltration BMPs. Half of the parking lot was assumed to be pervious pavement with the other half being driving lanes constructed out of standard pavement. Several raingardens could also be retrofitted into this site. SLL-21 is currently used as a boat storage area and is another site where opportunities are limited and pervious pavement may be a good option. Swale Enhancement Four swale enhancements were identified in the subwatershed. These sites would utilize existing swales and improve storage and vegetation to provide additional volume and water quality benefit. SLL-30 already has significant storage and volume retention; however vegetation enhancement would improve esthetics and promote infiltration along this trail corridor. Water Quality Treatment Pond Two potential water quality treatment features were identified. SLL-23 and SLL-46 are both low wet areas currently receiving storm water runoff. These could be constructed as either traditional NURP Ponds or wetland treatment basins. Additional storage creation via excavation and outlet modifications would be incorporated to provide addition treatment volume. Minnehaha Creek Watershed District Page 16

18 Table 8. South Lower Lake Catchment Summary and Analysis Description ID Catchment Area (ac) (lbs/yr) (ac-ft/yr) Capital ($) Capital per year Maintenance :TP ($/lb) : ($/ac-ft) Raingarden SLL % 92% $6,750 $225 $68 $293 $1,818 $2,274 Raingarden SLL % 94% $3,375 $112 $34 $146 $2,085 $2,609 Biofiltration SLL % 0% $7,625 $254 $76 $330 $1,405 None Raingarden SLL % 90% $4,750 $158 $48 $206 $1,103 $1,380 Raingarden SLL % 95% $5,750 $192 $58 $249 $1,794 $2,245 Raingarden SLL % 84% $6,625 $221 $66 $287 $642 $803 Biofiltration SLL % 84% $3,375 $113 $34 $146 $1,165 $1,458 Wetland Enhancement SLL % 0% $26,959 $899 $270 $1,168 $1,391 None Raingarden SLL % 74% $7,125 $238 $71 $309 $0 None Raingarden SLL % 96% $8,625 $288 $86 $374 $2,120 $2,652 Raingarden SLL % 90% $3,375 $112 $34 $146 $1,583 $1,981 Raingarden SLL % 10% $2,875 $96 $29 $125 $1,642 $1,251 Raingarden SLL % 88% $3,375 $112 $11 $124 $1,251 $1,565 Swale Enhancement SLL % 90% $10,375 $346 $35 $380 $824 $1,031 Raingarden SLL % 68% $11,375 $379 $38 $417 $753 $574 Raingarden SLL % 84% $2,875 $96 $10 $105 $1,510 $1,150 Raingarden SLL % 40% $5,750 $192 $19 $211 $540 $675 Raingarden SLL % 95% $8,625 $288 $29 $316 $1,583 $1,980 Raingarden SLL % 68% $2,875 $96 $10 $105 $528 $660 Pervious Pavement SLL % 80% $29,311 $977 $98 $1,075 $1,633 $1,244 Raingarden SLL % 99% $2,875 $96 $10 $105 $4,262 $5,332 Water Quality Pond SLL % 0% $10,053 $335 $34 $369 $0 None Tree Trench SLL % 74% $25,000 $833 $83 $917 $1,353 $1,693 Pervious Pavement SLL % 98% $29,792 $993 $99 $1,092 $3,132 $3,919 Biofiltration SLL % 0% $6,875 $229 $23 $252 $1,399 None Future Infiltration SLL % 86% $54,000 $1,800 $180 $1,980 $1,988 $1,514 Tree Trench SLL % 84% $395,000 $13,167 $1,317 $14,483 $2,525 $1,923 Pervious Pavement SLL % 86% $643,114 $21,437 $2,144 $23,581 $8,205 $6,249 Swale Enhancement SLL % 20% $3,000 $100 $10 $110 $677 $847 Biofiltration SLL % 0% $14,199 $473 $47 $521 $703 None Minnehaha Creek Watershed District Page 17

19 Description ID Catchment Area (ac) (lbs/yr) (ac-ft/yr) Capital ($) Capital per year Maintenance :TP ($/lb) : ($/ac-ft) Biofiltration SLL % 0% $12,250 $408 $41 $449 $565 None Pervious Pavement SLL % 86% $722,672 $24,089 $2,409 $26,498 $11,796 $8,984 Raingarden SLL % 55% $3,375 $113 $11 $124 $805 $613 Planter SLL % 86% $15,100 $503 $50 $554 $3,288 $2,504 Raingarden SLL % 10% $2,500 $83 $8 $92 $744 $567 Raingarden SLL % 55% $6,375 $213 $21 $234 $547 $684 Raingarden SLL % 84% $5,750 $192 $19 $211 $665 $833 Raingarden SLL % 74% $6,250 $208 $21 $229 $988 $753 Raingarden SLL % 55% $5,250 $175 $18 $193 $439 $550 Raingarden SLL % 55% $3,375 $113 $11 $124 $515 $644 Biofiltration SLL % 0% $11,000 $367 $37 $403 $698 None Biofiltration SLL % 0% $10,000 $333 $33 $367 $559 None Island Raingarden SLL % 86% $15,750 $525 $53 $578 $2,190 $1,668 Water Quality Pond SLL % 0% $13,480 $449 $45 $494 $751 None Raingarden SLL % 55% $5,250 $175 $18 $193 $619 $471 Raingarden SLL % 20% $5,375 $179 $18 $197 $2,827 $2,153 Raingarden SLL % 80% $5,375 $179 $18 $197 $2,340 $1,782 Swale Enhancement SLL % 84% $6,625 $221 $22 $243 $589 $736 Raingarden SLL % 80% $5,750 $192 $19 $211 $0 $642 Swale Enhancement SLL % 0% $4,750 $158 $16 $174 $425 None Raingarden SLL % 86% $160,875 $5,363 $536 $5,899 $3,866 $2,945 Raingarden SLL % 88% $3,875 $129 $13 $142 $1,275 $1,596 Raingarden SLL % 55% $2,875 $96 $10 $105 $460 $575 Raingarden SLL % 84% $4,750 $158 $16 $174 $700 $876 Raingarden SLL % 20% $3,875 $129 $13 $142 $726 $909 Raingarden SLL % 84% $3,875 $129 $13 $142 $1,111 $1,390 Raingarden SLL % 55% $3,375 $113 $11 $124 $555 $694 Raingarden SLL % 94% $8,625 $288 $29 $316 $1,388 $1,736 Raingarden SLL % 92% $7,625 $254 $25 $280 $1,064 $1,331 None? SLL % 0% $0 $0 $0 $0 $0 None Raingarden SLL % 40% $3,375 $113 $11 $124 $583 $730 Minnehaha Creek Watershed District Page 18

20 Description ID Catchment Area (ac) (lbs/yr) (ac-ft/yr) Capital ($) Capital per year Maintenance :TP ($/lb) : ($/ac-ft) Raingarden SLL % 68% $3,375 $113 $11 $124 $698 $874 Raingarden SLL % 97% $10,125 $338 $34 $371 $2,371 $2,967 Raingarden SLL % 68% $8,750 $292 $29 $321 $893 $1,118 Raingarden SLL % 92% $13,500 $450 $45 $495 $1,414 $1,769 Raingarden SLL % 98% $2,875 $96 $10 $105 $2,677 $3,350 Raingarden SLL % 90% $5,750 $192 $19 $211 $1,188 $1,486 Raingarden SLL % 68% $3,375 $113 $11 $124 $651 $814 Raingarden SLL % 94% $9,000 $300 $30 $330 $1,154 $1,444 Raingarden SLL % 40% $2,875 $96 $10 $105 $519 $650 Raingarden SLL % 68% $9,000 $300 $30 $330 $420 $526 Raingarden SLL % 98% $4,375 $146 $15 $160 $3,128 $3,914 Raingarden SLL % 88% $5,250 $175 $18 $193 $980 $1,226 Raingarden SLL % 95% $15,375 $513 $51 $564 $1,738 $2,175 Raingarden SLL % 20% $4,375 $146 $15 $160 $594 $744 Raingarden SLL % 68% $6,250 $208 $21 $229 $664 $830 Totals $2,519, $2,706 $3,023 Minnehaha Creek Watershed District Page 19

21 RECOMMENDATIONS The following recommendations are broken down by subwatershed and follow from the preceding cost benefit analysis. North Arm of Lake Minnetonka (NA-LM) 1) Park Lane/Ave Neighborhood Improvements (NA-1, 2 & 25). A series of neighborhood improvements including 3 swale stabilization and enhancement projects as well as a smaller scale wetland restoration and detention project are suggested downstream of Catchment NA-3. 2) Loma Linda Ave Neighborhood Improvements (NA-10 through NA-17). An infiltration basin (NA-15), wetland restoration (NA-14) and 6 raingardens are suggested in this residential neighborhood. 3) Cherry Place Ravine & Upstream Retention (NA-18 through NA-20). Swale improvements along CR 151 are recommended to provide infiltration retention and detention of runoff before discharge to the ravine north of Cherry Place. The ravine itself is recommended to be stabilized via a series of stair-stepped pools for further runoff retention and infiltration. 4) Shoreland Bluff Erosion. Consider further investigation of shoreland bluff erosion areas as reported by City of Orono. Grays Bay of Lake Minnetonka (GB-LM) 1) Cargill Campus Improvements (GB-22 through GB-25). Three larger raingardens serving parking and building rooftop areas have been identified for the Cargill site north of CR 16. Swale enhancement opportunities are also suggested to intercept and infiltrate runoff from CR 16 before discharge to the existing wetland basin. While limited with respect to volume control benefit, the existing wetland basin could be enhanced via native buffer (land cover is currently turfed to waters edge). Finally, MCWD may want to coordinate with Cargill to further investigate the campus south of CR16. 2) Crosby Road Infiltration Basin (GB-21). The majority of Crosby Road south of CR 16 discharges west as overland flow via an existing catchbasin into rear yard areas. While this drainage ultimately flows to a water quality pond, there appears to be sufficient opportunity and relief to intercept and infiltrate runoff higher in the landscape immediately downstream of the stormsewer outfall. 3) Ice Circle Neighborhood Raingardens (GB-31 through GB-36). One swale enhancement and 8 raingardens are suggested in this residential neighborhood. 4) Grays Bay Boulevard Neighborhood Raingardens (GB-13 through GB-17). A total of 10 raingardens are suggested in this residential neighborhood. 5) White Pine Dr. / Morton Rd. Neighborhood Raingardens (GB-40 through GB-50). A total of 12 raingardens are suggested in this residential neighborhood. Minnehaha Creek Watershed District Page 20

22 South Lower Lake of Lake Minnetonka (SLL-LM) 1) Excelsior Public Parking, Roadway and Blvd. Improvements (SLL-24 through SLL- 33). Several opportunities warrant further discussion with the City of Excelsior to explore collaboration with future public improvements. These opportunities include boulevard tree trenches along Lake St. (SLL-24), Water St. and 3 rd St. (SLL-28), pervious surface roadway (SLL-25), pervious public parking (SLL-29 & 33), and retrofits to existing depressional areas (SLL-30, 31 & 32). Further coordination with Excelsior is warranted to explore additional retrofit opportunities that may exist in concert with roadway improvement projects. 2) Excelsior Land Conservation Opportunity (SLL-27). An existing vacant parcel southwest of Water and Lake Street may warrant consideration for regional treatment of downtown drainage. 3) Bayview Event Center Parking Lot Improvements (SLL-43 & SLL-44). It is evident by signage and physical condition of this lot between Excelsior Blvd. and Lake St. that the northeast corner of the lot experiences periodic flooding. Opportunity exists to address onsite erosion, retrofit existing landscape islands and to enhance the existing low point to increase storage and retention or biofiltration of runoff. 4) Wekota Park Improvements (SLL-5 through SLL-8). A larger raingarden and two smaller raingardens (SLL-5 & 6) are recommended to serve the north parking lot and one smaller biofiltration (SLL-7) is suggested to serve the south parking lot. Siting of BMPs affords educational opportunity. Although less cost-beneficial in terms of runoff volume reduction, MCWD may want to consider restoration of the onsite wetland (SLL-8) to realize increased nutrient load reduction and additional environmental education opportunities. 5) Tonka Bay Road Improvements (SLL-15 through SLL 21). Several BMP opportunities exist along Tonka Bay Road including 1 larger swale enhancement (SLL- 15), a large raingarden (SLL-16), 6 residential raingardens (SLL-17 through SLL-20) and pervious parking (SLL-21). Additional residential raingarden opportunities exist in the neighborhoods in and around Tonka Bay Road. 6) Old Log Theatre Site Improvements (SLL-54). Pervious pavement, 5 raingardens and educational opportunities fit well with this frequented historic site. 7) Lodge Lane Neighborhood Raingardens (SLL-58 through SLL-67). Over a dozen curb-cut raingarden opportunities exist within this residential neighborhood. 8) Linwood Road Neighborhood Raingardens (SLL-74 through SLL-79). 10 or more curb-cut raingarden opportunities exist within this residential neighborhood. Several of these identified locations could be significantly larger than a standard 10-ft by 25-ft raingarden. 9) Sunrise Avenue Water Quality Pond (SLL-23). An easy retrofit via shallow excavation downstream of an existing curb-cut that exists within the Sunrise Ave. / Lakeview Ave. loop. Minnehaha Creek Watershed District Page 21