Preserve Stormwater Management 080413 Preliminary STORMWATER MANAGEMENT PLAN AND CALCULATIONS FOR The Preserve at Rice Lake Chanhassen, MN August 4, 2013 Prepared By: Kallio Engineering Inc. 10775 Poppitz Lane Chaska, MN 55318 612-418-6828 I hereby certify that this plan, report, or specification was prepared by me or under my direct supervision and that I am a duly licensed engineer under the laws of the State of Minnesota. ________________________ License No. 26909 Curtiss J. Kallio
STORMWATER MANAGEMENT PLAN AND CALCULATIONS Introduction This Preliminary Stormwater Management Plan addresses how this proposed project will meet the surface water management requirements of the City of Chanhassen as discussed below. This report is considered preliminary because changes ar e anticipated through city reviews. This site is very unique due to the topography a nd natural areas the developer is trying to preserve. This topography makes it difficult to collect and treat stormwater using only conventional ponding methods, and some alternative treatment methods w ill be discussed. Through city reviews, we are confident that effective stormwater treatment meeting city requirements will be achieved. Requirements The City of Chanhassen Surface Water Management requirements are summarized as follows: 1. Peak discharge rates: No increase in peak discharge rate may result from the proposed project for the 1- or 2-year storm, the 10-year storm and the 100-year storm event. Variances may be allowed if computations can be pr ovided which demonstrate no adverse downstream effects will result from the proposed system. 2. Water Quality Standard: Water quality treatment consistent with NURP criteria (90 percent removal of TSS and 60 percent removal of TP for a standard NURP particle size distribution). 3. Minor Drainage Systems: Storm sewer conveyance system shall be designed for a 10 year storm and be evaluated for a 100 year storm event. 4. Pond Outlet Structure : Designs shall provide rate controls that limit post-project rates to not exceed existing rates for 1- or 2-year, 10-year, 100-year events and 100-year/10-day snowmelt. Maximum inlet pipe velocity of 0.5 fps. 5. Emergency Overflow Spillway : Designed to convey storm flows for up to a 10.0-inch storm event shall be analyzed to ensu re the emergency overflow spillway will function as designed. Existing Site The 13.2 acre site currently consists of 3.6 acres of wetland and 9.6 acres of combined grass and tree area. There is no existing impervious area on the site. All of the runoff from this site ultimately drains to Rice Lake. The eastern 1.2 acres drain to wetland which then drains to a ditch at the northeast corn er of the site, while the remaining 12.0 acres drains to a ditch which passes through a culvert under the dr iveway to the north, and then drains to Rice Lake. This site also has 2.3 acres draining to it from MnDOT Right of Way, 25.16 acres draining to it from the development to the west, and 9.14 acres draining to it from the development to the north. This off-site or flow through drainage can not be easily separated from the site runoff, so it will modeled with the site runoff. The site is well vegetated and there are no signs of erosion or problem areas.
The majority of the soils on the site are the Lest er and Kilkenny series. These soils are formed on glacial till and moraines. The Lester soils typically have a hydrologic soil group B, but the city has required that a hydrologic soil group of C be used. The Kilkenny series has a hydrologic soil group of C. There are also Glencoe and Hamel loams on site within the wetland areas that will be assigned a hydrologic soil group D. Most of the building pads and street will be c onstructed within imported fill. Although the fill material would most likely be in the hydrologi c soil group B, a hydrologic soil group of C will be utilized. Proposed Improvements Proposed improvements to this site include subdivi ding the land into 16 lots. To accomplish this, 1300 feet of roadway and a cul de sac will have to be constructed. The road way has a curb back to back width that varies from 28 feet to 31 feet. The average lot size is 10,890 square feet. For calculation purposes, each lot was assumed to have 3500 sq. ft. of impervious surface. The 3500 sq . ft. is comprised of 2500 sq. ft. for house and landscaping and 1000 sq. ft. for driveway and walks. This averages to more than 30% impervious surface for each lot. The majority of the side yard high points are near the front of the houses, so most of the impervious surface per lot is in th e subcatchment to the rear of the house. The new impervious surface for this site is 2.46 acres. The impervious surfaces were assigned a curve number of 98. The yards were assigned a curve number of 74. To meet pre-development runoff rates and water qua lity standards, wet detention stormwater ponding is proposed. MODELING RESULTS Peak Discharge Rates The pre-development and post-development r unoff calculations and were performed using HydroCAD software which utilizes SCS TR-20 methodologies. All runoff calculations were performed using a Type II 24-hour rainfall distribution with a 2-year rainfall of 2.8 inches, a 10-year rainfall of 4.2 inch es, and a 100-year rainfall of 6.0 inches. Curve numbers were taken from the table using thei r respective soil group. The 10-day snowmelt calculations were performed using a rainfall of 7.2 inches and an AMC Condition of 4. The basin areas with their respective impervious ar ea, curve numbers, and time of concentration can be found in the HydroCAD results at the back of this report. Tables summarizing the HydroCAD inputs are also attached.
The pre-development and post development runoff cal culations are attached, but summarized below: Discharge 2-Year Storm (cfs) 10-Year Storm (cfs) 100-Year Storm (cfs) Point Pre Post Change Pre Post Change Pre Post Change D1 4.0 4.2 0.2 6.0 6.1 0.1 11.9 12.4 0.5 D2 5.8 5.8 0.0 12.5 11.9 -0.6 21.9 17.0 -4.9 D3 1.1 1.2 0.1 2.5 2.7 0.2 4.7 4.9 0.2 TOTAL 6.6 6.5 -0.1 14.1 13.2 -0.9 25.0 19.4 -5.6 The green cells above represent an increase in p eak discharge over existing while the pink cells represent a decrease from existing. As shown above, the post-development peak dischar ges are less than or equal to pre-development discharges for D2 and the overall total discharge for th e site. As mentioned in the previous report and as discussed at meetings, it is impossible to collect a nd rout the walkout lot rear yard drainage into the detention pond due to the elevations of the site. The following options would reduce D1 to predevelopment conditions: 1. Construct an outlet control device on the upstrea m side of the 30” culvert under the entrance street and utilize the upstream area for pond storage and bounce. 2. Construct an outlet control device on the upstream side of the 12” culvert located at D1. The large storage area upstream of this culvert could provide a significant amount of flood protection with a minimal amount of pond bounce. 3. Enlarge the existing stormwater pond (4D in the HydroCAD Report) that serves the townhome site to the west. The entrance street and boulevard runoff from this site could then be collected and conveyed to the enlarged pond. Implementation of the above options w ill be dependent on city preferences. The increase at D3 is primarily due to the propo sed bituminous trail required by the city. The increases are minor and the city code allows such increases provided that there are no detrimental downstream impacts. If the city requires D3 to meet pre-development conditions, a swale or some other rate control feature could be implemented into the grading design. A preservation/stormwater volume reduction area in the rear of Lots 3-7 has already been incorporated into the stormwater calculations to reduce the runoff. The preservation area would be planted with native grass and tree species and be lo cated in an Outlot to be owned by the city. The city code allows variances to these criteria if it is demonstrated that there will be no adverse downstream impacts as a result. The downstre am impacts will be discussed further below. Downstream Assessment/Impacts There are three discharge points shown in the HydroC AD model. Discharge 1 is the outlet of the 12” culvert just east of the large wetland area, Dischar ge 2 is the outlet of the 18” culvert under the driveway to the north, and Discharge 3 drains to a wetland to the east and then flows through a ditch to Rice Lake. Because the post development runoff of discharge 3 is less than or equal to existing conditions, the downstream impacts will not be analyzed.
From discussions with city staff, discharge D2 is of the most concern because it flows through a ditch on private land without an easement for maintenance if necessary. Based on the discharge table in the previous section, the post-development D2 is actua lly less than existing so it meets city criteria. Discharge D1 is larger than existing for each stor m event. If this were allowed to remain, it would pose no adverse downstream impacts because D1 drains to D2 and as previously mentioned, D2 is less than existing. The existing driveway to the north overtops fo r both pre-development and post-development conditions for a 100-year storm event. The calculation results have not yet been verified in the field by interviewing neighboring landowners. Energy dissipation will be added to the 12” culvert outle t at D1 at the city request. The pipe will also be cleaned and repaired as necessary. Water Quality Water quality computations for phosphorous (TP) and total suspend solid s (TSS) removal were computed using P8 Software. The wet detention po nds were analyzed with the following results: As shown above, the East Pond exceeds the TSS and TP removal requirements. Due to the limited are available for ponding, the West Pond does not meet the water quality requirements for phosperous removal. To mitigate this, it was discussed at a city meeti ng that iron filings could be using to increase the removal efficiencies. Bioretention which has proven water quality benefits was previously proposed, but was determined by the city that maintenance issues outweigh the benefits. Storm Sewer Design The storm sewer system was analyzed using Eagle point software and rational method for computing runoff at each inlet. The storm sewer was designed for a 10 year storm event. Storm sewer outlets velocities were reduced to prevent erosion. Riprap and geotextile filter fabric will be used at all storm sewer outlets. At the request of the city, the exis ting storm sewer at Tigua Lane was also analyzed and found to be undersized for a 10-year storm even t. See the attached storm sewer design summary sheets for details. The following emergency overflows meeting a 10 inch rainfall are provided: Variable East Pond (% reduction) West Pond (% reduction) TSS 95.5 92.2 TP 62.5 53.5
Location EOF Drainage from west Overtop proposed street to east at LP Elev. 897 LP Sta. 4+09 Overtop boulevard into wetland LP Sta. 9+71 Overtop to north between Lots 4 and 5 at Elev. 904 Pond Outlet The east pond outlet was designed with a 15” RCP outlet which follows city detail 3109. The velocity through the pond inlet pipe was limit ed to 0.5 fps for the 1-year storm event. The west pond outlet was designed with a 8” orif ice in a weir wall and a 12” RCP outlet which follows city detail 3109 as modified with the weir wall. The velocity through the pond inlet pipe was limited to 0.5 fps for the 1-year storm event. A pond emergency overflow will be provided with permanent protection in the final design. Attachments 1. Impervious Area Summary Table 2. Pre and Post Development Discharge Table 3. Pre Development Drainage Area and Curve Number Table 4. Post Development Drainage Area and Curve Number Table 5. Pre Development Time of Concentration Input Table 6. Post Development Time of Concentration Input Table 7. Permanent Pool Volume Calculations 8. Water Quality Summary 9. Storm sewer design summary 10. 2-yr, 10-yr, and 100-yr Pre-Development HydroCAD report 11. 2-yr, 10-yr, and 100-yr Post-Development HydroCAD report 12. 10-day snowmelt Pre-Development HydroCAD report 13. 10-day snowmelt Pre-Development HydroCAD report 14. Pre-Development Drainage Area Map 15. Post-Development Drainage Area Map 16. Storm Sewer Drainage Area Map 17. Storm Sewer Pipe Network
Impervious Area Summary Table PRESERVE AT RICE LAK E 8/4/2013 Impervious Area Summary Basis for Impervious Areas:Proposed Houses2500.0sq. ft Proposed Drives1000sq. ft.StreetsArea computed in AutoCAD WalksArea computed in AutoCAD HydroCADNo. ofNo. of BuildingDriveStreetsWalksTotal NewOtherTotalComments CatchmentBuildingsDrivesImp. (ac)Imp. (ac)Imp. (ac)Imp. (ac)Imp. (ac)Imp. (ac)Imp. (ac)S1.1100.060.000.000.000.060.000.06 S1.1A3.200.180.000.000.000.180.000.18 S1.2000.000.000.110.030.140.000.14 S1.30.720.040.050.290.060.440.000.44 S1.4000.000.000.000.000.000.080.08Pond S1.5000.000.000.000.000.000.000.00 S2.12.400.140.000.000.000.140.000.14 S2.20.600.030.000.000.000.030.240.27Pond S2.30.700.040.000.000.000.040.000.04 S2.40.500.030.000.000.000.030.000.03 S2.56.9140.400.320.590.091.400.001.40 S3.1000.000.000.000.000.000.000.00 TOTALS16160.920.370.990.182.460.322.78 New Impervious Surface
Pre and Post Development Discharge Table PRESERVE AT RICE LAK E 8/4/2013 Pre-Development and Post-Development Discharge Summary = Increase over existing = Decrease from existing HydroCAD Results Summarized in Table Below:Discharge PointPrePostChangePrePostChangePrePostChange D14.04.20.26.06.10.111.912.40.5 D25.85.80.012.511.9-0.621.917.0-4.9 D31.11.20.12.52.70.24.74.90.2 TOTAL6.66.5-0.114.113.2-0.925.019.4-5.6 2-Year Storm (cfs)10-Year Storm (cfs)100-Year Storm (cfs)
Pre-Development Drainage Area and Curve Number Table PRESERVE AT RICE LAK E 8/4/2013 Pre-Development Area and Curve Number HydroCAD Inputs HydroCAD Total CatchmentAreaCNAreaCNAreaCNAreaComments S1.14.44744.00798.44Incl. 1.20 acre MnDOT RW S2.12.40741.40793.80 S2.22.29740.80793.09Incl. 0.87 acre MnDOT RW S3.11.02740.40791.42Incl. 0.23 acre MnDOT RW FT4.11.02831.02Flow through - from west FT4.20.70830.70Flow through - from west FT4.31.39831.39Flow through - from west FT4.46.61836.61Flow through - from west FT4.50.86830.86Flow through - from west FT4.67.35837.35Flow through - from west FT4.73.44833.44Flow through - from west FT4.83.79833.79Flow through - from west FT5.19.14809.14Flow through - from north TOTALS44.456.600.0051.05 Area 1Area 2Area 3
Drainage Area and Curve Number Table PRESERVE AT RICE LAK E 6/8/2013 Post-Development Area and Curve Number HydroCAD Inputs HydroCAD Total CatchmentAreaCNAreaCNAreaCNAreaComments S1.10.78740.06982.28793.12 S1.1A0.52740.18980.40791.10 S1.20.06740.14980.20 S1.30.37740.44980.81Incl. 0.29 acre MnDOT RW S1.40.64740.08980.72Incl. 0.50 acre MnDOT RW S1.50.75790.75 S2.10.70740.14980.30791.14 S2.20.37740.27980.10790.74 S2.31.17740.04980.84792.05Incl. 0.67 acre MnDOT RW S2.40.73740.03980.76Incl. 0.19 acre MnDOT RW S2.51.47741.40982.87Incl. 0.43 acre MnDOT RW S3.10.88740.40791.28Incl. 0.23 acre MnDOT RW FT4.11.02831.02Flow through - from west FT4.20.70830.70Flow through - from west FT4.31.39831.39Flow through - from west FT4.46.61836.61Flow through - from west FT4.50.86830.86Flow through - from west FT4.67.35837.35Flow through - from west FT4.73.44833.44Flow through - from west FT4.83.79833.79Flow through - from west FT5.19.14809.14Flow through - from north TOTALS42.743.183.9249.84 Area 1Area 2Area 3
Pre Development Time of Concentration HydroCAD Input Table PRESERVE AT RICE LAK E Pre-Development Time of Concentration HydroCAD Inputs 8/4/2013 Sheet Flow Mannings Shallow Concentrated Flow Velocity Surface (n)Surface Description Factor (fps)Smooth surfaces0.011Paved20.3 Fallow0.05Unpaved16.1 Cultivated: Residue<=20%0.06Grassed Waterway15.0 Cultivated: Residue>20%0.17Nearly Bare & Untilled10.0 Grass: Short0.15Cultivated Straight Rows9.0 Grass: Dense0.24Short Grass Pasture7.0 Grass: Bermuda0.41Woodland5.0 Range0.13Forest w/Heavy Litter2.5 HydroCAD CatchmentLengthSlopeManningsLengthSlopeManningsLengthSlopeVel. FactorLengthSlopeVel. Factor S1.12000.1000.244100.0207.00 S2.12000.1000.242500.0207.00 S2.22000.0150.244200.0257.00 S3.1600.2500.241400.0550.241600.0157.00 FT4.1800.2000.151200.0250.24 FT4.2800.0800.24 FT4.3500.0200.151500.01020.30 FT4.42000.0600.153300.02015.00700.02020.30 FT4.52000.0250.15200.20015.00 FT4.6800.0200.152600.01020.30 FT4.71900.0400.151900.01015.00 FT4.82000.0350.153500.04015.00 FT5.12000.0300.154000.04015.003800.02020.30 Sheet Flow 1 Shallow Concentrated Flow 2 Sheet Flow 2Shallow Concentrated Flow 1
Post Development Time of Concentration HydroCAD Input Table PRESERVE AT RICE LAK E Post-Development Time of Concentration HydroCAD Inputs 6/8/2013 Sheet Flow Mannings Shallow Concentrated Flow Velocity Surface (n)Surface Description Factor (fps)Smooth surfaces0.011Paved20.3 Fallow0.05Unpaved16.1 Cultivated: Residue<=20%0.06Grassed Waterway15.0 Cultivated: Residue>20%0.17Nearly Bare & Untilled10.0 Grass: Short0.15Cultivated Straight Rows9.0 Grass: Dense0.24Short Grass Pasture7.0 Grass: Bermuda0.41Woodland5.0 Range0.13Forest w/Heavy Litter2.5 HydroCAD CatchmentLengthSlopeManningsLengthSlopeManningsLengthSlopeVel. FactorLengthSlopeVel. Factor S1.12000.0600.201400.0307.00 S1.1A1000.0800.15 S1.2100.0200.155000.02020.30 S1.3800.1200.24300.0400.241700.0150.01 S1.42000.1000.24200.33015.00 S1.5700.2500.241300.0200.24300.02015.00 S2.1800.1500.15800.0200.24 S2.21300.1200.20 S2.32000.0150.244200.0257.00 S2.4500.2500.241500.0400.151100.02015.00 S2.5700.2500.241100.0200.15900.02020.30 S3.1600.2500.241400.0550.241600.0157.00 FT4.1800.2000.151200.0250.24 FT4.2800.0800.24 FT4.3500.0200.151500.01020.30 FT4.42000.0600.153300.02015.00700.02020.30 FT4.52000.0250.15200.20015.00 FT4.6800.0200.152600.01020.30 FT4.71900.0400.151900.01015.00 FT4.82000.0350.153500.04015.00 FT5.12000.0300.154000.04015.003800.02020.30 Sheet Flow 1 Shallow Concentrated Flow 2 Sheet Flow 2Shallow Concentrated Flow 1
Permanent Pool Volume Calculatons PRESERVE AT RICE LAK E Permanent Pool Volume Calculations8/4/2013 ContourArea (ac)V ol. (ac-ft)887.00.010Pond Bottom 888.00.0100.010 889.00.0200.015 890.00.0300.025 891.00.0500.040 892.00.1000.075NWL Total0.165 ContourArea (ac)V ol. (ac-ft)886.00.060Pond Bottom 888.00.0900.150 890.00.1400.230 892.00.1900.330 894.00.3100.500NWL Total1.210 West Pond Permanent Pool Volume East Pond Permanent Pool Volume
Water Quality Summary PRESERVE AT RICE LAK E 8/4/2013 Water Quality Summary VariableEast Pond Removal %P0%P10%87.2 P30%94.1 P50%97 P80%99.5 TSS95.5 TP62.5 TKN52.9 CU65.7 PB84.8 ZN10.9 HC84.8 VariableWest Pond Removal %P0%P10%76.8 P30%88.9 P50%96.0 P80%99.6 TSS92.2 TP53.5 TKN44.0 CU58.0 PB79.2 ZN8.1 HC79.2