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HomeMy WebLinkAbout2 - App. B. - Stormwater Design Report 10-16-2020 DESIGN REPORT STORMWATER MANAGEMENT NORTON RANCH SUBDIVISION, PHASE 5 Prepared for: Norton Properties, LLC 63020 NE Lower Meadow Road, Suite A, Bend, OR 97702 Prepared by: C&H Engineering and Surveying, Inc. 1091 Stoneridge Drive, Bozeman, MT 59718 (406) 587-1115 ON T A/V� COTE �o • No. 60593PE ' w = Project Number: 161141 %i'S/ONAL``�����` September 2020 11iRevised (October 16, 2020) INTRODUCTION The proposed Norton Ranch Subdivision, Phase 5 is a 138-lot subdivision located on a 79.31-acre parcel in the East Half and the West Half of Section 9,Township 2 South,Range 5 East of P.M.M., Gallatin County, City of Bozeman. A combination of site grading, curb and gutter, storm inlets, and piping will be used to manage stormwater runoff on the site. Supporting stormwater calculations are attached to this report. A Drainage Area Map is included in Appendix A. Calculations for each individual drainage area (total area, weighted C factor, and time of concentration) are included in Appendix B. STORM SEWER FACILITIES DESIGN Storm sewer facilities were sized for the 25-yr storm using Manning's Equation. For each inlet, the contributing area, weighted C factor, and time of concentration were calculated. These values were input into Manning's Equation to check capacity and flow characteristics for inlets, storm drain pipes,and curb gutters.All curbs are designed to maintain 0.15' freeboard per C.O.B.Design Manual Section IV.C.5. For the purposes of this report, each pipe section was named to match the associated upstream structure. Pipe sizing calculations are included in Appendix D. RETENTION/DETENTION POND DESIGN All ponds have been sized according to City of Bozeman Design Standards. Retention ponds are sized to capture the entire volume of the 10-year 2-hour storm event. They are designed with a maximum depth of 1.5 feet, and maximum side slope of 4:1. Detention Ponds are sized to limit discharge to pre-development rates for the 10-year storm event. Calculations used for sizing each pond can be found in Appendix C. GROUNDWATER The Geotechnical Investigation Report for Phase 5 encountered groundwater at depths varying from 3.5 to 5.5 feet below ground surface. The seasonally high groundwater was measured during the spring of 2018. Seasonally high groundwater levels were determined to be 0.5' to 2' below ground surface. Shallow groundwater elevations have been encountered throughout the Norton East Ranch Subdivision. The developer is aware of these elevations and all structures are constructed as slab-on-grade. The infrastructure plans for this development will account for the high groundwater condition on site. All streets will be constructed above existing grade to ensure all stormwater ponds are installed above the SHGWL. Dewatering for utility installations and foundation construction is expected and will be accounted for in any construction permits. Crawl space and basement foundations are not recommended within this subdivision—a note is included on the plat with this recommendation. Detention Pond 91 Detention Pond #1 is located in the northwest corner of the site within Park 5B. This detention pond will be replacing the temporary retention pond installed with Phase 3B that is located just north of Babcock Street. Detention Pond #1 receives runoff from Drainage Areas 1-8, 11-13, and the existing stormwater system from Norton East Ranch, Phase 3B (see Appendix F), totaling 63.43 acres. The pre-development time to concentration for the pond was calculated to be 50 minutes and the pre-development runoff rate for the 10-yr storm event was calculated to be 9.09 cfs. In order to limit discharge from the detention pond to pre-development runoff rates the proposed outlet structure for the pond will have a 17.8"weir installed. The outlet pipe(Pipe#Outlet Pipe) was sized for the 25-yr storm event using the maximum time of concentration of the contributing drainage areas (21.4 minutes for DA 0). The pipe will discharge into Aajker Creek. The required pond volume was calculated to be 37,081 cubic feet. The provided pond volume is 47,688 cubic feet at an effective water depth of 1.5' above the seasonal high groundwater level. The groundwater elevation was found to be located at 4778.3' in the vicinity of the detention pond and the bottom of the pond is set at 4778.8'. A profile showing the proposed detention pond, inlet pipe, outlet pipe, and groundwater levels can be found in Appendix E. In the case of a storm exceeding the 10-yr design storm, runoff will overflow the outlet structure top grate into the outlet pipe and flow into the Aajker Creek. Supporting calculations for the pond sizing can be found in Appendix C. Groundwater Influence within Detention Pond #1 It is known that there is currently groundwater influence within the Phase 3 stormwater system installed in 2017 contributing to the existing retention pond to be replaced with this development. Because of this, groundwater influence calculations have been included within the proposed detention pond. As a conservative measure, the groundwater infiltration rate was set at 300 gal/ac/day. • Groundwater Infiltration Rate = 300 gal/ac/day • 1 Gallon= 0.13368 ft3 • Groundwater Infiltration Rate = 0.02785 ft3/ac/min As an additional conservative measure, this infiltration rate was applied to not only the entirety of the Phase 3 development that will contribute to the proposed detention pond, but also over the entire Phase 5 development contributing to the detention pond for a total of 63.37 acres. This results in a groundwater infiltration rate of 0.03 ft3/sec (1.765 ft3/min) This infiltration rate was added to the runoff volume calculations within the proposed detention pond as the detention pond calculations are on a per minute basis. The groundwater was found to have minimal impact on the proposed detention pond due to the high time of concentration and the presence of an outlet structure. The outlet structure will help mitigate the groundwater influence as it will provide a hydraulic conduit for the groundwater to continue into Aajker Creek rather than accumulate and occupy pond capacity as it currently does within the existing temporary retention pond that is being replaced with this detention pond. Additionally, all storm manholes and inlets within the Phase 3 development will be inspected for any leakage and/or standing groundwater during the first phase of development for Norton Phase 5. If any leakage is found, the pipe between the two suspect inlets/MH will be TV inspected and any cracks will be fixed. Retention Pond #2 Retention Pond 42 is located just east of the terminus of Laurel Parkway, directly south of the existing lift station. It receives runoff from Drainage Areas 9 and 10, totaling 5.55 acres. Runoff from Drainage Areas 9 and 10 is conveyed via surface flow and gutters to proposed inlets located within the proposed curb. The groundwater elevation was found to be located at 4782.1' in the vicinity of this retention pond and the bottom of the pond is set at 4782.7'. The required pond volume was calculated to be 7,891 cubic feet. The proposed pond is designed to store 8,021 cubic feet of water. This pond may be temporary and incorporated into the future development plans for Lot RI once that development layout is decided upon. Supporting calculations for the required pond volume can be found in Appendix C. Drainage/Retention Swale 93 The drainage/retention swale #3 is located along the northern boundary of the proposed subdivision. It receives runoff from Drainage Areas 14 and 15, totaling 3.15 acres. Runoff from Drainage Areas 14 and 15 is conveyed via surface flow and gutters to proposed inlets which discharge into the proposed swale. The swale has a shallow slope of 0.8% to the west and will ultimately discharge into Aajker Creek. The shallow slope was utilized so that the drainage swale can act as a retention pond with a bottom width of 2',water depth of 1.5', and 4:1 side slopes. This cross-sectional area in conjunction with the length of the swale was used to determine its retention volume. Due to the shallow slope, stormwater will have ample time to be treated and settle prior to discharging into Aajker Creek. The required swale volume was calculated to be 2,572 cubic feet. The proposed swale is designed to store 9,000 cubic feet of water. The proposed swale will have an overtopping berm installed on the far west side to ensure that during storm events exceeding the design storm, the runoff will not flow north into the Lakes property or flood the proposed houses. Supporting calculations for the required retention swale volume can be found in Appendix C. A detailed exhibit showing proposed elevations of the Swale is included in Appendix E.