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HomeMy WebLinkAbout005_StormwaterReport WESTLAKE PARK IMPROVEMENTS – STORMWATER DESIGN REPORT Project No. 18098.18 City of Bozeman Parks and Recreation P.O Box 1230 Bozeman, MT 59771 March 25, 2022 WESTLAKE PARK IMPROVEMENTS DRAINAGE REPORT BOZEMAN, MONTANA CERTIFICATION I hereby state that this Final Drainage Report has been prepared by me or under my supervision and meets the standard of care and expertise which is usual and customary in this community of professional engineers. The analysis has been prepared utilizing procedures and practices specified by the City of Bozeman and within the standard accepted practices. ____________________________________________ ___________________________ Tim Pirtz, PE Date 03/25/2022 March 25, 2022 Project No. 18098.18 STORMWATER DESIGN REPORT FOR WESTLAKE PARK IMPROVEMENTS PROJECT OVERVIEW & BACKGROUND The purpose of this drainage plan is to present a summary of calculations to quantify the stormwater runoff for the Westlake Park project. LOCATION The lot is 5.902 acres of the Westlake’s 4th Addition, Block 1. It is bordered to the north by W Tamarack Street and to the west by North 5th Avenue. EXISTING SITE CONDITIONS The existing project area contains the remains of a dirt mountain bike track after construction of North 5th Avenue improvements. The remaining area is undeveloped land. There is an existing BMX bike park on the south end of the property and a Children’s Memorial Garden on the east side of the project area. PROPOSED PROJECT Project improvements include construction of a new mountain bike course, asphalt pathways, and a concrete plaza area. Approximately 90,000 (~2.1 acres) will be improved, including landscaping. HYDROLOGY All design criteria and calculations are in accordance with The City of Bozeman Design Standards and Specifications Policy, dated March 2004. The site stormwater improvements have been designed with the intent to meet the current City of Bozeman drainage regulations for the entire site to the extent feasible. P:18098_18 Westlake BMX Track Landscape and Design 1 (03/23/22) CS/trp PRE-DEVELOPMENT Existing topography generally slopes south to north towards W Tamarack Street. There is a large topsoil stockpile on the north end of the site. There is currently no stormwater collection system or storage facilities located within the project area. There is a 30” corrugated metal pipe (CMP) storm sewer running underneath the sidewalk along North 5th Avenue. Runoff calculations for pre-development conditions are shown in Appendix C. POST-DEVELOPMENT The Rational Method provided in The City of Bozeman Standard Specifications and Policy was used to calculate the runoff volumes for the 10-year and 25-year, 2-hour storm event. The rational drainage coefficient “C” was calculated for each contributing area using 0.9 for impervious areas and 0.20 for pervious areas. For the purposes of this report, the post construction storm water system will consist of one drainage basin (See Appendix B, Exhibit B Post-Developed Sub-Basins). Basin 1 includes sub-basin areas 1 through 4. The storage requirements for this project will be met by using the greater of the 10-year storm event using the modified rational or the direct runoff from the first 0.5-inches of precipitation. The direct runoff from the first 0.5-inches of precipitation is calculated as follows. Precipitation = P = 0.5-inches Drainage coefficient = c =0.23 Assume 23% of the total precipitation is converted to direct runoff Direct runoff = Q = 0.23 * 0.5-inches = 0.115-inches Direct runoff volume = (0.115/12) * (1.768acres*43560sf/acre) =738cf The required storage from the 10-year storm using the modified rational method is 957cf as shown in the calculations in Appendix D. An ADS Stormtech (or approved equal) chamber system that will key into native gravels estimated to infiltrate at 6.8 in/hr per Infiltration Rates for USCS Soils Table for GP soils (Appendix B) will be used to detain the storm water. For this design, an infiltration rate of 3.4 in/hr will be used, which is half of the estimated rate, to be conservative. The depth of the existing gravels was estimated to be at a depth of 7-8 feet based on observations of soil profiles during construction of the 5th Avenue Sewer Improvements project constructed along the west boundary of this project in 2020. The required storage volume for the 10-year storm is calculated using the Modified Rational Method with a discharge rate calculated from the 3.4 in/hr over an area equal to the chamber system footprint. For storms larger than the 10-year event, runoff will overflow through a new 12-inch pipe that will tie into an existing manhole located in the northwest corner of the site. Runoff calculations for post-development conditions and ADS Design are shown in Appendix D. P:18098_18 Westlake BMX Track Landscape and Design 2 (03/23/22) CS/trp PIPES All storm drainage pipes were sized to handle peak flow resulting from a 25-year storm event. The Rational Method was used to calculate peak flow. The Manning Equation was used to determine the flow full capacity of each pipe. See Appendix E for pipe sizing calculations. Appendices Appendix A –Stormwater Basins Appendix B – Stormwater Chambers Design Details Appendix C – Pre-Development Calculations Appendix D – Post-Development Calculations Appendix E – Pipe Sizing Calculations Appendix F – Surface Improvements O&M Manual Westlakes 4th Addition Park Plat I-16 Stormwater Design Report Project No. 18098.18 APPENDIX A Stormwater Basins EXHIBIT ANORTH030SCALE:1" = 60'6030 EXHIBIT BNORTH030SCALE:1" = 60'6030 APPENDIX B Stormwater Chambers Design Details Westlakes Park Improvements – Stormwater Design Report Project No. 18098.18 H‐1 | Page  This Section adds to the infiltration testing section of the Montana Post‐Construction Storm Water BMP Design Guidance Manual; however, this section shall take precedence over any discrepancies with the practices or results of the water quality Guidance Manual. One of the following methods should be used to determine the design infiltration rate.  Design Infiltration Rate Using the USCS Classification (non‐field measured – This method is only applicable to proposed infiltration systems with less than 5,000 square feet of infiltration area)  Encased Falling Head Test (field measured)  Pilot Infiltration Test (field measured)  Borehole Infiltration Test (field measured) Design Infiltration Rate Using the USCS Classification For infiltration systems with less than 5,000 square feet, a design infiltration rate can be selected from Table 1: Infiltration Rate Ranges for USCS Soils based on the least‐permeable soil layer encountered within 10 feet of the base of the proposed infiltration system. The design infiltration rates presented in Table 1: Infiltration Rate Ranges for USCS Soils represent the ranges of infiltration rates for each soil classification. The minimum infiltration rate shall be selected as the design infiltration rate. Table 1: Infiltration Rate Ranges for USCS Soils Soil Description USCS Range of Typical Infiltration Rates (inches/hour) min max* Well graded gravel, sandy gravel GW 1.30 137.00 Poorly graded gravel, sandy gravel GP 6.80 137.00 Well graded sand, gravelly sand SW 0.80 68.00 Poorly graded sand, gravelly sand SP 0.50 68.00 Silty gravel, silty sandy gravel GM 1.63 13.50 Clayey sands SC 0.05 0.78 Silty Sand SM 0.24 0.70 Clayey gravel, clayey sandy gravel GC 0.04 0.50 Inorganic Silts of low plasticity ML 0.04 0.14 Clay CL 0.00 0.01 Inorganic Silts of high plasticity MH 0.00 0.01 Inorganic clays of high plasticity CH 0.00 0.01 *If proposing to use maximum rates, a detailed explanation shall be provided why the maximum rates apply to development. APPENDIX C Pre-Development Calculations Westlakes Park Improvements – Stormwater Design Report Project No. 18098.18 P:18098_18 Westlake BMX Track Landscape and Design 1 (03/23/22) CS/trp Basin 1 Pre-development =1.871.1 − / / C = 0.2 (City of Bozeman Design Standards and Specifications Policy Table I-1) Cf = 1.00 (2 to 10 year) D = 400ft S = 1.5% =1.87 1.1 − 0.2 ∗ 1.0 400/ 1.5/ Tc = 29min = C = 0.2 (City of Bozeman Design Standards and Specifications Policy Table I-1) i =Y=0.64X-0.65 = 0.64(29/60)-0.65 = 1.03 cfs A = 1.768 acres = 0.20 ∗ 1.03 " ℎ$∗ 1.768 &’$() Qpre = 0.36 cfs APPENDIX D Post-Development Calculations Westlakes Park Improvements – Stormwater Design Report Project No. 18098.18 P:18098_18 Westlake BMX Track Landscape and Design 1 (03/23/22) CS/trp Basin 1 Post-development Impervious Area = 143ft2 + 3,529ft2 = 3,672ft2 = 0.084 acres Pervious Area = 34,299ft2 + 6,061ft2 + 16,642ft2 + 19,823ft2 = 73,353ft2 = 1.684 acres Total Area = 77,025ft2 = 1.768 acres Post-development C = (0.9(0.084 acres) + 0.2(1.684 acres))/1.768 acres = 0.23 Cf = 1.0 Tc = 29min (same as pre-development) = I = Y=0.64X-0.65 = 0.64(29/60)-0.65 = 1.03in/hr A = 1.768 acres = 0.23 ∗ 1.03 ℎ∗ 1.768 Qpost = 0.42 cfs Chamber Type = SC-740 Gravel Footprint Area = 500.13ft2 (See ADS System Parameters) Infiltration Rate = 500.13ft2 * 3.4 in/hr * 1hr/3600sec * 1ft/12in = 0.0394 cfs Although the flowrate of 0.36 cfs is allowed to discharge based on pre-development conditions, the design will infiltrate into the native gravels with no discharge for the 10-year storm. This will treat the first 0.5 inches of precipitation. The maximum storage is calculated using the Modified Rational method with a 10-year storm and a Post-developed Tc of 29 minutes for Basin 1, and the required volume was found to be 957 cubic feet as shown on the following table. P:18098_18 Westlake BMX Track Landscape and Design 2 (03/23/22) CS/trp Basin 1 Storage Calculation with Infiltration (10-year) Tc(min) 29 Detention Area (sf) 500.13 Post C 0.23 Perv Rate (in/hr) 3.4 Cf 1 Area(ac) 1.768 Infiltration Rate 0.0394 cfs Duration Intensity Qp Incoming Outgoing (min) (in/hr) (cfs) Vol(cf) Vol (cf) 5 3.22 1.31 392.63 11.81 380.8 10 2.05 0.83 500.43 23.62 476.8 15 1.58 0.64 576.73 35.43 541.3 20 1.31 0.53 637.82 47.23 590.6 30 1.00 0.41 735.08 70.85 664.2 60 0.64 0.26 936.90 141.70 795.1951 90 0.49 0.20 1079.75 212.56 867.1980 120 0.41 0.17 1194.13 283.41 910.7270 200 0.29 0.12 1427.91 472.35 955.5627 210 0.28 0.12 1452.50 495.96 956.5386 212 0.28 0.11 1457.33 500.69 956.6419 214 0.28 0.11 1462.12 505.41 956.7157 216 0.28 0.11 1466.89 510.13 956.7604 218 0.28 0.11 1471.63 514.86 956.7766 220 0.28 0.11 1476.34 519.58 956.7645 300 0.22 0.09 1645.63 708.52 937.1120 400 0.19 0.08 1819.95 944.69 875.2649 500 0.16 0.07 1967.79 1180.86 786.9292 600 0.14 0.06 2097.45 1417.04 680.4196 800 0.12 0.05 2319.64 1889.38 430.2629 The chamber system provides 1,076 cubic feet of storage (See ADS System Parameters), which exceeds the required volume of 957 cubic feet. Runoff from larger storm events will fill up the available storage and the excess will spill out of the outlet pipe into the existing storm drain manhole at the corner of 5th Avenue and Tamarack Street. APPENDIX E Pipe Sizing Calculations Westlakes Park Improvements – Stormwater Design Report Project No. 18098.18 P:18098_18 Westlake BMX Track Landscape and Design 1 (03/23/22) CS/trp Basin 1 Post-development Impervious Area = 143ft2 + 3,529ft2 = 3,672ft2 = 0.084 acres Pervious Area = 34,299ft2 + 6,061ft2 + 16,642ft2 + 19,823ft2 = 73,353ft2 = 1.684 acres Total Area = 77,025ft2 = 1.768 acres Post-development C = (0.9(0.084 acres) + 0.2(1.684 acres))/1.768 acres = 0.23 Cf = 1.10 (25-year) Tc = 29min (same as pre-development) = I = Y=0.78X-0.64 = 0.78(29/60)-0.64 = 1.24 cfs A = 1.768 acres = 0.23 ∗ 1.24 ℎ∗ 1.768 Qpost = 0.50 cfs Chamber Type = SC-740 Gravel Footprint Area = 500.13ft2 (See ADS System Parameters) Infiltration Rate = 500.13ft2 * 3.4 in/hr * 1hr/3600sec * 1ft/12in = 0.0394 cfs 12-inch SDR 35 PVC pipe flowing full capacity: =1.49 ( !/#)%&/! n = 0.013 = ’ ! = 0.785ft2 ( = 2’r = 3.14ft R=A/P=0.785ft2/3.14ft= S=1.2% =1.49 0.013 0.785+,!(1.5708+,!/#)0.012&/! Q=3.91cfs All pipes are 12-inch with at least 1.2% slope; therefore, they are adequate to handle the 25- year storm event. APPENDIX F Surface Improvements O & M Manual Westlakes 4th Addition Park Plat I-16 Stormwater Design Report Project No. 18098.18 March 25, 2022 Project No. 18098.18 STORM DRAINAGE FACILITY MAINTENANCE PLAN FOR WESTLAKE PARK IMPROVEMENTS OVERVIEW NARRATIVE The purpose of this maintenance plan is to outline the necessary details related to ownership, responsibility, and cleaning schedule for the storm drainage facilities for Westlake Park. This plan has been completed in accordance with The City of Bozeman Design Standards and Specifications Policy, dated March 2004. The site stormwater improvements have been designed with the intent to meet the current City of Bozeman drainage regulations for the entire site to the extent feasible. Specific site information and criteria are described below: I. Ownership of Facilities City of Bozeman Parks and Recreation Department City of Bozeman Parks and Recreation Department will own all stormwater facilities which includes the storm chambers, inlets, and piping within the site boundary. II. Inspection Thresholds for Cleaning Infiltration Chamber If sediment in the Isolator Row exceeds 3 inches or grate is more than 25% clogged with debris, clean grate and/or structure and vacuum Isolator Row. Structures Inspect structures and inlet grates for debris and sediment. P:18098_18_WestLake_BMX_Track18098.18_O&M 2 (03/23/22) CS/trp III. Cleaning Infiltration Chamber To clean grate of structure, remove and dispose of debris clogging the grate. To clean structure, use catch basin vacuum to remove sediment and debris. To clean Isolator Row use JetVac. Structures Remove trash, obstructions, debris, sediment and vegetation to prevent clogging of the grate or pipes. Clean out if sediment fills 60% of sump or comes within 6-inches of a pipe. IV. Schedule Infiltration Chamber Inspection: Every 6 months or after a storm event larger than 0.5-inches of precipitation. Vacuum Isolator Row: Every 5 years or as needed based on inspection. Structures Inspection: Every 6 months or after a storm event larger than 0.5-inches of precipitation. V. Responsible Party City of Bozeman Parks and Recreation Department City of Bozeman Parks and Recreation Department will be responsible for the inspection and maintenance of all stormwater facilities located within the project limits. I agree to the above operation, maintenance and replacement schedule detailed above. Signature: __________________________________________ City of Bozeman Parks and Recreation Department Representative