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Iq:I <br /> Consulting <br /> Engineers <br /> Page 12 of 13 <br /> 5.7 Water Quality Pump Station and Wet Well Calculations (Basin M) <br /> A pump station will be integrated into the high-flow bypass structure within Basin M to pump the <br /> water quality flow rate. The pump station is designed as a duplex system with variable frequency <br /> drives. Each pump shall each be designed to deliver the water quality flow rate of 1,593 gpm. A flow <br /> meter will be installed to measure and record performance. The pumps will alternate in operation <br /> and will not operate simultaneously. <br /> The internal dimensions of the wet well are roughly 10 feet long by 4.75 feet wide. The pumps will <br /> be controlled by float switches with Pump On at elevation 6.87 and Pump Off at elevation 0.00. The <br /> stored water volume will be roughly 2,400 gallons. The total time between successive pump starts <br /> (operating cycle) will be 6 minutes. See Appendix F for pump station and wet well calculations. <br /> 5.8 Conveyance and Backwater Calculations <br /> Basin A and M were modeled with EPA Stormwater Management Model (SWMM) using Autodesk <br /> Storm and Sanitary Sewer Analysis 2018 to verify conveyance sizing and analyze backwater conditions. <br /> Hydrologic Approach <br /> The following hydrologic parameters were used in the model: <br /> - Design Storm (Conveyance): 25-year cumulative storm for Snohomish County, WA, with a total <br /> rainfall amount of 2.60 inches using a SCS Type 1A 24-hr storm distribution <br /> - Design Storm (Backwater): 100-year cumulative storm for Snohomish County, WA, with a total <br /> rainfall amount of 3.20 inches using a SCS Type 1A 24-hr storm distribution <br /> - Model: EPA SWMM method <br /> - Routing Method: Hydrodynamic Link Routing <br /> - The site was analyzed for the full buildout condition (100% paved) <br /> o SCS Curve Number=98 <br /> o Manning's n =0.015 (asphalt) <br /> o Depression Depth =0.08 inch <br /> o Average Slope = 1-3% <br /> Hydraulic Approach <br /> The following hydraulic parameters were used in the model: <br /> - Routing Method: Hydrodynamic Link Routing <br /> - Manning's n =0.012 to 0.009 depending on pipe material <br /> - Tail Water Elevation: Mean Higher High Water elevation plus 2 feet = 11.06 (NAVD88 Datum) <br /> Model Results <br />