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Thomson Residence <br />Stormwater Site Plan Report <br />March 5, 2021 <br />Section IV, Page 2 <br />3. Sheet flow dispersion in accordance with BMP T5.12 or concentrated flow dispersion in <br />accordance with BMP T5.11 are infeasible because the required lengths for vegetated flowpath <br />and buffers cannot be met. <br />Flow Control <br />No Low Impact BMPs area feasible for the site and flow control is required per Section 6.10 of the Everett <br />Manual. A detention pipe was sized according to the combined sewer flow control performance standard, <br />which states that stormwater discharges shall match developed discharge durations to existing site <br />durations for the range of existing discharge rates from the 10-year peak flow through the full 100-year <br />peak flow, and peak runoff rates from the developed site shall not exceed existing site runoff rates in the <br />25-year and 100-year return intervals. A 16' long x 36" diameter detention pipe will be utilized to meet <br />flow control thresholds for this project. See the attached WWHM printout for the model used to size the <br />detention pipe. The outlet of the detention pipe is connected to the existing combined sewer system <br />along the west side of Rucker Ave via conveyance pipes and catch basins. <br />Conveyance Sizing <br />Stormwater conveyance pipes were sized based on the 25-year recurring interval peak flow rate. This <br />calculation was performed using the rational method. See the calculations below. <br />250 4th Avenue South, Suite 200 <br />Edmonds, WA 98020 <br />ENGINEERING ph.425.778.8500 1 f.425,778.5536 <br />www.cgengineering.com <br />