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STORMWATER SITE PLAN 8/18/2023 <br /> SOUTHWEST STORAGE YARD 5 <br /> Solutions. Appendix B has the WWHM2012 calculations for both basin inputs as well as the water quality flows.These <br /> were provided to Contech who in turn provided system sizing. <br /> Basin #1 <br /> 1-18" PSORB cartridge in a steel catch basin <br /> Water Quality Flow: 0.023cfs <br /> Peak 100yr flow: 0.282cfs <br /> Required Minimum Drop (inlet to outlet): 0' <br /> Assumed Depth (rim to outlet) =2.3' <br /> Level of Treatment: Basic and phosphorus <br /> Basin #2 <br /> 1—27" PSORB cartridge in a steel catch basin <br /> Water Quality Flow: 0.037cfs <br /> Peak 100yr flow: 0.445cfs <br /> Required Minimum Drop (inlet to outlet): 0' <br /> Assumed Depth (rim to outlet) =3.05' <br /> Level of Treatment: Basic and phosphorus <br /> The access point descends down to the existing roadway and is not tributary to the filter system due to grade. It will use <br /> a Compost Amended Vegetated Filter Strip to provide Enhanced Treatment of runoff.The basin is 0.03 ac.WWHM was <br /> used to calculate a filter strip. The result was a 3 ft wide 4:1 strip along the edge of pavement.The WWHM output is <br /> presented in Appendix B. <br /> MR 7: FLOW CONTROL <br /> The site discharges to a flow control exempt water body, namely the Snohomish River system.There were concerns <br /> about the operation of the tide gate/storage ditches in relation to the outlet of the stormwater treatment system and <br /> also a concern about providing compensatory storage for the increase in runoff volume due to the change from pasture <br /> grasses to proposed impervious. <br /> TIDE GATE STORAGE SYSTEM <br /> The tide gate storage system was studied under the previous work for Sea-Con to the east. See that report for expected <br /> heights of 6 month flow for protecting the treatment system. It should be noted that the site will be below the 100 year <br /> flood elevation.The filter systems will need to be cleaned out and provided with new cartridges after each flood event <br /> due to expected high levels of sedimentation. <br /> DIKE DISTRICT COMPENSATORY STORAGE <br /> The flow path to the east connects to an area of additional storage that was created to off-set increases in storm runoff <br /> volumes due to the development. As stormwater backs up behind the tide gate during a storm/high tide combination, <br /> the large conveyance channel will provide for temporary storage and rising volumes will backfill into the additional <br /> storage area.There is additional storage available to compensate for this site as well. <br /> PAWork\Pi ojccts\zou\u Ib2Sea Con\CE\D06\ST0RMWATER SITE PLAN SMALL.a_ <br />