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k if <br /> RUNO�FC�NTROL <br /> Streambank Erosion Control <br /> Mitigation for stream bank erosion roncems is required when a project nrr_poses to add a net nea <br /> 5,000 square feet or more of impervious surfaces. The deve!�p�nent of this parcel includes the <br /> addition of a net new 2,945 square feeL The ur^!=�i adds 6,488 square feet of impervious surface <br /> but it alsc removes 3,543 square feFt ot existing impervious surface and changes it to pervious,thus <br /> the net total of.�dded imp^.rcious surface is or�ry 2,945 square feet This project is exempt from <br /> providing stre�mCank erosion control features. See Figure F-01 in Appendix A!or a map ot the added <br /> and re;��oved impervioussurfaces. <br /> Runoff Treatment <br /> Bes[Managnment Practices(BMPs)to remove sediments and other p�llulants from runoff are <br /> required when a project prcGoses to add or replace 5,000 square feet or mcre of pollu.ion generating <br /> impervious surfaces(PGISj. The redevelopment on[his parcel has a riet quantity of new or replaced <br /> PGIS o(8,496 square feet. See Figure F-02 in Appendix A for a map of these areas. Due to site <br /> constraints,treatment will be provided for an equivalent minimum PGIS area of 8.496 square feet. <br /> This allows for the treatment BMPs to be added as a retrofit to the existing storm drainage system <br /> rather than trying to specifically collect and treat runoff only from the newly paved surfaces. 7his <br /> project proposes to use a canister media filtei system as the prirriary runoff treatment BMP. Per the <br /> city requirements,a pretreatment device must be added upstream of the media filter. A Hydrodynainic <br /> separator with Washington State Department of Ecology(WSDOE)general use level designation for <br /> pretreatinent w�il be used to ineet[his requirement. The filter and pretreatment ha��e been placed <br /> on-site in a locaticn where they will collect and treat runoff from 9,015 square feet of PGIS,which <br /> exceeds the minimum area required. <br /> Buth the media filters and the hydrodynamic separator are sized based on the water quality flowrate <br /> for the area draining to them. A water quality flowrate was calculated using the computer program <br /> WWHM3 for all lhe area draining to lhe pretreatment and filter system,including the non-PGIS areas. <br /> From WWHM3,ihe�va[er quality flowrate for an on-line facility using 15-minu[e time steps is 0.034 <br /> cubic feet per second (cfs), or 15.3 gallons F;er minute(gpml;see Appendix A for calculations. <br /> BMP Slzing <br /> Pr[atreatmeo: <br /> Per the City of Everett's requirements,a preVeatment device is required if the filter media cartridges <br /> are used for basic[realment. Due to the site constraints,the CDSA hydrodynamic separator by <br /> r,ontech Stormwater SoWcions has been chosen to provide pretreatment. The CDS system has <br /> General Use Level Designation for PretreaUnent fr�m the WSDOE. The CDS Model 2015-4 is approved <br /> �i for a water quality fio�vrate of 0.7 cfs,and allo�vs drainage to enter the top of the manhole to function <br /> as an inlet. <br /> � <br /> � <br /> CommuNly 7ranslt 2 <br /> Kasr b P;nti Op��rabni�f3,,s�, <br />