2312 W CASINO RD 2025-07-25 - Laserfiche WebLink

RUNOFF CONTROL Streambank Erosion Control Mitigation for stream bank erosion concerns is required when a project proposes to add a net new 5,000 square feet or more of impervious surfaces. The development of this parcel includes the addition of a net new 2,945 square feet. The project adds 6,488 square feet of impervious surface but it also removes 3,543 square feet of existing impervious surface and changes it to pervious,thus the net total of added impervious surface is only 2,945 square feet. This project is exempt from providing streambank erosion control features. See Figure F-01 in Appendix A for a map of the added and removed impervious surfaces. Runoff Treatment Best Management Practices(BMPs)to remove sediments and other pollutants from runoff are required when,a project proposes to add or replace 5,000 square feet or more of pollution generating impervious surfaces(PGIS). The redevelopment on this parcel has a net quantity of new or replaced PGIS of 8,496 square feet. See Figure F-02 in Appendix A for a map of these areas. Due to site constraints,treatment will be provided for an equivalent minimum PGIS area of 8,496 square feet. This allows for the treatment BMPs to be added as a retrofit to the existing storm drainage system rather than trying to specifically collect and treat runoff only from the newly paved surfaces. This project proposes to use a canister media filter system as the primary runoff treatment BMP. Per the city requirements,a pretreatment device must be added upstream of the media filter. A Hydrodynamic separator with Washington State Department of Ecology(WSDOE)general use level designation for pretreatment will be used to meet this requirement. The filter and pretreatment have been placed on-site in a location where they will collect and treat runoff from 9,015 square feet of PGIS,which exceeds,the minimum area required. Both the media filters and the hydrodynamic separator are sized based on the water quality flowrate for the area draining to them. A water quality flowrate was calculated using the computer program WWHM3 for all the area draining to the pretreatment and filter system, including the non-PGIS areas. From WWHM3,the water quality flowrate for an on-line facility using 15-minute time steps is 0.034 cubic feet per second(cfs), or 15.3 gallons per minute(gpm);see Appendix A for calculations. BMP Sizing Pretreatment Per the City of Everett's requirements,a pretreatment device is required if the filter media cartridges are used for basic treatment. Due to the site constraints,the CDS®hydrodynamic separator by Contech Stormwater Solutions has been chosen to provide pretreatment. The CDS system has General Use Level Designation for Pretreatment from the WSDOE. The CDS Model 2015-4 is approved for a water quality flowrate of 0.7 cfs,and allows drainage to enter the top of the manhole to function as an inlet. Community Transit Kasch Park Operating Base 2