2320 CALIFORNIA ST 2025-07-25 - Laserfiche WebLink

WC! The southern basin of the project includes the proposed 16,000 building footprint and additional new parking areas which will create more than 5,000 square feet of impervious surfacing. According to Table 1-1.1 from the COE Manual all stormwater controls and a Stormwater Site Plan and Drainage Plan are required. This report is organized following Minimum Requirements #1 through #10 outlined in the 2005 DOE Manual. These requirements are listed in Section 5, Summary of Minimum Development Requirements, of this Stormwater Site Plan along with a response of how these requirements shall be met. This organizational method meets City of Everett requirements while providing some additional information. 2.2 Design Criteria North Basin In accordance with the City of Everett Design Manual water quality will be provided for the re- developed parking area. Rain gardens or as they are often referred to as bio-retention cells will be incorporated into the new parking lot landscape areas. These facilities have been designed according to the Low Impact Development Technical Guidance Manual for Puget Sound. A small portion of the proposed parking lot will be treated by a Filterra bioretention system. This system has Washington State Department of Ecology approval for General Use Level Designation (GULD). All water quality facilities were designed using WWHM3 in accordance with the LID Technical Guidance Manual for Puget Sound. South Basin In accordance with the City of Everett Design Manual the south basin's hydrologic analysis was performed using the StormSHED software (formerly Waterworks), using SBUH or SCS TR-55 methods for hydrograph generation. An existing detention and water quality pond recently constructed for the south storage yard will be modified to provide detention and water quality for all tributary areas. ' 2.3 Design Assumptions North Basin The following assumptions were used to design the water quality treatment rain gardens: • Design infiltration rate of 3 inches/hr(12 in/hr initial & infiltration reduction factor of 4). • Under-drains and an overflow system will convey runoff to the existing conveyance system. ' • 18 inches of imported soil will be placed above the under-drain system. • Imported soil will have a CEC>or equal to 5 meq/100grams of dry soil • Imported soil will have 8-10 percent organic matter content. • Imported soil will have 2-5 percent fines passing the 200 sieve. • Rain gardens will capture and treat the water quality design storm volume, or 91% of the ' runoff volume predicted by a continuous runoff model (WWHM3), with an overflow sized to convey larger storm events. • The 4x4 filterra bioretention system is able to treat 0.140ac of contributing drainage area for enhanced treatment. DAVID EVANS AND ASSOCIATES,INC. OPERATIONS CENTER ADDITION TUNE,2010 STORMWATER SITE PLAN PAGE 2