1304 80TH ST SW EVERETT SCHOOL DISTRICT 2018-01-02 MF Import

Everett School District— Bus Facility March 24, 2006— Storm Drainage Report Page 9 e V. ANALYSIS AND DESIGN OF REQUIRED RUNOFF CONTROL BMP'S Performance Standards: Runoff control requirements for detention and water treatment for this site were evaluated using the 1997 City of Everett Stormwater Management Manual. Flow Control System: Flow control for the site is required since greater than 5,000 square feet of additional or new Impervious area will be added under proposed conditions. Peak runoff rates under developed conditions will be significantly less than under existing conditions because Flow control is provided for the entire site. Runoff control will be provided in the form of underground large 8 foot diameter CMP detention pipe. The CMP detention pipe will be located on the project site and will be privately owned and maintained. Flow control calculations were performed using the SBUH method that has been approved by the City of Everett for analysis. The Flow control facility was designed to match the existing 100-Year and 10-Year, 24-hour storm and 50% of the 2-Year, 24- hour storm peik runoff. Bypass area peak runoff rates were subtracted from the existing area runoff rates to provide additional storage. An SBEC volume correction factor was applied as a streambank erosion control BMP. The SBEC volume correction or enlargement factor of 45% was applied after sizing the facility to account for the approximate 87% impervious area of the 5.4-acre area analyzed for runoff control. The total required detention volume for a live storage depth of 4.25-feet is approximately 16,837-cubic feet. The total provided detention volume is 16,879-cubic feet. Existing conditions were taken prior to original development in 1980. Proposed conditions are to be the conditions after the development of the bus facility project site. Discharge of all site runoff will be at the natural location into the wetland on neighboring property located at the northeast corner of the project site. A rip-rap rock pad will provide for energy dissipation at the facility outfall. Overflow from the detention facility will enter the Flow control structure riser, which is designed to convey the 100-year undetained overflow (SBUH method). The following table provides existing, developed, bypass and allowable storm release rates. The detention vault and flow control structure will be designed to release the detained stormwater equal to or less than the allowable release rates shown below.