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Overview <br /> The project is in an area that drains to the combined storm drainage and sanitary sewer <br /> system. The City has directed that sewer system upgrades will be required in lieu of <br /> stormwater detention. The existing 6-inch diameter sewer main located in the adjacent <br /> alley will be replaced with a 12-inch diameter pipe per the City direction. <br /> Stormwater detention calculations are therefore not applicable. The following is a <br /> rational method calculation of the 100-year flow from the developed site. The existing <br /> site is about 30 percent impervious and will be close to 100 percent impervious after the <br /> development. <br /> 100-year rainfall intensity for Everett for duration of 2 minutes is: <br /> Iloo = 3.80 inches(Fig. 1-5.1,City of Everett Stormwater Management Manual) <br /> C=0.9 for impervious, A=0.62 acres (27,076 square feet) <br /> Q1oo=CAI <br /> =(0.9)(0.62)(3.80) <br /> =2.12 cubic feet per second <br />