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919 E. Marine View Drive Apartments #13-4517 <br /> DRAINAGE DESIGN & CALCULATIONS <br /> The site is located within the City of Everett, Washington. The 2010 City of Everett Stormwater <br /> Management Manual is applicable. <br /> Conveyance Design <br /> The conveyance system is designed to accommodate peak discharge from the 100-year, 24- <br /> hour storm based on post-development site conditions. <br /> In accordance with isopluvial maps for 25-year, 24-hour and 100 year, 24-hour precipitation, <br /> precipitation at the site is 2.75 and 3.25 inches, respectively. <br />• Runoff coefficients for pavement/roofs and lawn are 0.90 and 0.25, respectively. <br /> The conveyance system was analyzed using the Rational Method utilizing a Excel spreadsheet <br /> performing the necessary conveyance calculations. Backwater analysis was also performed, <br /> using the maximum water surface elevation at the detention tanks as the tailwater condition. <br /> The calculations indicate that the conveyance system adequately accommodates runoff from <br /> the 100-year, 24-hour storm. Backwater calculations indicate adequate freeboard at the various <br /> drainage structures. The conveyance and backwater calculations are included in this section of <br /> the report. <br /> Flow Control Design <br /> The City of Everett Stormwater Management Manual requires that detention be provided to limit <br /> the peak developed rate of runoff from the 2-year, 24-hour design storm to 50 percent of the <br /> existing condition 2-year, 24-hour design storm while maintaining the existing condition peak <br /> runoff rate for the 10-year, 24-hour and 100-year, 24-hour design storms. Stream protection <br /> duration criteria also needs to be met. <br /> Below is a summary of the land cover types and areas used for hydrologic modeling. <br />. Land Cover Type Pre-Developed Area (ac) Developed Area (ac) <br /> C, Forest, Mod 0.587 0 <br /> Lawn, Mod 0 0.205 <br /> Road, Mod 0 0 <br /> Rooftops/Driveways 0.154 0.529 <br /> Total 0.741 0.734 <br /> The required and designed detention tank volumes and release rates are summarized below. <br /> Live Stor. Vol. (CF) Release Rate (CFS) <br /> Storm Event Required Designed Required Designed <br /> 2 (1/2) 2,140 2,140 0.054 0.031 <br /> 10 3,400 3,400 0.087 0.051 <br /> 100 5,375 5,375 0.133 0.082 <br /> March 25,2014 Prepared by:Group Four, Inc. <br />• <br />