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• • <br /> COFFMAN L. <br /> 's , .:1,r <br /> ENGINEERS .� <br /> 140 <br /> 464o <br /> Memo 4� �c1sT04-1,40 <br /> 31owAL <br /> 03/25/2020 <br /> Date: March 25, 2020 Project: Boeing Flightline <br /> Building 45-005 <br /> To: City of Everett Drainage Calculations <br /> Company: PWD/Building <br /> Project No.: 182659 <br /> C: Kevin Hanefeld From: Allison L Hazen , PE <br /> Todd Haberlack <br /> CONVEYANCE SYSTEM CALCULATIONS <br /> The stormwater system has been designed using the Rational Method to calculate design peak flow <br /> rates. <br /> Rational Method Peak Flow Calculations <br /> The Rational Method peak flow rate is calculated using the following equation <br /> Q= C*i*A <br /> where: <br /> Q= peak flow[cfs] <br /> C= composite Rational Method runoff coefficient <br /> A= contributing area [acre] <br /> = average rainfall intensity, in inches per hour, for a particular storm duration <br /> The rainfall intensity is found using the following equation,with contributing basin's time of concentration <br /> used as the storm duration: <br /> i = mire <br /> where <br /> To=time of concentration <br /> m,n =dimensionless coefficients(from Table 4-3.1(1)-1 Rainfall Coefficients of the City of <br /> Everett Design and Construction Standards and Specifications for Development, <br /> Volume 1) <br /> Peak rainfall intensity is calculated using the following equation: <br /> = P* i <br /> where <br /> P=24-hour precipitation for a given return frequency[in]** <br /> I = peak rainfall intensity[in/hr] <br /> **Precipitation values are from Table 4-3.1(D) Design Storm Hyetograph 24-hour Design Storm Totals of <br /> the City of Everett Design and Construction Standards and Specifications for Development,Volume 1) <br /> RUNOFF COEFFICIENT <br /> The composite Rational Method runoff coefficient is calculated by the following equation: <br /> C= (C1*A1 + C2*A2..Cn*An)/At <br /> where: <br /> C= Curve number for a contributing area <br /> =Total Contributing Area <br /> 1101 Second Avenue,Suite 400 I Seattle,WA 98101 <br /> Phone 206.623.0717 www.coffman.com <br />