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CITY OF EVERETT STORMWATER MANAGEMENT MANUAL <br /> �. SCS Curve Number Equations <br /> The rainfall-runoff equations of the SCS method relate a land area's runoff depth(precipitation excess)to <br /> the precipitation it receives and to its natural srorage capaciry az follows: <br /> Qd — �PR'O.ZS��I(PR+O.HS� for PR z 0.25 <br /> = 0 for PR<01S <br /> wherc: <br /> Qd = runoff depth in inches over the arca, <br /> pR = prccipitation dcpth in inches over t6e area,and <br /> S = potential maximum na[ural deten[ion,in inches over the area,due to <br /> infiltration,storagc,etc. <br /> The area's potential maximum detention, 5, is related ro its curve number,CN: <br /> S = (1000/CN)- 10 <br /> The combination of the above equations allows fur estimation of the total runoff volumc by computing total <br /> runoffdepth, Qd,given the total prccipitation depth,PR. <br /> When developing thc[unoff hydrograph,t6e above cquation for Qd is used to compute the incremeotal <br /> runoff depth, for each time interval, from the incremental precipitation depth given by the design stortn <br /> hyetograph. This time distribution of runoff depth is often rcterted to as thc prccipitation exccss and <br /> .�, provides thc basis for synthesizing thc mnoff hydrograpL. <br /> Time of Concentretion and Travel Time <br /> The following discussion is based on the methods described in Chapter 3, SCS TR-55(5). The time of <br /> concentration(T�) is the time it takes for runoff to travel Gom Ihe hydraulically most dislant point of the <br /> watershed to thc watershcd oudet. Thc T�is computed by sumnung all�he travel timcs for consccutivc <br /> components of thc drainage comeyance system. The T� influcnces the shape and peak oCthe runoll <br /> hydrograph. Urbanization usually decreases the T�,which incrcases peak discharge. <br /> Water moves through a wamrshcd as sheet flow,shallow concentrated floa�,open channel Flow,or some <br /> combination of these typcs of flow. The type of flow that occws is best detcrmined by field inspec�ion. <br /> Travcl timc(Ti)is thc ratio of[lo�v length to (low vclociry: <br /> T, = U(60V) <br /> whcrc: <br /> '[� = travcl time(minutcs) <br /> L = flow length(feet) _ <br /> V = avcrage vclocity(feeVsec) <br /> 60 = conversion factor from seconds to minutes <br /> Thc T�is the sum of T�values for thc various consecutivc flow segments. <br /> Tc = Tu +T�+...T�m <br /> Volume 1—Minimum Technical Requirements <br /> 5-7 <br />