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<br />Figures 4.4.1.3-la and 4.4.1.J-lb can be used also for selected <br />vegetative complexes to estimate the curve number, The curve number <br />must be chosen carefully in order for I:he method to produce meaningful <br />results. <br /> <br />All the curve numbers presente.d are for average soil-moisture condi- <br />tions, which are referred to as Antecedent ]1oistu1'e Condition II (ANC II). <br />In exceptional cases, the engineer may want to look at hydrologic <br />responses under very dry (AMC I) or very wet (AMC III) conditions. Pro- <br />cedure for this selection of curve num':,ers ITwy be found in the SCS <br />National Engineering Handbook, Section 4--Hycirology CU. S. Dept. of <br />Agriculture, Soil Conservation Service, 1972;. <br /> <br />4.4.1.4 Time of concentration. The time of concentration (Tc) <br />is the time it takes for runoff to travel from the hydraulically most <br />distant part of the watershed to the point of reference. It is a basic <br />element involved in determining tir.ling and magnitude of a flood hydro- <br />graph. Although the Soil Conservation Service has several techniques <br />for detennining the time of concentratlon, it is recommended that the <br />travel time approach be used for storm-drainage dE,sign in Larimer County. <br />It must be noted that Fig. 4.2.4-1 is lOt recommended for use with the <br />SCS procedure. <br /> <br />The travel time (Tt) for overland flow consists of the time it <br />takes water to travel from the uppf:rmost part of the watershed to a <br />defined channel or inlet of the storm sewer system. This type of flo" <br />is significant in very small watersheds becallse a high proportion of <br />travel time is due to overland flo>l. The velocit)' of overland flo" can <br />vary greatly with the surface cover and tilla,ge (Fig., 4.4.1.4-1). If <br />the slope and land use of the overland flo" segment are kno"", , the <br />average fll)" velocity can be read from Fig. l,.4.1.4-1. The travel time <br />is then computed by dividing the total overla,nd fJ ow length by the aver- <br />age velocity. ' <br /> <br />Travel time from the storm se\oier or road. gutt.er to the main open <br />channel is the sum of travf:l times in .!ach icdividual component of the <br />system between the uppermost inlet and the oc,tlet. In most cases aver- <br />age velocities can be used "ithout a significant loss of accuracy. During <br />major storm events, the sewer system may be fully taxed and additional <br />overland fIo" may occur, generally at a significantly lower velocity <br />than the flow in the storm sewers. By using average conduit sizes and <br />an average slope (excluding any vertical drops in the: system), the <br />average velocity can be estimated using Manning's formula. <br /> <br />Since the hydraulic radius of a pipe flowing half full is the same <br />as when flowing full, the respective velocities are equal. 'Fravel time <br />may be based on the pipe flowing full or half full. The travel time <br />through the storm se"ers is computed by dividing the length of flo" by <br />the average velocity. If flo" is principally in shallow road gutters, <br />the curve for overland flow in paved areas sho,"", in Fig. 4.4.1.4-1 can <br />be used to determine average velocity. <br /> <br />LCS-"'M l{anual <br /> <br />4.4-"-Ll <br /> <br />April 1979 <br />