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• <br />• <br />and tp 4.63A0.422SLp-0.46~L~WJ(1.133 (g) <br />where SLP =difference in elevation in feet, divided by <br />flood-plain distance in miles, between <br />watershed outlet and mos[ distant porn[ <br />on the watershed, <br />and LAW =watershed length-width ratio. <br />Storm hydrographs are computed by con- <br />volving unit hydrographs with incremental <br />source runoff. To compute incremental source <br />runoff, the mass rainfall curve is broken into <br />equal time increments, and the Soil Conserva- <br />tion Service (SCS) rainfall-runoff relationship <br />(3) is applied. The SCS rainfall-runoff relation- <br />ship is expressed in a set of numbered curves. <br />The SCS National Engineering Handbook (3) <br />provides detailed instructions for selecting the <br />proper curve number. <br />Hydrographs computed by this procedure <br />compazed closely with measured hydrographs <br />from 34 watersheds located in Texas, Oklahoma, <br />Arkansas, Louisiana, Mississippi, and Tennessee. <br />The watershed azeas ranged from 0.5 to 25 <br />square miles. ' <br />Flood Routing <br />Streams and valleys <br />The variable storage coefficient (VSC) flood• <br />routing method (S) was selected for HYMO. <br />u <br />q <br />s <br />:a <br />tot <br />Ct 2Tt +or (9) <br />i 1800/Vll + VptJ <br />T L / <br />r L x SLPo 1'h <br />x\LxSLPo+Dit-DOt J (10) <br />s <br />xi,, <br />• Figure 2. Relationship between dimensionless shape <br />parameter and recession constant/time to peak. <br />7-6-1-4 <br />aoc <br />m <br />_ o x • a s io ~ <br />n <br />Figure 3. Relationship between dimensionless shape <br />parameter n and watershed parameter B. <br />The VSC method has been revised (6) to <br />account for the variation in water surface slope <br />during a flood. The revised VSC method is about <br />as accurate as the implicit method (1) and has <br />the general applicability of simpler storage <br />methods. Although an iterative solution is used, <br />the VSC method requires little computer time <br />and is free of convergence problems. <br />The VSC routing equations aze <br />tot <br />C2 2T2+ct. (8) <br />T L <br />x =Cisoo (viz + vos J <br />L x SLPo ~ '/+ <br />X L x SLPo +Dfz.. p~z (ii) <br />