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I II II fl !II t r,1 !.I- il 11.1 II II I :!,) , I J IJ ,J ,. IW . .. I. .. II - T 3. "i" equals ratio from Figure 111-3 times the one hour rainfall. B. Soil Conservation Service Method Runoff calculations for drainage basins encompassing 20 acres to 25 square miles shall be analyzed by the method developed by the Soil Conservation Service (SCS). This procedure, as outlined in the SCS publication "Procedures for Determining Peak Flows in Colorado," latest edition, is a graphical solution to a summation of small triangular hydrographs, each incremental hydrograph represented by the equation: qp = 484 6D 2 A U"O) + 0.6 Tc where: qp = peak runoff for an increment of time, D A = drainage area, square miles 0 = storm runoff for increment of time D, inches 6D = time increment, hours Tc = time of concentration, hours e Figures 111-4 and 111-5 show the peak runoff in cfs per square mile per inch (CSMjinch) of runoff resulting from this incremental summation for Type IIA and Type II storms respectively for a given time of concentration. The time of concentration is defined similarly to Tc for the rational method and may be calculated in the same manner based on mean flow velocities. Lacking defined channelization of flow, Figure 111-6 is a nomo- graph to solve the equation for determining Tc for overland flow: Tc = (n. ~ L3r.385 where: Tc = time of concentration, hours L = length of watercourse, miles H = elevation difference, feet The amount of runoff in inches (0) is a function of rainfall, soil type, land use, cover condition, antecedent moisture condition, and conservation practices within the contributing drainage area. The SCS method of calculating L,e ratio of runoff to rainfall is based on the computation of a weighted curve number (CN) for the 19