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<br />In addition to the applications referred to above, the S-curve procedure <br />is useful in modifying unit hydrographs to represent more conservative peak values, <br />or to reflect moderate changes in rainfall distribution. <br /> <br />4. SNYDER'S METHOD. <br /> <br />The empirical relations developed by Franklin F. Snyder have proven to <br />be particularly useful in the stuqy of runoff characteristics of drainage areas where <br />streamflow records are not available, as well as in modifying or supplementing <br />available runoff records to serve specific purposes. The following terms are used in <br />the equations: <br /> <br />t = lag time from midpoint of unit rainfall duration, t , <br />p , <br />to peak of unit hydrograph, in hours. <br />t, = unit rainfall duration equal to 1." in hours. <br />5.5' <br />t R = unit rainfall duration other than standard unit, t R' <br />adopted in specific study, in hours. <br />t pR = lag time from midpoint of unit rainfall duration, t R' to <br />peak of unit hydrograph, in hours. <br />q p = peak rate of discharge of unit hydrograph for unit rainfall <br />duration, t " in c.f.s./sq. mi. <br />q pR = peak rate of discharge of unit hydrograph for unit rainfall <br />duration, t R' in c.f.s./sq. mi. <br />Q p = peak rate of discharge of unit hydrograph, in c.f.s. <br />A = drainage area in square miles. <br />L C8 = river mileage from the station to center of gravity of the <br />drainage area. <br />L = river mileage from the given station to the upstream limits <br />of the drainage area. <br />e t and e p = coefficients depending upon units and drainage basin <br />characteristics. <br /> <br />The following equations are the most frequently used: <br /> <br />t p = e * (L * L ) * * 0.3 <br /> t C8 <br />t , = tp / 5.5 <br />q p = (e p * 640 ) / tp <br />t pR = (640 * e p ) / tpR <br /> <br />(3) <br />(4) <br /> <br />(5) <br /> <br />(6) <br /> <br />7-29 <br />