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<br />An S-curve hydrograph may be computed by tabulating a series of identical unit <br />hydrographs, arranged with origins spaced progressively one unit-rainfall duration apart of <br />the time scale and accumulating the ordinates for specific times. With tile unit Ilydrograph <br />values known, the S-curve hydrograph would be computed by steps. During the first <br />period of unit-rainfall duration, the unit hydrograph and S-curve values are identical. The <br />S-curve values for the first period are added to unit hydrograph discharqes for the second <br />interval to obtain corresponding $-curve ordinates. The process is continued until the S- <br />curve discharge rate is equal to the basic rate of rainfall excess. The operations involved <br />are more apparent if the reverse procedure is considered, assuming that the S-curve <br />hydrograph values were originally known. Discharqe values represent the rates of runoff <br />that would result from a uniform continuous rate of rainfall excess of 1 inch per unit-rainfall <br />duration beginning at time zero. <br /> <br />In accordance with the unit hydrograph principle, the ordinates of an S.curve <br />hydrograph representing runoff from a uniform continuous rainfall excess rate of 1 inch per <br />12 hours may be multiplied by 2 in order to obtain values applicable to a rainfall excess <br />rate of 1 inch per 6 hours. Accordingly, S-curve hydrographs developed frorn runoff data <br />for unit storms of vcrious durations may be adjusted to apply to any unit rainfall duration <br />desired, within praci.icallimits. <br /> <br />In addition to the applications refermd to above. the S-curve procedure is useful <br />in modifying unit hydrographs to represent more conservative peak values, or to reflect <br />moderate changes in rainfall distribution <br /> <br />4. SNYDER'S METHOD. <br /> <br />The empirical relations developed by Franklin F. Snyder have proven to be <br />particularly useful in the study of runoff characteristics of drainage areas where streamflow <br />records are not available, as well as in modifying or supplementing available runoff records <br />to serve specific purposes. The following terms (Chow. 1%4) are usee! in the equations: <br /> <br />t = lag time from midpoint of unit rainfall duration, t , to peak of unit hyelrograph. <br />p , <br />in hours. <br />t, = unit rainfall duration equa.l to L, in hours. 5.5' <br />t R ~ unit rainfall duration other than standard unit, t R' adopted in specific study, <br />in hours. <br />t pR = lag time from midpoint of unit rain!all duration, t R' to peak of unit hydrograph. <br />in hours. <br />q p ~ peak rate of discharge of unit hydroqraph for unit rainfall duration, t " in <br />c.t.s./sq. mi. <br />q pR ~ peak rate of discharge of unit hydrograph for unit rainfall duration. t R' in <br />c.f.s./sq. mi. <br />Q p = peak rate of discharge of unit hydrograph, in cJ.s. <br />A ~ drainage area in square miles. <br />L o. ~ river mileage from the station to center of gravity of the drainage area. <br />L = river mileage from the given station to the upstream limits of the drainage <br /> <br />Colorado Flood <br />Hydrology i'v[anual <br /> <br />1'<39 <br /> <br />fR'JFT <br />