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e e e EM 1116-2-1405 31 Au( 59 as the character of basic data permitted, The topography of the basins considered varied from rolling slopes to relatively steep hills several hundred feet in height above the prineipal stream ehannels. c. With but a few exceptions, it was found that unit hydrographs required to reproduee the major flood hydrographs had peak diseharge ordinates consistently higher than those eomputed from records of minor floods in which the areal distribution of rainfall was approximately uniform. In the majority of basins considered, the peak ordinates of unit hydrographs derived from major flood hydrographs, representing runoff volumes greater than approximately 5 inches depth from the drainage area, were 25 to 50 percent higher than values computed from records of minor floods, in which the runoff was from one to two inches. The variations were greater in a few instances. The differences were not proportional to the volumes of flood runoff, but apparent.ly were the result of a number of faetors, some of which had greater influences during certain floods than in others. d. The following probably represent the principal reasons for the observed differences between unit hydrographs derived from minor and major flood hydrographs referred to above: Differences in areal distribution of rainfall-The minor flood rises analyzed resulted from rainfall of approxim~tely uniform areal distribution. Precipitation during the major floods usually covered t.he entire drainage area, but in most instances the rainfall intensity and accumulated amounts varied over the ares. If the volume of rainfall excess during the ma.jor storm were proportionately heavier in the lowe,r portion of the basin, or near the principal stream channels, the concentration of runoff would be higher than represented by the unit hydrographs derived from the minor flood rises, Difference.. in hydraulic relati01l.,-During minor flood rises, the hydraulic gradients in natural streams are usually relatively low, because of the series of pools that exist in the channels. As the stage increases during major floods, the pools tend to drown out and, in basins less than several hundred square miles in area, the channel conveyance is usually increased substantially. It is to be expected th<:refore that unit hydrographs dcrived from major flood hydrographs for small streams would have higher peak discharge ordinates than those derived from minor floods. In large basins in which high quantities of overbank storage occur at flood stages, the channel eonveyance may decrease with rises in stage but the higher efficiencies of flow in tributary streams tend to counteract sueh decreases. 25. SELECTION OF UNIT HYDROGRAPHS FOR DESIGN-FLOOD COMPUTATIONS. a. In the majority of hydrologic studies pertaining to the determination of maximum probable flood hydro, graphs, reliable data for thc determination of unit hydrographs are 'limited to records of relatively minor flood occurrences, For reasons discussed in paragraph 24, it is usually just.ifiable to assume that a unit hydrograph applieable to the most intense periods of rainfall during a design storm would have a higher peak discharge ordinate, and would represent a higher eoncentrat;on of runoff, than might be indie.ate.d by unit hydrographs derived from minor floods. If adequate and reliable hydrologie records arl' available for floods that resulled from rainfall intensities and areal distributions reasonably eomparable to those to 1>1' expeeted during the design storm, uncertainties regarding the proper unit hydrograph values for use in estimating the de,sign flood runoff are substantially redueed. Howe\'er, it, is frequent.ly necessary to modify unit hydrographs derived from availabll' hydrologie records to repr('~nt highef rates of runoff, in order that thf'ir use in computing th(" desi~ flood discharges may aggurc conservative fesults. b. The general procedure outlined in paragraph 23 is suitable for modifying a given unit hydro, graph to represent a higher peak discharge, The method was followed in arbitrarily modifying unit hydrograph No. I-A, plate No. la, figure b, to obtain Kos, I-B and I-C, whieh have peak ordinates 25 and 50 percent higher, respectively, than :\0. I-A, The widths of eaeh of the three unit hydrographs at ordinates equal to 50 and 75 percent of the maxima, wem obtained from Curves A-A' and B-B', of plate Ko. 7. c. The unit hydrographs selected for estimating ratt's of runoff from successive unit periods of the design storm should be applieable to the areal distributions of rainfall that are assumed to oceur during: thp fpspprtivp pf"fiods, As a gi'llefal rule, it is ordinarily satisfaetofY to consider that rainfall 15