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<br />EM 1110-2,]405 <br />31 Aug 59 <br /> <br />major variations in rainfall distribution, either by analysis of actual rainfall-runoff records, or by llSP <br />of synthetic methods. <br />c. The' term 11m'i-ra.infall duration rpfers to the duration of runoff-producing rainfall, or rainfall <br />excess, thut results in a unit hydrograph. Th(' unit h.vdrograph ff'sulting from a 6-hour unit-rainfall <br />duration is T('ferred to as a o-hour unit h:"drograph. Thl' term lag, as us('d hf'fCin, is the lC'ngth of time <br />from the midpoint of the unit rainfall dura-tion to the peak of th,' unit hydrograph. <br />d. Thr un'it-rainfall duratio'ft selected for a. unit hydrograph should not (>xceed the ppriod during <br />which the design storm rainfall is assumed to lw approximau'},v uniform in illU'nsity in various portions <br />of the drainage area UIH]f'r study. Inasmuch as valley storagr tpnds to rliminatp. thr cfff'cts of minor <br />variations in rainfall intensity, some'A:hat longer unit.-rainfall durations than an' othcr'\\-Tise desirable <br />are suital)l{' for basins having large valley storage capacities. .-\ 6-hour unit-rainfall duration is suitable <br />and convenient for most studies rplating to drainage arras larger than approximatrly 100 square milrs. <br />Only in approximate studie's should unit-rainfall durations longpr than 12 hours he adopted, inasmuch <br />as major changes in the' areal distribution of rainfall may oeeU!; during longer illtprvals. {l'or drainage <br />areas of less than approximately 100 square miles, values equal to about Oil(' half of the lag appear to <br />be satisfactoI'.Y. <br />e. All references made herein t,o unit hydrograph discharge rates, or ordinates, refer to instan- <br />taneous discharge va.I\H:'s at the instant designated. In order to accuratrly define the shape of a specifie <br />unit hydrograph, any convenirut serif'S of discharge ordinates may be tabulated. The interval of time <br />between the discharge ordinates tabulated has no relation whateVp.f to Hw uTI'it-rail{fall duration of the <br />particular unit hydrograph. For C'xample, discharge ordinates separatr(j by {i-hour intervals of time <br />were tabulated in column 2 of plate :\0. 6 for a unit hydrograph resulting from a 12,hour unit,rainfall <br />duration. Discharge ordinatf's at 12-hour interva.ls onl.Yl might have been used to define the same <br />12-hour unit hydrograph. However, if a 12,hour interval had been used, diseharge values of the final <br />hypothetieal hydrograph (column 10, plate :\0. 16) would have been knowIl ouly at 12,hoUl' intervals, <br />and it would have been necessary to intrrpo'9.h' 'for interrnediatp vahlf's. COIlsidcrablf'. care would ho..ve <br />been required in sketching the final hydrograph through known points in ord,'r to assure that the correct <br />flood volume was represented. By using 6-hour interval~, a suffirirnt Ilumlwr of points wt're,; ohtain('d <br />to permit an accurate plotting of the final hydrograph, although the ordinates at 12-hour intervals were <br />not changed. It should be observed that references to a i2-I",ul' unit hydro!val,h infers one that results <br />from a unit rainfall excess of 12 hours' duration, regardlC'ss of the interval of time he tween discharg'p <br />ordinates used in tahulating the unit hydrograph values. <br />f. A unit hydrograph derived from actual rainfall-runoff records for a particular basin rcprf'sf'Tlts <br />an integration of the many influences that aff'pC't runoff under Uw prevailing eonditions. Application <br />of such a unit hydro graph to conditions diffrring from the original is a proc('ss of extrapolal.ion. Tlw <br />validity of the extrapolation should be ('hrekrd h~T ('v err mrfins availablp. A study of unit hydrographs <br />derived for a large numlwf of basins in which a varjrty of va]]ry storagp charactC'risties, hasin configura- <br />tionsl topographical features, and met-eorological ('onditions al'P l'P[n'('srntrd, off(\rs it basis for estimating: <br />the reJative efIE'ets of predominating influf'tI('Ps. <br />[J. Appendix TV prf'sents tll(' gl'nrral approach and associated forms for usp. in df'tf'l'mination of <br />unit hydrographs ffom obsf'rvrd streamflow r('('ords. Division and Distri('t. Engin{,(,fs arp ff'quf'stNl t(l <br />use thf's(, forms in submitting to thp Chipf of Enginc('rs all rdiahlr dnta 011 unit hydrograph ell.terminH- <br />tions that arr asspmhlcd in their I'(.spp(.tivr offices in (,oIltl('(,tioIl ,,,'ith pl'Ppu.ratioIl of pl'ojeet n'ports. <br />h. Thre(' gPIlPral mrthods are ordinarily availnhle for df'v('loping' unit hydrog'l'aphs. In df'tailpd <br />hydrological studil's, pach of thl' following pl'oe('dul'cs may 1)(' lIs('1i to advantage: <br />By ana.l~~sis of rainfall-runoff rpcords for isolat{'eJ "unit storms." <br />By analysis of minfllll-rulloff !'('cords for major storms. <br />B:'T computation of s~Tnthetic unit hydrographs from (1) dil'('d analogy with basins of similar <br />dwraet{'ristic's and (2) indired unalogy with a ]nrgp nurn}wl' of otll('" hnsins through thr <br />applicution of l'mpirical l'plutiollships. <br /> <br />8 <br /> <br />e <br /> <br />. <br /> <br />e <br /> <br />. <br /> <br />e <br />