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<br />EM 1110-2-1405 <br />31 Aug 59 <br /> <br />c{'nlrations of flood ."uuoff and small vall('y storage capu('ities, the critical flood would ,.('suIt from <br />inlt'Ilse rainfall for n'}ativply short durations, In such cases, runoff from rainfall for IOllgt,f' durut.ions <br />would uol add app"l'('iably 10 II", flood pl'ak, but would simply prolon~ thl' flood, On tlil' oll",r hand, <br />in hasins ('haruc!t'ri7..pd by large natural or artifieial storage ('apa('iti(,:3~ t he high rate of runoff from <br />sh0l1-dllration. high~inlplIsit:.v rainfall would he modulated by storagp to a large t'xt{'nt. and the Plf('ct <br />of the longer-duration rainfall would be of greater importance. For use in computing hypothrtical <br />hydrographs, rainfall qU8ntitiC"S for convenient unit periods of time arC' scaled from the adopt-pel en- <br />vC"loping (h'pth-duratioll curve. A unit p{'riod of six hours is a convpuicnt standard for an'as larg('r <br />thnn approximately 100 sq'-mrc miles, The scqm'ncc of rainfall incrempnts IH'('pssar'y to giv<, the eritieal <br />rat<' of runoff can be arbi.trarily ueterminpd by trial applications of the unit hydl'ograph to various <br />groupings of the iIH'f('m{'nts, inasmuch as it has been shown by records that the maximu~ rainfall <br />intC'118iti('8 ran o('('ur (war the beginning, middle or ('Bel of the prC'cipitat.ion period. A typical arl'ange- <br />TnC'nt of rainfall incf('mC'nts is shown in the hyetograph of plate No. 16, <br />d. The following proc(~dure may be used to obtain an estimate of the maximum probable volume <br />of rainfall <,xcess to be expeded from a design storm resulting in rainfall quantities COff('sponding to <br />th(' adopted envdoping depth-duration curve: <br />TJH~ minimum initial loss and infiltration index values to be cxpt.c.ted, in the basin under <br />study, during the Sf'ason in which the design storm is likely to occur are estimated by analyses <br />of hydl'Olo~il' reeords for the partieular basin. (See par. 14b.) <br />Rainfall quantities during the first periods of the design storm are assum{'d to lw lost. to runoff <br />until the aceumulative rainfall is equal to the initial loss, alld thereafter a uniform rate of loss <br />('qual to the minimum infiltration index is assumed. The computations are illustl'ut.l'<1 by <br />data labulaled beneath the hyetographs in plate No. 16. <br />e. In estimating rainfall-cxecss quantities in the matter outlined above it is assurm'd ,that. rainfall <br />CXCf'SS during successive periods of the design storm would have approximately the same areal dis- <br />tribution as oc('urred during the storm considered in, deriving the infiltration i ndpx. If such all assump- <br />tion cannot be made without the risk of serious error, either Method II or [I I should he followed in <br />preparing the design storm estimate, in order that the effects of probable variations in rainfall continuity <br />and areal distribution {~aIl be more closely approximated. The probabilit,y of significant ('rrors in <br />E'stimating infiltration losses normally increases with an increase in the size of drainage ar('11 involved <br />and with increase in infiltration capacities in the basin, <br /> <br />34, METHOD II. TRANSPOSITION OF RECORD STORM, RAINFALL-EXCESS ESTIMATE. <br />Information available for an aeiual storm of reeord may fonn a reasonable basis for the spillway desi~n <br />storm estimate, both with regard to rainfall quantities and intensity variations, The data may be <br />utilized conveniently in the following manner: <br />a. Superimpose an outline of the given drainage basin on the total-storm isohyetal map of the <br />seleeted re~"Ord storm in sueh manner as to plaee the hi~hest rainfall quantities in a position that would <br />result in the maximum flood runoff. <br />b. Construct a net.work of Thiessen polygons for precipitation stations in and near the basin in <br />its transposed position; prepare mass rainfall curves for the respective stations and complete the rainfall <br />analysis in exaetIy the manner followed in eomputing infiltration in<liees from aetual floods. (See part <br />I of plate No.4.) <br />c. Increase or decrease the observed rainfall values as considered necessary to assure an adequate <br />design storm rainfall estimate for the purpose involved. <br />d. On the basis of data obtained by analysl'.s of aetual floods in the given basin and similar areas, <br />scleet minimum values of initial loss and infiltration indiees eonsidered applieable to the desi~n storm, <br />and estimate eorresponding rainfall-exeess quantities in the manner followed in deriving infiltration <br />indiees (see part II of plate No.4). <br /> <br />24 <br /> <br />e <br /> <br />e <br /> <br />e <br />