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<br />j. Plate 11. '!'1JIe-D18tr1butiClll ot ............- 6-Bour SPS Ba1lltaU <br />Ana.lyeea of major st01'1lUl approach1Jl@: SPS intRllties over area.e ot a tev <br />hund:,.,;d S'luar.. mile8 or larger shov that the I1lte ot ra1ntall is W1uaJ.ly <br />fairly unitorm dur1.llg the M...1........ 6-haur period ot the stona, Vith exceptlon <br />of w~enn1ttent short-perlod variatlona whlch usual.ly produce only ~ <br />_riations in strelUll-nov rates at dovnatreliJll points after valley storege <br />,.has modulated loc&l variatlons in surface 1'\IIIQff. Ra1Iltall ratell during <br />less inteue 6-hour perioda ot the stO:nD are se1l8r&lly more erratlc, and <br />may tollov many different lIequences and rate chlln6es In different storma. <br />Hovever, studies indicate that aasUlllptlon of unltom ra1ntall intendtlell <br />dur:1ng succeed ve 6-hour perlods ot the SPS, Vi th exception ot the IIIlI.X1DB <br />6-hr. period or SPS est1lllates appl1cable to ~ dra.1nll8e ba.e1l:la, vould <br />give satistactory flood discharge ellt1lllates lt reasonable cOI18ervatiem is <br />exercised in est1lllat1ng infiltration loases aDd unit h~pIIa. In <br />order to alleure sat'e est1lllates ot peak dischargee to be expected trolll SPS <br />rainfall over drainll8e areas lees than approxiJllately 300 squa:re miles OIl <br />the average, the l1li'.71.."", 6-hour rainfall of the SPS should be broken cloYD <br />into shorter unit perioda and higher intendt1ee aasUllllld tor tbe shorter <br />1ntervals according to criteria presented on Plate 11. <br /> <br />k. Plate 12, SPS Isob:yetal Pattern. Studies ot isob:yet&l <br />patterns resulting from .storms in \/blch the IIIAld.llIuIII 6 or 12-hour ra1ntall <br />1s gener&lly comparable to SPS values reveal tbat tbe patterns over a tev <br />tb.ousand square miles or leas are usually elliptlc&l in shape and that the <br />longer a.xis may be orlented in tmy direction, unleBll topographical 1ntluencee <br />ILre pronounced. Within these general 11Jll1ts, a Vide variety ot patterna <br />might be selected Vith equ&l validity. To s1lllp11ty SPS ccmsputations in <br />Which areal distribution of precipitation ba.e an 1mport8l1t 1ntluence on <br />'ron-off rates and volumes, a typic&l isoh:yet&l pattern 1B Jresented on Plate <br />12. The application ot this isoh:yetl!ol pattem is explained later. <br /> <br />1. Plates 13, 14 and 15. These relate to illustrative <br />examples, as discussed here1l1a:rter. ' <br /> <br />2-03. PREPARATIOl'l OP SPS :l:STIMA!l'ES FOR SMALL DRAIl'IAGE llASIIIS (Il!:SS <br />'1'lWI IOOO SQ. MI.) <br /> <br />a. General. The preparation ot IL SPS estilate tor a aal.l <br />dra1n&ge ba.ein utllizilll( generalized criteria presented on Plates 2 and 9 <br />to 12 involves the tolloving steps: <br /> <br />(1) Interpolate trom generaliZed leoh:yetB on Plate 2 the <br />SPS ind"x ra1ntall cOM"'spond1ng to tb" location ot tbe drainage ba.ein <br />und"r study (reteM"'d to here1na:rter as "project basin"). <br /> <br />(2) EDter Plate 9 Vitb the SPS IIldex Raint&ll value <br />d"tenll1n"d 111 step (1) 10 obtain tb" SPS Iadex Ra1lltall Ratios correspond1ng <br />to 24, 4.8, 12 and 96-hour periods and lID area "qual to the total area ot <br />the project buin. Multiply these ratios by th~ SPS Index RlUntall to <br />obtain o.""rag~ depths of rainfall Vithin an 1sohyet&l area equal to th~ <br />dra~" area ot th~ proj~ct basin, and ,d"hnll1ne incremental 24-hour <br />valu"s by subtraction. <br /> <br />10 <br /> <br />e <br /> <br />e <br /> <br />e <br />