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<br />UI. SPS ESTIM.U'I!l3 JOB LAMB DRAJJW]B RASIl'IS <br /> <br />e <br /> <br />:-01. GERERAL. The badc princ1plee involved in tbe pnparstion ot <br />SPS ~",d SPF eet1Jllatee tor lArge drlUABge basine are the e_ as thoee <br />applicable to basins less than 1000 square 1II11es 1n area, ..mieh have been <br />discussed in Sl!ct1on n. However, geDeralizat10ne ot criteria become IIlOre <br />d1ff~cult as the size or area increases. wnl!reas SPF d1scharge est1mates <br />for 8IIl&l.l areas are usus.l.ly gOV'llnled largely by the IIllI.lC1mImI 6 or 12-hour <br />M!1nfall associated with a severe thUD.deretollll situat1on, floods ot SPF <br />cate~o~ on large dra1nage basins are generally the result of a succeesion <br />of relatively distinct rainfall events. Although the intens1ty and quant1ties <br />of ra1ni'all are important factors in the production ot a flood in a large <br />drainage basin, the loea~10n of successive 1ncrements of ra1nts.l.l 1n the <br />basin. a.nd the syncron1zation ot intense burst. ot ra1nfs.l.l with progression <br />of 1"UIlo~, are of equal or gre!Lter importance 1n =y cases than q~tity <br />of tcta: precipitation. For e~amp1e, the total rainfs.l.l over the Kansas <br />R1 ver basin dur'..ng the period 25- 31 May 1903, which produced an est1Jllated <br />peak discharge ot 260 ,000 cts on the Kansas River at Kansas C1ty, vas sJJnoat <br />1dentica: with the total prec1pitation that occurred over the basin 9-12 <br />July 1951 to produce a peak discharge of 510,000 cfs. Other examples of <br />a similar nature might be cited. Accordingly, &election ot a SPS for a <br />large basin cannot be predicated on a statistical ane.l;yBis ot precepitatioa <br />q~tit1es alone, but lllUst be based on a review of hydromet.eorological data <br />tor several outstanding storms of record in the basin and adjacent regions <br />in rel!Ltion to hydrologic characteristics of the basin under study. Cons- <br />iderat10n of major floods of record and histor1cal account should also play <br />an important part in the select10n of SPF criteria and tinal es-t1mate. <br /> <br />e <br /> <br />3-02. EXAMPLE: SPS BSTDlATE FOR ,CUMIlERLAIm RIVER ABOVE OLD HICKORY <br />DAM SITE (11,700 SQ. MI.). <br /> <br />a, Select10n of Model Storm for SPS Est1Jllate. FolloVin8 1s <br />a chronologic41 out11ne of the procedure used: <br /> <br />(1) Using the Corps ot &lg1neers publicat10n "Stom Ra1n- <br />fs.l.l in t.he Un! ted States, Depth-Area-Durat10n Data", approX1Jllately 30 stOl'llUl <br />of record \I1th1n a fe" hundred miles of the Cumberland basin "ere tentatively <br />listed for transpos1tion. . To limit. the number, only those storms that <br />produced rainfall exceed1ng an average of 6 inches over 5,000 sq. mi. llere <br />included. The average depth of rainfall over 5,000 and 10,000 sq. mi. \laB <br />listed for purpose of compar1son. <br /> <br />(2) Following a preliminary inspect10n of data referred <br />to above, it "as l!vident th&t winter-season storms llere preilO"'i """tly the <br />lIOet severe for the area 1nvolTed, &l:1d that around cond1tions would be IIlOst <br />favorable for flood runoff during the \I1nter. AA:cordingly'- the list of <br />storms cited in sub-paragraph ~ vas reduced by eliminat1ng the less important <br />Summer and Fall Storms, and also certain "inter storms that did not show <br />1eohyetal patters oriented favorably for transposit1on to the Cumberland <br />basin. The f~n&l 11st of 19 storms considered tor transposit1on is shown <br />in table A of PlAte 15. <br /> <br />14 <br /> <br />e <br />