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<br />..13Q 7' <br /> <br />. <br /> <br />. <br /> <br />1- <br /> <br />the probable maximum flow for this stream, Rainfall over the Black <br />Squirrel Creek watershed produced a peak flow of 141,000 c,f.s. These <br />two small streams produced most of the flood on the Arkansas River from <br />Pueblo to Las Animas, Jimmy Camp Creek was the primary contributor to <br />the flood On Pountain Creek which resul ted in the flood of record at <br />Pueblo. The contribution from Illack Squirrel Creek caused Chico Creek <br />to produce 50,900 c. f. s. at the mouth. The Chico Creek and Pountain <br />Creek flows combined to give a peak of 102,000 c.f.s. in the Arkansas <br />River at Avondale. <br /> <br />36, FLOOD FREQUENCY ESTIMATES.- Exceedence frequencies of floods <br />for existing river conditions and conditions resulting from control by <br />Pueblo Reservoir were prepared to permit evaluation of benefits credit- <br />able to the project for prevention of flood losses. The principles used <br />in the computation of flood probahilities were based on procedures of <br />statistical analysis. Since flood losses in the Arkansas River subbasin <br />from Pueblo to John Hartin Reservoir are more closely related to flood <br />peaks than flood volumes, only peak frequencies were developed. <br /> <br />" <br /> <br />37. Data used to develOp the frequency curves from Pueblo to Las <br />Animas \~'crc obtaineJ from records at six main stem gaging stations and <br />from Pountain Creek at the mouth. These stations on the Arkansas River <br />were at Pueblo, Avondale, Nepesta, POlder, Lil Junta and Las Animas. <br />Datn. from records at gagin~ stations \':erc extended and 3.ojustE:;l for ad- <br />ditional drainaGe area to develop peak frequency curves below all of the, <br />major tributaries. The frequency relationship for a typical reach down- <br />stream from Chico Creek is shown on plate 6. <br /> <br />38. STANDARD PROJECT FLOOD. - Runoff from the June 1921 storm was <br />determined to be of standard project flood magni tude. Por the original <br />studies of ti,e flood control requirements at Pueblo Reservoir, it was <br />determined that an increase of SO percent in the 1921 flood hydrograph <br />as it occurred 'vou1d be equivalent to the standard project flood, During <br />-subsequent-s tudies- for-the -Repo2't-on -Revi~ew Sfi2'-veY-fo2' -nood-Con t2'O I and - - -, <br />Allied Pu2'poaes, A2'Kansas Rive2' and T2'ibuta2'ies above John Martin Dam, <br />Colo2'ado, it was further determined, from a transposition of the June <br />1921 storm, that the standard projcct flood volume would be about SO <br />percent more than the June 1921 flood as it occurred. HOliever, the peak <br />discharge from this transposition would be about twice that of the actual <br />flood or 170,000 c.f.s. The standard project flood hydrograph at Pueblo <br />Dam wi th a vo I tune of 140,000 acre-feet and a peak discharge of 170,000 <br />c.f.s. is shown on plate 5. <br /> <br />39. DESIGN FLOOD.- The maximum flood of record at Pueblo occurred <br />in June 1921. The flood had an estimated peak discharge of 103,000 <br />c.f.s. and a volume of approximately 100,000 acre-feet. Following this <br />flood, the Pueblo Conservancy District constructed a floodway through <br />the urban area which is capable of safely discharging 110,000 c.f.s. <br />with a minimlID1 of 3 feet freeboard, To supplement the floodway, the <br />Pueblo Conservancy District also constructed llarrier Dam, a detention <br /> <br />.1 <br /> <br />12 <br />