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<br />1 nf C value seie'~,80 for me Bonny Dam subbasin was 1,5, based on field investigations of the
<br />arte c lar~e ::.JdS::- \Nltr~ ;-eio:lveiv fiat slopes and sparse vegetation. Loss rates ranged from 0.15
<br />w 0,25 In /h iCOCiS;anli in each of the nine subbasins, The DUH selected was an average of two
<br />j,menslo~less gr3phs derived from observed data from the flood of August 23, 1969, on tributaries
<br />of the Republican River upstream from Bonny Dam [37],
<br />
<br />The final PMF hydrograph computed for the Bonny Reservoir site has a peak of 613,400 ft3/s and
<br />a 150-n volume of 957,200 acre-ft for the 1,612 mi' contributing area, The recorded peak of
<br />103,000 ft'/s dUring the 1935 event represents 17 percent of the PMF peak, Plate 20 shows a
<br />comparison of the Bonny Dam PMF hydrograph peak with the historic 1935 flood peak for the
<br />Bonny Dam subbasin,
<br />
<br />18. Rapid Creek near Rapid City, South Dakota
<br />
<br />On the morning of June 9, 1972, a series of thunderstorms began congregating over the eastern
<br />slope of the Black Hills near Rapid City, South Dakota. Upper air conditions held these thunder-
<br />storms in a quasi-stationary position for nearly 13 h. Showers began in the late afternoon and
<br />continued at an average rate of 2 in/h, until about 2:00 a,m. on June 10, 1972,
<br />
<br />,"t 6:30 p,m, on June 9, Rapid Creek was reported to be rising rapidly. Just after midnight, after
<br />riSing 15,77 ft, the flood crest reached downtown Rapid City, In its course, the flood claimed 237
<br />lives (confirmed), destroyed 770 permanent homes and 565 mobile homes, and caused more than
<br />$160 million In damages, Reports listed major damage to an additional 2,035 permanent homes
<br />and 785 mobile homes and destruction of 5,000 automobiles [38].
<br />
<br />Water in the river crested 7,69 ft higher than had previous been recorded, with a peak discharge
<br />of 31,200 ft' /s (estimated) - 12 times greater than the previous record, This represented the
<br />uncontrolled drainage from approximately 54 mi' upstream to Pactola Dam. Only the presence of
<br />Pactola Dam kept the damage from being many times more devastating, impounding the drainage
<br />of 312 mi' upstream,
<br />
<br />Rainfalls on June 2,3. and 5, of 0,81 in, 061 in, and 2.49 in, respectively, gave rise to saturated
<br />antecedent conditions In calculating the PMF, similar antecedent conditions were assumed, pro-
<br />ducing an initial loss of 0 in and a constant loss rate of 0,2 in/h,
<br />
<br />The baSin extends from Pactola Dam (EI. 5200 ft) 22.46 mi downstream to gauging station
<br />06412500, Just above Canyon Lake lEI. 3423), The terrain is steep, rough, and covered with
<br />timber and varYing degrees of brush and grass undergrowth. This topography and the results of
<br />other hydrologiC studies in the area led to the assumption of C = 1,8 for purposes of computing
<br />a theoretical lag time in the PMF derivation,
<br />
<br />The PMP for this study was developed from information contained in HMR 55 [2], The 6-h pre-
<br />cipitation calculated using these procedures was 20,7 in, An isohyetal pattern for the 1972 storm
<br />shows an average of 8 in actual rainfall in this area, which compares favorably with the 6-h PMP,
<br />
<br />45
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