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07/20/04 TUE-16:42 FA% 303 761 2802 W W Wheeler <br />FLOOD HYDROLOGY MANUAL <br />(d) For PMF's generated by PMP falling on a snowpack, the antecedent <br />meteorologic condition is assumed to be the snowfall that accumulates <br />and forms the snowpack. It is further assumed that runoff resulting <br />from the melting of the snowpack will occur during and somewhat <br />prior to the probable maximum rainfall. For the majority of drainage <br />basins, those with areas smaller than about 3,000 square miles, current <br />practice is to develop a 100-year balanced snowmelt hydrograph and <br />then superimpose the probable maximum rainflood onto it in such a <br />manner that the peak flow of each hydrograph occurs at the same <br />point in time. For basins larger than 3,000 square miles, it has been <br />frequently found that the snowmelt will peak earlier than it is mete- <br />orologically reasonable for the probable maximum rainstorm to occur. <br />In such situations, an'evaluation of the timing of the two events should <br />be requested of and performed by experienced hydrometeorologists. <br />Application of the western snowmelt equation discussed in section <br />4.3(a) is not currently used. However, when definitive criteria for <br />application of this equation have been developed and generally ac- <br />cepted among the Federal water resource development agencies, it is <br />expected that the equation will replace the 100-year balanced hydro- <br />graph approach. <br />4.5 Foss Dam Example <br />As an example of probable maximum flood hydrograph determinations, <br />consider the Washita River Basin above Foss Dam in Oklahoma. As shown <br />on figure 4-15, this dam and its 1,490-square mile tributary drainage <br />basin are located in west-central Oklahoma, with the basin centroid lo- <br />cared at latitude 35°40'N. and longitude 99°45' W. The upper reaches <br />of this drainage basin extend about 30 miles into Texas. For hydrologic <br />reasons, the basin was divided into four subbasins as shown on figure <br />4-15. This subdivision will result in the generation of four component <br />flood hydrographs that will require routing and combining to obtain the <br />total flood hydrograph at the dam, This routing and combing will con- <br />tinue this example in chapter 5. <br />Probable maximum storm precipitation values were obtained for this <br />basin using procedures described tomprehensively in HMR 51 and 52 <br />(35,18]. The process for obtaining these values is identical with Example <br />No. la shown on pages 108 through 126 of HMR 52. For brevity, the <br />actual PMP computations for the drainage basin above Foss Dam are <br />not included in this manual; however, the results of the computations <br />for the four subbasins are shown in table 4-20. <br />The ftrst step in the rainfall-to-runoff conversion process is to derive <br />unit hydrographs for each of the four subbasins shown on figure 4-15. <br />The salient physical properties shown in table 4-20 for each subbasin <br />were determined using a USGS quadrangle map. <br />118 <br />f~1005 <br />C <br />C <br />f <br />C <br />f <br />l <br />C <br />C <br />C <br />