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07/20/04 TUE 16:42 FA% 303 761 2802 W W Wheeler [~j003 FLAOD HYDROLOGY MANUAL of discussion among representatives of the Bureau N WS, COE, and SCS, and future ac[ion of this group will be the development of suitable criteria. In view of this lack of criteria, the standard practice of the Bureau is to combine the rain generated part of the PMF hydrograph with a snowmelt chat could reasonably be expected to occur at the time of year chat the probable maximum rainfall occurs. Naturally, this practice is only used to those areas where snowpacks of sufficient magnitude to be considered do occur. It should be noted that since the melting snowpack tends to satisfy the infiltration losses, the losses to the rainfall increment are min- imal. Current practice is to apply a loss of 0.05 inch per hour to the PMP when generating the PMF rainflood hydrograph. Such losses apply only to the area assumed co be covered by the snowpack. The Bureau currently uses a 100-year snowmelt flood to account for snowpack. A frequency analysis of the maximum annual snowmelt vol- ume is performed using the procedures discussed in chapter 7. The nor- malperiod ofrunoff selected is 15 days; however, in large drainage basins with significant areas where snowpack accumulates, this period may ex- tend to 30, or 60 days or, in the case of the Coiorado River above Hoover Dam, the period may extend through two yearly runoff seasons.•The 100-year snowmelt flood is then distributed over time using either the largest snowmelt flood of record as the basis for distribution or by using the balanced flood hydrograph approach as discussed in chapter 7. The resulting snowmelt hydrograph is generally expressed in terms of mean daily flows for the 15-day period with diurnal fluctuations being neglected. For drainage basins Tess than about 3,000 square miles, the rainflood hydrograph is superimposed on the snowmelt hydrograph with the rain assumed to occur during the day or days of the greatest snowmelt. This assumption is made so that the maximum rain occurs during the warmest period. The resulting combined rain-on-snow flood represents the prob- able maximum event. For larger basins, the melting snowpack may form the antecedent flood discussed in section 4.4. ~' 4:4 Antecedent Floods iu When developing a PMF hydrograph, it is normal practice to consider antecedent conditions, both meteorologic and hydrologic. The resulting flood hydrograph, in combination with the PMF hydrograph, is generally referred co as the PMF series. The concept of an antecedent event is based primarily on the meteorological factors discussed in chapter 3. The occurrence of antecedent precipitation, either in the form of rain or snow, is the basis for assuming wet or saturated ground conditions and adopting minimum or ultimate infiltration losses in developing PMF hydrographs. 116