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07/20/04 TUE 18:42 FA% 303 761 2802 W W Wheeler HYDROGRAPH DETERMINATIONS Currently and with two exceptions, none of the Federal water resource development agencies have criteria that reflect definitive hydrologic and hydrometeorologic antecedent storm and flood studies. The two excep- tions are antecedent precipitation criteria adopted by the Bureau and COE for the State of Texas, and by the Tennessee Valley Authority for the Tennessee River Basin. Meteorological studies leading to the devel- opment of these criteria were performed by the Hydrometeorological and Special Studies Branch, Office of Hydrology, Natronal Weather Ser- vice. Documentation of the studies and the resulting criteria are con- tained in HMR 56, "Probable Maximum and TVA Precipitation Estimates With Areal Distribution for Tennessee River Drainages Less than 3,000 Square Miles in Area" [60], and NWS Technical Memoran- dum "Precipitation Antecedent to the 24-Hour Probable Maximum Pre- cipitation for Small Basins in Texas"[61 ]. Since no similar criteria other than the Texas criteria are currently avail- able for the 17 Western States where the Bureau has primary interest, certain provisional criteria have been adopted as established by hydra meteorlogists and hydrologic engineers who have given specific prelim- inary consideration co the governtng meteorlogical factors. Therefore, until such time as definitive studies are conducted, most probably in an interagency effort, the following criteria are used by the Bureau: (a) For PMF's generated by general PMP events in areas east of the Sierra Nevada and Cascade Ranges, excluding areas covered by the previously mentioned NWS reports, the antecedent flood is estimated by either converting 100-year prectpttation co a flood hydrograph or developing a balanced 100-year hydrograph using statistical analyses of runoff data. If 100-year prectpttatton method is used, the time .between the end of antecedent rainfall and the beginning of the PMP event is assumed to be 3 days. Tf the balanced flood hydrograph ap- proach is used, a time interval of 3 days between the peak of the ~ancecedent flood hydrograph and the begtnning of the PMP event is used. :~~. ~•(b) For PMF's generated by general PMP events west of the Sierra Nevada and Cascade Ranges, the same criteria given in (a) apply except . • that the time intervals are 2 rather than 3 days. ~~`~ (c) For PMF's generated by local PMP events in the entire region west of the 103rd meridan, no antecedent event is used. Meteorological conditions are such in this region that hydrometeorologists do not consider it reasonable to assume an event of 100-year magnitude to precede the probable maximum event. However, it is reasonable to assume that a storm of sufficient magnitude has occurred to satisfy initial infiltration losses and provide for minimum or ultimate infil- tration loss conditions at the onset of the probable maximum local storm event. X004 117