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<br />000817 The IOo-yUI ~,,~~~~~~nt: Fact OJ Fiction
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<br />GREAT BASIN PRECIPITATION DISTRIBUTION
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<br />SOUTH WEST MOUNTAIN DISTRIBUTION COMPARISON
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<br />FiJ::,UIC 4. Precipitation Distribution for thc Grcat
<br />Basin and Southwest Mountain Rcgions.
<br />
<br />HYDROGRAPH COMPARISONS
<br />
<br />To evaluate the technique presented by Frederick, nood
<br />flows from ungaged watersheds resulting from 100-year
<br />'24.hour events were developed for various regiuns of the
<br />western United Stales. The SCS Curve Number Method was
<br />utilized with a Type I or Type II distribution to create
<br />flood hydrographs thai were compa,ed to flood hydro-
<br />graphs resulting from 0.90 probability level distribution
<br />based upon F7redericks' work and assuming Quartile I
<br />slorms occur. All watersheds were assumed to be I square
<br />mile in area, have a CN \':Jlue of 86, excepl for the Pacinc
<br />Northwest (West) where CN = 70, a drainage length of
<br />5,000 fl. and a,slope of 10 percent.
<br />Figure 5 illustrates the difference in the hydrographs for
<br />Parker, Colorado. which is representative of the Front Face
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<br />Rockies (South). Notice that the peak flow is approximate-
<br />ly twice as large utilizing the Frederick method as com-
<br />pared to the Type II distribution. , Figure 6 illustrates a
<br />flood hydrograph for Gillette, Wyoming, which is repre-
<br />sentative of the Front Face Rockies (North). Once again,
<br />the peak nows diller in magnitude but not as severe as
<br />Parker, Colorado. Within the Pacific Northwest (East)
<br />section, the roles are reversed with respect to peak flows
<br />depending on the distributions used. Figure 7 illustrates
<br />the flood hydrograph for a 1 DO-year 24-hour event in Spo-
<br />kane, Washington, where the Type II distribution yields a
<br />higher peak flow than the Frederick method. Finally, when
<br />the Pacific Northwest (West) is analyzed (see Figure 8) with
<br />regard to the flood fl~ws from I OO-year 24-hour event, the
<br />technique developed by Frederick illustrates a substantial
<br />difference in peak value flows occurs. Figures 5 through 8
<br />illustrate that when determining peak flood nows, if storms
<br />do not have time distributions representative of the area
<br />under investigation. substantial errors can result. Conse.
<br />quently. it is important that when the hydrologic engineer
<br />determines peak flood nows, a time distribution that ade.
<br />quately represents how precipitation falls on an area be used
<br />to ensure accuracy.
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<br />TIME 1110 !lOURS
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<br />figure S. lOO.Ycar 24.Hour Hydrograph for ParKer, Colorado.
<br />
<br />SUMMARY AND CONCLUSIONS
<br />
<br />For the Western United Slates, the hydrologic engineer
<br />must realize that a 24.hour climatic event is the accull1u1:'l..
<br />lion of precipitation for 24 hours aflcr IneaSllre11lenl of
<br />initial rainfall. Since severe nood damage from ungaged
<br />watersheds can occur during the thunderstorm season, it is
<br />essential to adequately represent the time distribution of
<br />precipitation for the area being analyzed. A time distribu-
<br />tion of precipitation developed by Frederick can better
<br />simulate climatic conditions in the Western United States
<br />
<br />I
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<br />
<br />533
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