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<br /> <br />u <br /> <br />Salls <br />Soils infonnation was obtained from the Logan County Soil Survey <br />(U.S, Soil Conservation Service, 1976). Calculations of the weighted curve <br />n(ll1ri)ersareincludedintheTechnical Addendum (Leaf. 1983). In general. <br />soils on the four Sand Creek watersheds are in hydrologic soil group C. <br />They inclUde the ~eld.Platner-Ascalon and ~anter-Ascalon-Vona units. <br />which formed in eolian and alluvial sand and sandy loam materials. Land <br />use on the watersheds varies from gra~ing to the cultivation of dryland <br />crops. Soils are in good hydrologic condition. and curve numbers vary <br />from 74 for rangeland to 83 for crop land. The weighted curve number <br />for the basin is 78. An antecedent moisture condition II (AMe II) was <br />assumed in the analysis. <br /> <br />",8 <br />Q<< <br />Wo:: <br />:I: 0 <br />"'-' <br />0::0 <br />wu <br />!;(<!l <br />';:2 <br />",-' <br />wo:: <br />wW <br />.. a:: I- <br />"" U'" <br />~C1a:: <br />=>2<< <br />.E'<J: W <br />u.. "'2 <br /> <br />Precipitation <br />Precipitation frequency curves were developed for the Sterling area <br />usingdataandproceduresfromNOAAAt1as2{seeMi1leret.al,1973). <br />Precipitation duration curves for 5,10,50,100, and SOO-year recurrence <br />interval stonnsareshown1nF1gure 9 To estimate rainfall amounts for <br />storm durations shorter than one hour, the following lQ-minute interval <br />adjustment were used: <br /> <br />End of Period <br />(minutes <br />10 <br />20 <br />30 <br />40 <br />SO <br />60 <br /> <br />Percent of 1-Hour <br />Depth <br />8 <br />11 <br />35 <br />16 <br />11 <br />9 <br /> <br />These 10-minute intervals also provided <br />to develop the design hydroqraphs which <br />wellwiththerainfall1ntensltycurves <br />Hoskins and Associates (1957). <br /> <br />therainfal1 distribution pattern used <br />follow. The above compares reasonably <br />prepared for the City of Sterling by <br /> <br />,. <br /> <br />Desiqn HydrographS and Peak Flows <br />The U.S. Soil Conservation Service prtlcedure (see U.S. Soil Conse rva_ <br />tion Service, 1972) was used in computing design hydrographs, storm <br />runoff volumes, and peak flows for Sand Creek and waterSheds $-1 through <br />14 <br /> <br />" <br />85,'"' <br />"'0'" <br />..,., <br />~i" <br />:<L <br />