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• Precipitation: <br />P10 - 1.7 in. <br /> <br />~• <br />P25 = 2.1 in. <br />(10 year - 24 hr. percipitation event from <br />the Precipitation - Frequency Atlas of the <br />Western United States, Volume 111 - Colorado). <br />(25 year - 24 hr. precipitation event from <br />the Precipitation - Frequency Atlas of the <br />Western United States, Volume 111 - Colorado). <br />Assume Soil Group B <br />Streets and roads: gravel CN = 85 <br />Assume Antecedent Moisture Condition (AMC) II <br />S = 1000 - 10 <br />CN <br />(SCS Method) <br />S = 1000 - 10 = 1.76 <br />.85 <br />Runoff: (SCS Method) <br />2 <br />410 = (P 10-0.25) _ (1.7-(0.2)(1.76)2 = 0.58 in. <br />P10+0.8S 1.7+0.8 (1.76) <br />2 <br />425 - (P25-0.25) _ (2.1-(0.2)(1.76)2 = 0.87 in. <br />P25+0.85 2.1+0.8 (1.76) <br />Peak Discharge: <br />For calculation of the peak discharge use "Procedures for <br />Determining Peak Flows in Colorado. Incorporates and Suppli- <br />ments Technical Release No. 55 - Urban Hydrology for Small <br />Watersheds". Refer to Chapter 5 - Tabular and Graphical Methods <br />of Determining Peak Discharges. <br />