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I I I I I I I I I I I I I I I I I I I 4.3 Hydrologic Analysis A complete analysis of the hydrologic cycle needs to be determined. The drainage basin and its corresponding area for the watershed leading to the community are defined. A stream-gauging station in the area is used to obtain information on the flood heights and discharges. if such a station exists. The United States Geological Survey (USGS) has more than 7,IX>O of these stream-gauging stations, some of them in operation since before 1900 (Perry. 2000). The basin charactcrL'itics are established for the floodplain area that is being analyzed. The IOO-year peak flow along with peak flows for other frequencies as needed. is determined with one of the following methods: Regional regression equations. statistical analysis of gage data. or rainfalVrunoff. The choice is made on the amount of infonnation that is available from recorded gauging stations. The relative locations of the gauging stations are a factor for deciding a method to be used. The final results are then compared with published data. 4.3.1 Analysis of/he Magnuude and Frequency of Floods in Colorado, USGS, 2000 The regressIon analysis developed by the USGS was the ordinary least-squared and the generalized least-squared methods. Flood magnitudes as a function of the drainage basin characteri..tics were expressed through multiple regression equations. Base 10 logarithmic transfonnations are used to nonnalize the data and to obtain linear relationships. The equation og Ql = log K + a log A + b log B + ...n log N here: Ql = The estimated nood magnitude (cfs) K = Regression constant A. B. .. .N: Values of drainage basin characteristics a. b. ...n = Regression coefficients :quation I (USGS, 2000) The values for each of the variables listed above can be found by referring to the USGS document. 18