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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I. . . . . . . . . ENGINEERING METHODS Hvdroloeic Analvsis Table 2 lists the streams and reaches for which approximate or detailed floodplain boundary information is contained in the September 30, 1988 PIS. Discharge rates for streams studied by detailed methods are presented in the 1988 PIS and generally include the 10-, 50-, 100- and 500- . year events, Hydrologic analyses and discharge rates for approximate flooding sources are not documented by FEMA in the 1988 PIS, As previously discussed, Colorado statutes require that this information be provided in order for floodplain studies to be "designated" and thus made legally usable for floodplain regulation, This section of the report documents loo-year discharge values developed for approximate stream reaches using various methodologies and sources of data. loo-year discharge rates for approximate flooding sources in Pitkin County are presented in Table 3 for one or both of the following methods of computation: L Rel!.ional Regression, Regional regression equations for river basins in Colorado have been developed by the CWCB and are documented in Guidelines for Determining 100- Year Flood Flows for Approximate Floodplains in Colorado (CWCB, January 1999). Regression equations presented in this publication compute lOO-year peak discharge rates and are based upon data from floodplain studies previously designated by the ewCR Regression equations have been developed for nine major river basins within the state and a number of subbasins, The appropriate equation from these guidelines for all drainage basins within Pitkin County is the Upper Colorado River Subregion and is as follows: Q = 79,5(A)o.724 Equation 1 A = Drainage area in square miles (1<A<950 sq mi) Q = lOO-year peak discharge in cfs 2, Frequencv Analysis, Stream gage frequency analysis has been carried out for USGS gaging station locations where periods of record were judged to be of adequate length to produce meaningful results. Frequency analyses were carried out according to the document entitled Guidelines for Determining Flood Flow Frequency, Water Resources Council, Bulletin l7B (1981), Table 4 presents a list of all USGS gaging stations located on approximately studied streams within Pitkin County, Frequency analysis was not carried out for gages with less than lO-years of record, the minimum recommended by Bulletin #17R For those locations with gage records longer than 10 years, computed lOO-year frequency discharges are presented in Table 3, Analysis of the available data and methods of lOO-year flow computation are provided in the following paragraphs, Conclusions and recommendations on appropriate approaches to computing loo-year flows are also provided for each stream reach studied by approximate methods. 4