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FLOOD00209
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Last modified
11/23/2009 10:50:40 AM
Creation date
10/4/2006 9:09:08 PM
Metadata
Fields
Template:
Floodplain Documents
Designation Number
543
County
El Paso
Community
Unincorporated El Paso County
Title
Flood Insurance Study - Pitkin County, CO, and Incorporated Areas
Date
9/1/1988
Designation Date
3/1/2001
Contract/PO #
&&
Floodplain - Doc Type
Floodplain Report/Masterplan
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<br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />I. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br /> <br />ENGINEERING METHODS <br /> <br />Hvdroloeic Analvsis <br />Table 2 lists the streams and reaches for which approximate or detailed floodplain boundary <br />information is contained in the September 30, 1988 PIS. Discharge rates for streams studied by <br />detailed methods are presented in the 1988 PIS and generally include the 10-, 50-, 100- and SOO- . <br />year events. Hydrologic analyses and discharge rates for approximate flooding sources are not <br />documented by FEMA in the 1988 PIS. As previously discussed, Colorado statutes require that <br />this information be provided in order for floodplain studies to be "designated" and thus made <br />legally usable for floodplain regulation. This section of the report documents loo-year discharge <br />values developed for approximate stream reaches using various methodologies and sources of <br />data. <br /> <br />loo-year discharge rates for approximate flooding sources in Pitkin County are presented in <br />Table 3 for one or both of the following methods of computation: <br /> <br />1. Rel!.ional Regression. Regional regression equations for river basins in Colorado have been <br />developed by the CWCB and are documented in Guidelines for Determining 100- Year Flood <br />Flows for Approximate Floodplains in Colorado (CWCB, January 1999). Regression <br />equations presented in this publication compute 100-year peak discharge rates and are based <br />upon data from floodplain studies previously designated by the CWCB. Regression <br />equations have been developed for nine major river basins within the state and a number of <br />subbasins. The appropriate equation from these guidelines for all drainage basins within <br />Pitkin County is the Upper Colorado River Subregion and is as follows: <br /> <br />Q = 79.5(A)o.724 <br /> <br />Equation 1 <br /> <br />A = Drainage area in square miles (1<A<950 sq mi) <br />Q = 100-year peak discharge in cfs <br /> <br />2. Frequencv Analysis. Stream gage frequency analysis has been carried out for USGS gaging <br />station locations where periods of record were judged to be of adequate length to produce <br />meaningful results. Frequency analyses were carried out according to the document entitled <br />Guidelines for Determining Flood Flow Frequency, Water Resources Council, Bulletin 17B <br />(1981). Table 4 presents a list of all USGS gaging stations located on approximately studied <br />streams within Pitkin County. Frequency analysis was not carried out for gages with less <br />than lO-years of record, the minimum recommended by Bulletin #17B. For those locations <br />with gage records longer than lO years, computed 100-year frequency discharges are <br />presented in Table 3. <br /> <br />Analysis of the available data and methods of lOO-year flow computation are provided in the <br />following paragraphs. Conclusions and recommendations on appropriate approaches to <br />computing loo-year flows are also provided for each stream reach studied by approximate <br />methods. <br /> <br />4 <br />
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