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. . . PREFACE This Hydrology Addendum is part of a program by the Colorado Water Conservation Board (CWCB) to help make the use of floodplain maps prepared by the Federal Emergency Management Agency (FEMA) easier for local governments to use. This CWCB report provides limited engineering information to support the approximate floodplain delineations on the maps. The official floodplain information for Custer County are the Flood Hazard Boundary Maps (FHBM's) dated August 13, 1976. The FHBM's were prepared by FEMA, however this study does not include any engineering information to describe the basis of the approximate floodplains. The initial purpose of this hydrology addendum, therefore, is to provide the county with limited supporting information. This addendum includes a description of a hydrologic analysis that was performed for the county as part of the CWCB's technical assistance program. The hydrologic analysis is consistent with the approximate floodplain information shown on the FHBM panels. In addition, this addendum gives the county some basic information about floodplain management and it gives some alternatives that could be considered in formulating and implementing its floodplain management objectives. A second function of this report is to assure that the FHBM's meet the basic technical requirements for designation and approval by the CWCB. Without a hydrologic analysis, the approximate floodplain information on the FHBM panels for Custer County cannot qualify for CWCB designation. Colorado statutes require that local governments obtain from the CWCB the designation and approval of floodplain information prior to its use for regulatory purposes. This report provides a means for the FIS to meet that statutory objective. The FHBM was prepared prior to the completion of a CWCB document entitled Guidelines for Determining lOO-year Flood Flowsfor Approximate Floodplains (Guidelines). The Guidelines provide the necessary hydrologic information in support of the IOO-year floodplain boundaries identified on the FIRM panels. The hydrologic information contained in the Guidelines was used in preparing this report and it supports the subsequent designation of the Custer County FHBM's.