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. I . . PREFACE This hydrology addendum is part of a program by the Colorado Water Conservation Board (CWCB) to make approximate 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 delineations on the maps, and it provides floodplain management information to guide the local governments in the use of the maps. The official floodplain information for Chaffee County is the Flood Insurance Study (PIS) with its associated Flood Insurance Rate Maps (FIRMs), dated March 4, 1987. The PIS was prepared by FEMA, and includes engineering information for those streams that were studied in detail. However it does not include any engineering information to describe basis of the approximate fluodplain mapping, The initial purpose of this report, therefore, is to provide the County with limited supporting information for the approximate floodplain delineations, 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 FEMA mapping, In addition, the 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 PIS meets the basic technical requirements for designation and approval by the CWCB. Without a hydrologic analysis, the approximate floodplain information in the Chaffee County FrS cannot qualify for CWCB designation. Colorado statutes require that local governments obtain from the CWCB the designation and approval of floodplain information for its use in regulatory purposes. This addendum provides a means for the PIS to meet that statutory objective, The FIS was prepared prior to the completion of a CWCB document entitled Guidelines for Determining lOO-year Flood Flows for Approximate Floodplains (Guidelines). The Guidelines provide the necessary hydrologic information in support of the approximate I DO-year floodplain boundaries identified on the Flood Insurance Rate Map (FIRM) panels, The hydrologic information contained in the Guidelines was used in preparing this addendum and it supports the subsequent designation of the Chaffee County FIS.