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. . . PREFACE This Hydrology Addendum (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 FEMA maps, and it provides floodplain management information to guide the local governments in the use of the maps. The official floodplain information for Jefferson County is the Flood Insurance Study (FIS) dated July 4, 1989. The FIS and accompanying Flood Insurance Rate Maps (FIRMs) were prepared by FEMA and they include both detailed and approximate floodplain information. The approximate information does not include any engineering data to describe the basis of the unnumbered Zone A delineations. 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 available hydrologic information for major flooding sources within or adjacent to the County. In addition, the Addendum gives the County some basic information about floodplain management and it provides some alternatives that could be considered in formulating and implementing floodplain management objectives. Much of the general information within this Addendum has been extracted from the Jefferson County FIS. A second function of this report is to assure that the FrS meets the basic technical requirements for designation and approval by the CWCB. Without the inclusion of basic engineering information, the approximate floodplain delineations shown on the Jefferson County FIS and FIRMs 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. 1bis Addendum provides a means for the FIS to meet that statutory objective. As new and more detailed floodplain information becomes available, the approximate floodplain areas shown on the FIS panels will become superseded. The CWCB will designate and approve new information, and rescind old information as appropriate.