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<br />. <br /> <br /> <br />. <br /> <br />CHAPTER 4 <br />FLOODPLAIN <br />ADMINISTRATION <br />AND <br />aELlNEATION <br /> <br />~ECTION 2.0 <br />FLOODPLAIN <br />DELINEATION <br /> <br />COLORADO STATEWIDE <br />DRAINAGE AND FLOODPLAIN MANAGEMENT CRITERIA MANUAL <br /> <br />reseNoir formed by the dam will be filled to the elevation of the dam's <br />emergency spillway. The project engineer should coordinate with <br />appropriate govemment agencies and CWCB in determining whether a non- <br />flood control dam should be included in the analysis or not. <br /> <br />If existing dams are not included in the hydrologic analysis, discussions <br />should be provided in the report describing the dams and reasons why they <br />were not considered in the analysis. <br /> <br />2.10 ALLUVIAL FAN FLOODING <br /> <br />Alluvial fan flooding is quite different than a riverine flooding, and consequently, the <br />alluvial fan floodplains should be studied and delineated based on a different set of <br />criteria. Alluvial fan flooding can be characterized by unpredictable flow paths, mud- <br />flows, high flow velocity, and erosion and sediment deposition. Alluvial fans typically <br />do not have a well-defined channel capable of conveying a 100-year flows, although, <br />it is not unusual to have smaller defined channel(s). Typically, flood flows do not <br />spread over the entire alluvial fan surface, but are conveyed down from the apex to <br />the toe of the fan by a network of old and new flow paths/channels. <br /> <br />For detailed discussions on the floodplain analysis of active or semi-active alluvial <br />fans, readers are referred to the following publications: <br /> <br />. National Research Council, Alluvial Fan Flooding, 1996 <br />. Federal Emergency Management Agency, Guidelines for Determining Flood <br />Hazards on Alluvial Fans, February 23, 2000 <br />. Federal Emergency Management Agency, FAN. An Alluvial Fan Floodina <br />Computer Program & User's Manual, September 1990. <br /> <br />2.11 FLOODPLAIN DELINEATION STUDY REPORT <br /> <br />A floodplain delineation study is required in order to delineate and designate new <br />floodplains or modify existing floodplains. All floodplain information shall be <br />developed by a qualified hydrologist or hydraulic engineer under the direct <br />supeNision of a professional Civil Engineer registered in the State of Colorado, or by <br />an employee of a state or federal govemment agency that has executed a <br />memorandum of understanding or other written agreement with CWCB. The <br />memorandum of understanding shall be based on a written statement, which <br />demonstrates to the satisfaction of CWCB their equivalent qualifications to perform <br />such work in Colorado. <br /> <br />The floodplain delineation study shall be professionally prepared, legible and able to <br />be reproduced. <br /> <br />2.11.1 OUTLINE FOR A FLOODPLAIN DELINEATION STUDY <br /> <br />In general, the following points should be addressed through actual analysis <br />or through reference to adopted studies: <br /> <br />1. A description of the floodplain and channel areas (I.e. vegetation, <br />slopes, constrictions & etc.). <br /> <br />2. A description of the contributin9 drainage basin(s). <br /> <br />AUGUST 2002 <br /> <br />FLOODPLAIN DELINEATION <br /> <br />CH4-215 <br />