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<br />7 <br /> <br /> <br />topographic maps, soil maps and surveys, aerial photographs, documented reports of flooding <br />and debris flow, and/or field reconnaissance. The major advantage of using geomorphic evidence <br />is that it is physically and process-based. This method is superior to traditional engineering <br />modeling for delineating flood hazards because the magnitude of the 100-year flood peak <br />discharge is not the only important factor of the hazard. Other factors are (1) debris and sediment <br />in the floodwater, (2) changing flow paths due to sedimentation and erosion, (3) unconfined <br />floodwater in various flow paths, and (4) movement of sediment by later floods. <br /> <br />Risk-Based Analysis. The National Research Council (NRC, 1996) recommended <br />consideration of the use of Guidelines for Risk and Uncertainty Analysis in Water Resources <br />Planning (USACE, 1992) for specific guidelines on how to apply method for analyzing active <br />alluvial fans. The principles of risk-based analysis provide a framework for a more general and <br />realistic way to identify areas subject to active alluvial fan flooding. <br /> <br />FAN Program. FEMA has issued a draft "Guidelinesfor Determining Flood Hazards on <br />Alluvial Fans" dated February 23,2000. These guidelines include an alluvial fan modeling <br />program entitled "FAN". As a supplement to using geomorphic evidence, this program can be <br />used to evaluate active fluvial conical-shaped alluvial fans. <br /> <br />FLO-2D Model. For more complex as well as simple systems, the FLO-2D model can be <br />used to predict the flow depths and the flow boundaries on active alluvial fans with or without <br />debris flow. It is a two-dimensional, finite difference, flood routing model which can simulate <br />clear water, mud, and debris flooding over unconfined alluvial fans and flood plains. The <br />process-based FLO-2D model is practical for delineating debris flow fans because of the <br />multifunctional capability of the program. It should be noted that there are other computer <br />models available, however, the Corps of Engineers considers this program to be reliable for most <br />alluvial fan modeling problems. <br /> <br />Combination. Flexibility in choosing a method to define an active alluvial fan is <br />necessary since there is no clear analytical technique for evaluating and estimating the extent of <br />its flood hazard. Backwater models by USGS, NRCS, and others may be used for assessing <br />active alluvial fans. Site-specific evaluation is very important since the characteristics of two <br />fans are often quite different. The analytical process requires knowledge and the ability to apply <br />judgement regarding the hydraulic analyses and qualitative interpretations of geologic and <br />geomorphic evidence concerning the recent history and probable future evolutions of channel <br />forms, as well as flooding and sedimentation processes. For that reason, the combination of an <br />appropriate engineering modeling and the geomorphic evidence may be more reliable to <br />approximate the extent of hazard boundaries. <br /> <br />Existing or Planned Development. Since flood damages and loss of life are possible <br />anywhere on an active alluvial fan, the area must be planned, managed, and regulated based on <br />the degree of development in the area. For that reason, Step 3 also includes a determination of <br />the extent and type of development on the active alluvial fan, <br />