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<br />Guidelines and Specifications for Flood Hazard Mapping Partners [April 2003] <br /> <br />shall review coastal analyses carefully to determine whether all factors that are currently <br />considered in determining flood hazards have been considered. <br /> <br />Taking into account the multiple variables that can affect alluvial fans and their flooding <br />characteristics-including climate, fan history, vegetation, and land use-FEMA developed an <br />approach to identify and map flood hazards on alluvial fans that accounts for site-specific <br />conditions. The approach, documented originally in Guidelines for Determining Flood Hazards <br />on Alluvial Fans (FEMA, 2000) and detailed in Appendix G of these Guidelines, addresses <br />recommendations in a 1996 report prepared by the National Research Council Committee on <br />Alluvial Fan Flooding (National Research Council, 1997). For alluvial fan areas that were <br />identified and mapped before FEMA issued Guidelines for Determining Flood Hazards on <br />Alluvial Fans, Mapping Partners may want to consider an approach to evaluating alluvial fan <br />hazards other than the one used for the effective FIRM. <br /> <br />ComparinQ Recent FloodinQ Events to Effective Map <br /> <br />As part of determining flood data update needs, the Mapping Partner performing the Mapping <br />Needs Assessment shall compare the flood hazards shown on the effective FIRM to any <br />documented out-of-bank flooding that has been estimated by the community or a State or Federal <br />agency to be approximately equal to the I-percent-annual-chance flood, However, the Mapping <br />Partner shall exercise care not to assume that a mapping error exists on the FIRM on the basis of <br />historical flood events. The return frequency of flooding can vary greatly from stream to stream <br />or from one part of a stream to another, depending on the distribution of rainfall over the <br />drainage basin(s). For example, if precipitation is localized, flooding on a small tributary may <br />approach the magnitude of the I-percent-annual-chance event, but the flooding on the larger <br />receiving stream may be a much smaller magnitude event. Conversely, the main stream could be <br />experiencing flooding from rainfall in the upper watershed that does not affect the lower <br />tributaries, causing less severe flooding on the tributaries than the main stream. <br /> <br />The Mapping Partner performing the Mapping Needs Assessment may fmd that documentation <br />of observed rainfall amounts and high-water marks, including any photographs of flooding <br />events within the community, may be useful information to review. Anecdotal information on <br />flooding is not considered reliable unless it is combined with surveyed high-water marks and <br />includes the date and time of the high-water mark observation. The Mapping Partner also may <br />find that information about the performance of bridges and culverts during the flood event is <br />useful, particularly whether the carrying capacity of the bridge openings or culverts were <br />adequate or were exceeded or whether any bridge openings or culverts were clogged with debris <br />or ice. Photographs of bridges and culverts during flooding also may be useful. <br /> <br />AssessinQ Factors That Affect HvdroloQic Analvses <br /> <br />One of the primary components in riverine flooding analyses is the hydrologic analysis. The <br />methodology for hydrologic analyses is discussed in Section 3.0 of the FIS report. (Refer to <br />Appendix J of these Guidelines for further information regarding FIS reports.) Floodplain and <br />watershed conditions can change that would affect these analyses. <br /> <br />1-16 <br /> <br />Section 1.2 <br />