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<br />1--- <br /> <br />Guidelines and Specifications for Flood Hazard Mapping Partners [April 2003] <br /> <br />study methods may be used regardless of the current flood insurance risk zone designation. <br />Considerations for the use of detailed coastal study methods are similar to those for detailed <br />riverine study methods. <br /> <br />Detailed Study-Alluvia/Fan <br /> <br />This data update method entails the evaluation and mapping of alluvial fan flood hazard areas. <br />Through the approach for alluvial fan flooding identification and mapping documented in <br />Appendix G of these Guidelines, the Project Team will be able to identify whether (1) the area <br />under study is an alluvial fan and (2) which portions of this area, if any, are characterized by or <br />subject to active alluvial fan flooding. After these steps, various methods unique to different <br />situations can be employed to analyze and defme the I-percent-annual-chance flood within the <br />areas of alluvial fan flooding identified on the alluvial fan. Thus, the approach for the <br />identification and mapping of alluvial fan flooding can be divided into three stages. <br /> <br />. Stage I-Recognizing and characterizing alluvial fan landfoffils; <br /> <br />. Stage 2-Defining the nature of the alluvial fan environment and identifying a.ctive and <br />inactive areas of the fan; and <br /> <br />. Stage 3-Defining and characterizing the l-percent-annual-chance flood within the <br />detmed areas. <br /> <br />Approximate Study-Riverine or Lacustrine <br /> <br />This data update method entails using topographic data, typically without bathymetry or <br />bridge/culvert opening geometry, to conduct approximate hydrologic and hydraulic analyses. An <br />approximate analysis results in the delineation of a I-percent-annual-chance floodplain but does <br />not include the determination ofBFEs or base flood depths. <br /> <br />Generally, approximate-study methods are appropriate for areas where no flood hazards have <br />been identified but which are thought to be floodprone. If these areas are experiencing light to <br />moderate development and these trends are expected to continue, then approximate-study <br />methods are appropriate. Likewise, approximate-study methods may be used for areas that were <br />already mapped based on an approximate study and where development is minimal to moderate, <br />but where experience indicates that the current SFHA delineation is inadequate. Approximate- <br />study methods are not to be used for flooded sources that have already been studil~d using <br />detailed study methods. <br /> <br />Redelineation-Riverine <br /> <br />This data update method involves no new analyses. This method uses effective information <br />(Flood Profiles and data tables from the FIS report, BFEs from the FIRMs, and supporting <br />hydrologic and hydraulic analyses) and new topographic data that are more up-to-date and/or <br />detailed than those used to produce the effective FIRM to redelineate the floodplain boundaries. <br />Redelineation of effective I-percent-annual-chance floodplain boundaries that were based on a <br />detailed study is appropriate when the discharges and BEEs are determined to be appropriate, the <br />floodplain boundary delineations is inadequate, and updated topographic data are available. It is <br /> <br />1-39 <br /> <br />Section 1.3 <br />