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<br />Guidelines and Specifications for Flood Hazard Mapping Partners [April 2003] <br /> <br />Use Effective Information-Alluvial Fan <br /> <br />This is the least expensive method of updating data for alluvial fan areas. No new analyses or <br />floodplain mapping are required; rather, the effective NFIP data are used "as-is." Mapped flood <br />hazard areas on the effective NFIP map that are not being updated through a reanalysis or <br />redelineation are carried over to the updated FIRM. For Flood Map Projects that entail <br />converting a manually produced FIRM to a digital FIRM, the effective information must be <br />digitized and fitted to the selected base map. <br /> <br />Before making a final decision to use the effective information, the Project Team shall review <br />the guidance provided in Appendix G of these Guidelines to determine whether (1) the <br />previously mapped area is, in fact, an alluvial fan and (2) which portions of this area, if any, are <br />characterized by or subject to active alluvial fan flooding. The Project Team shall then <br />determine whether the approach that was used in developing the flood hazard information shown <br />on the effective FIRM was appropriate or one of the other approaches discussed in Appendix G <br />may be more appropriate. If the Project Team determines that another approach may be more <br />appropriate, then the Project Team shall not use the effective information and shall instead <br />perform a detailed study as discussed later in this subsection. <br /> <br />Detailed Study-Riverine <br /> <br />This data update method entails using topographic data, channel bathymetry, and bridge/culvert <br />opening geometry to conduct detailed hydrologic and hydraulic analyses and floodplain <br />mapping. Detailed-study methods involve the determination and publication of BFEs. <br />Normally, a regulatory floodway will be determined if a flooding source is studied by detailed <br />methods. If a regulatory floodway along a particular flooding source has been developed and is <br />shown on the FIRM, and if the flooding source is being restudied, the new detailed study must <br />include the regulatory floodway. <br /> <br />Detailed-study methods may be used regardless of the current flood insurance risk zone <br />designation. They may be used to update a previous detailed study, to upgrade the analysis of an <br />area previously studied using approximate methods, or to map the SFHA in areas that were <br />previously unmapped. <br /> <br />If areas are experiencing or expected to experience moderate to dense development, then detailed <br />studies are important to provide BFEs and regulatory floodways to regulate safe construction in <br />these areas. This applies to residential, industrial, or commercial areas where growth is <br />beginning and/or subdivision is underway, and where these trends are likely to continue. They <br />include areas that are likely to be developed within 5 years following the completion of the <br />detailed study. <br /> <br />Detailed Study-Coastal <br /> <br />This data update method entails using transects and offshore bathymetry to conduct detailed <br />erosion, wave height, and wave runup analyses and prepare floodplain mapping. Detailed <br />coastal methods involve the determination and publication of BFEs and designation of the <br />coastal high hazard areas (V zones). As for detailed riverine study methods, detailed coastal <br /> <br />1-38 <br /> <br />Section 1.3 <br />