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<br />11 <br /> <br /> <br />Managing Existing Development <br /> <br />General guidelines for managing alluvial fan areas include the following: <br /> <br />· A trained and qualified geologist, hydrologist, or engineer should determine the alluvial <br />fan landforms and identify areas at risk from flooding as described in steps I through 3. <br />Where appropriate, the active and inactive portions of the fan should be delineated on a <br />map. <br /> <br />· The areas of the fan which require immediate flood protection should be identified, <br />taking into account the location and density of existing development. Regulatory <br />guidelines for existing structures should be set. For areas at risk from alluvial fan <br />flooding, future development should be restricted. <br /> <br />· Mitigation measures for an alluvial fan should be site- and application-specific for <br />mitigating the associate hazards. The range of flood management and mitigation <br />measures available should be analyzed, taking into account the identified hazards, <br />existing structures, anticipated future development, cost versus benefit to the locality, and <br />public acceptance. The final decision should be based on community goals reached <br />through consensus and coordination among local government officials, planners, resource <br />managers, engineers, landowners, residents, and the development community. <br /> <br />. Local government officials should ensure effective flood plain management by adopting <br />and enforcing regulatory controls applicable to Federal, State, and local standards. <br /> <br />. Communities should initiate public awareness programs on alluvial fan hazards, <br />encourage the purchase of Federal flood insurance, and promote preventive measures to <br />reduce risk (FEMA 1989). <br /> <br />· Inspection/maintenance requirements for structural mitigation measures such as debris <br />basins, channels, debris fences, or floodwalls can help guarantee their effective <br />functioning and operation. <br /> <br />Structural measures can be used to protect existing development from flooding on <br />alluvial fans. The selection of measures should be site- and application-specific based on the <br />size and density of the development. At times, it may be more economical to protect the entire <br />fan with a large-scale project when the area is highly developed and densely populated. In most <br />cases, however, site-specific or subdivision protection may be more economical and desirable, <br />depending on requirements set by Federal, State, and local criteria; physical and environmental <br />constraints of the structural measures; public acceptance; construction and maintenance costs; <br />and other factors. The intended life of the structural measure and the frequency of flooding are <br />critical factors when selecting a plan to protect against alluvial fan flooding. The importance of <br />these factors is directly related to the erosion/deposition potential of the fan. <br /> <br />For large-scale protection, levees, channels, detention basins, debris basins, or debris <br />dams can be adopted as structural measures. The structures can be designed to intercept and <br />