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<br />Wenachee, Washington, mapped alluvial fan areas and adopted special regulations <br /> <br />The maps were later sustained in court (see Figure 3-3). <br /> <br /> <br />Rancho Mirage, Whittier, Palm Desert, and Palm Springs, California have adopted <br /> <br />alluvial fan regulations. <br /> <br /> <br />Clark County, Nevada and the City of Las Vegas have adopted a master plan for <br /> <br />an alluvial fan system and regulations to implement the plan. <br /> <br /> <br />OPTIONS FOR COMMUNITY ACTION <br /> <br />Policy and Program Elements <br /> <br /> <br />A community with alluvial fan flooding should adopt a policy and program which <br /> <br />contains the following elements: <br /> <br />I. A statement that alluvial fans are much more hazardous than shallow flood- <br /> <br /> <br />ing areas or normal riverine floodplains due to the combined erosion and <br /> <br />flooding problem; <br /> <br /> <br />2. A mapping program, perhaps as an overlay system for existing land use base <br /> <br /> <br />maps; <br /> <br /> <br />3. Special standards for siting and construeting on fan areas to address veloc- <br /> <br /> <br />ity, debris and erosion; <br /> <br /> <br />4. A master drainage and development plan for the fan as a whole, including <br /> <br />eareful design and siting of roads, drainageways and other public works on <br /> <br />the fan; <br /> <br /> <br />5. Construction of debris basins or other engineering measures for the fan, <br /> <br />partieularly where existing development is at risk. <br /> <br />C-4 <br /> <br /> <br />Mapping <br /> <br />At a minimum, alluvial fan areas should be identified on flood maps as high risk <br /> <br /> <br />areas. Some alluvial fans have already been identified on flood insurance study maps as <br /> <br /> <br />"shallow flooding" areas. Fans so designated should either be remapped or an overlay map <br /> <br /> <br />should be prepared to indicate areas where water velocities, debris, erosion and ehannel <br /> <br /> <br />migration are potential problems. <br /> <br />It may be possible to identify alluvial fan areas at modest eost using existing air <br /> <br /> <br />photos, soils maps and topographic data. Alluvial fans are often highly visible in arid and <br /> <br /> <br />semi-arid areas due to their distinctive shape and the presence of boulder trains. In <br />