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<br />forested areas, fans may be more difficult to map. Topographic and soils maps may be <br /> <br /> <br />used to identify areas with steep slopes and alluvium. <br /> <br /> <br />An outline of alluvial fan areas can, even without more detailed identification of <br /> <br /> <br />risk zones within the fans, be an important "red flag" for land use decisions. Once out- <br /> <br /> <br />lined, fans can be zoned as high risk areas, Developers can be required to conduct de- <br /> <br />tailed studies of the flood risk and design accordingly. <br /> <br /> <br />Engineering methods, although quite expensive, are available for mapping fans in <br /> <br />more detail and determining the relative risks within the fans, With these maps, zoning <br /> <br /> <br />regulation can be quite specific. However, site-specific studies and master planning will <br /> <br /> <br />still be needed as new development is proposed. <br /> <br />Regulations <br /> <br /> <br />Regulations for alluvial systems should have two principal goals: to prevent accel- <br /> <br />eration or diversion of runoff and increased erosion, and to insure that individual struc- <br /> <br /> <br />tures and infrastructures are adequately protected from high velocity flows, debris and <br /> <br /> <br />erosion. <br /> <br /> <br />If the fan is undeveloped, future flood damages can be avoided by prohibiting de- <br /> <br /> <br />velopment. Development should only be permitted if a master plan has been prepared. An <br /> <br />alluvial fan master plan should show the drainage system, roads, grading and filling <br /> <br /> <br />needed for drainageways, debris walls and other flood protective measures, such as bank <br /> <br /> <br />stabilization, erosion control measures and floodways to be maintained as open space. <br /> <br /> <br />Where the fan is in multiple ownership, the community should prepare the master plan. <br /> <br /> <br />Developers can be required to implement their portion as a condition of plat approval or <br /> <br />building permits. Las Vegas takes this approach. Where the fan is in single ownership, <br /> <br /> <br />regulations may require the developer to prepare a master plan for the fan as a whole. <br /> <br /> <br />Zoning, subdivision controls or grading codes can be adopted to limit development <br /> <br /> <br />densities, impervious surfaces and modifications to natural topography. <br /> <br /> <br />To help protect individual structures, include the following provisions in your <br /> <br /> <br />building codes, zoning regulations, grading codes and subdivision regulations: <br /> <br /> <br />I. Prohibit building in areas where velocities exceed a selected threshold level <br /> <br /> <br />(e.g" 7 feet per second). <br /> <br /> <br />2. Require that structures in other areas be elevated on stabilized fill or rein- <br /> <br /> <br />forced pilings to a height above the IOO-year flood elevation, taking into ac- <br /> <br /> <br />count debris as well as water elevations. <br /> <br /> <br />C-6 <br />