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<br /> <br /> <br />ALLUVIAL FAN HAZARDS IN THE UNITED STATES <br /> <br />Flood hazards in the American West are often greatly under- <br />estimated due to the dry conditions, lack of rainfall and ab- <br />sence of defined watercourses. Ironically, western floods are <br />qutte severe and powerful, exhibtting unpredictable flow paths <br />and high velocities that usually occur with Itttle advance <br />warning time. These floods can cause considerable erosion in <br />some areas while depostting large amounts of sediment and <br />debris in others. <br /> <br />With rapid growth continuing throughout the West (particularly <br />in many major metropolttan areas) hillside building sttes have <br />become more popular as the supply of prime developable land <br />becomes depleted. This has resulted in an increasing amount <br />of development occurring in floodplain areas called alluvial <br />fans. <br /> <br /> <br />Figure 1. <br />Dens. dBllelopm.ntonalluvlsl lans/n Ih. South MountalnafH ofPhOBnlx, <br />Arizona. Not. IJut numBrouB branching channws. <br /> <br />Alluvial fans are triangularor fan-shaped, gently-sloping land- <br />forms which typlly the floodplain management dilemma facing <br />many western states today: fans provide attractive develop- <br />ment sites due to their commanding views and good local <br />drainage, yet harbor all the severe flood hazards which <br />endanger arid western communities. <br /> <br />Development pressure on alluvial fan areas is intensifying, <br />creating a crttical need to provide guidance to communtties, <br />developers and cttizens on how to safely accommodate growth <br />while protecting life and property from flood hazards. To <br />address this need, the Federal Emergency Management <br />Agency (FEMA) has designed this brochure to increase aware- <br />ness of alluvial fan flood hazards, and to provide general <br />guidance on the techniques and strategies for minimizing <br />losses from these hazards when building and developing. <br />FEMA is the agency which administers the National Fiood <br />Insurance Program (NFIP), which enables property owners <br /> <br />within participating communities to purchase insurance to <br />cover flood losses affecting structures and their contents. <br /> <br /> <br />Flgu,..2. <br />Damag9d horn.. on. fIIn In Ocotlllo WtN'/s, CaJlfomJa, which 116. In Hst.rn <br />S.n OhJgo County. Tropical Storm K.thltH.In cau..d .xt.ns/vB flooding <br />Septsmbflr ~1 '. 1976, thlHfllmd throughout RivlIrs/ch and Imp6rlal coun- <br />rles 101M north and NsI. TM eh.nmll bfllldJng shown In the photo Is <br />typical ollllluvl.1 "m "ooding, and thus II Is difncun to predict thelocat/on <br />of floodflows. <br /> <br />Where Are Alluvial Fens Found? <br /> <br />In the Untted States, alluvial fans are typically found along the <br />base of mountain fronts in the western states of California, <br />Arizona, Nevada, Colorado, Utah, Idaho, New Mexico, Wyo- <br />ming, Montana, Oregon and Washington. Here, the infrequent <br />but intense storms typical of arid and semi-arid climates <br />combined with abrupt changes in topography create the nec- <br /> <br />1 <br />