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<br /> <br />CHAPTER 3. ALLUVIAL FANS <br /> <br />THE HAZARD <br /> <br />Alluvial fans are fan-shaped deposits of rock and soil which eroded <br />from moulllainsides and accumulated on the valley floors. The deposits <br />are narrow and steep at the head of the valley, broadening as they <br />spread aut onto the valley floor n hence the name fans. Rain runs off <br />of steep valley walls, gaining velocity and carrying large boulders and <br />ather debris, When the debris fills the runoff channels an the fall, flood <br />waters spill aut alld cut Ilew challllels. The process is thell repeated, re- <br />sultillg ill shiftillg challllels and combined erosioll alld floodillg prob- <br />lems Ol'er a large area. <br /> <br />Flood and erosion problems on fans are often complicated by the activities of man, <br /> <br /> <br />Roads act as drainage channels, carrying high velocity flows to lower portions of the fan. <br /> <br /> <br />Fill, leveling, grading and structures can divert waters and cause new and unexpected pat- <br /> <br />terns of flooding and erosion. <br /> <br /> <br />Alluvial fans occur principally in dry, mountainous areas, They are common in <br /> <br /> <br />California, Nevada, Utah, Arizona, Idaho, New Mexico, Wyoming, Montana and Washing- <br /> <br /> <br />ton, Some fans are also found in Alaska, West Virginia, Kentucky and Tennessee. <br /> <br /> <br />Flooding on alluvial fans causes greater damage than clear water flooding for sev- <br /> <br /> <br />eral reasons: <br /> <br /> <br />I. Floodwaters move at high velocities due to steep slopes and lack of vegeta- <br /> <br /> <br />tion. Velocities of 15 to 30 feet per second are common. At these velocities, <br /> <br /> <br />water has tremendous erosive force and damage potential. <br /> <br /> <br />2. Floodwaters contain large amounts of sediment and debris including boul- <br /> <br /> <br />ders and trees. <br /> <br />3. Floodwaters are not confined to a single channel. The channels meander, <br /> <br /> <br />threatening development over a broad area. <br /> <br />EXISTING MITIGATION EFFORTS <br /> <br />Often alluvial fans have been mapped and regulated as shallow flooding areas re- <br />quiring minimal flood protection through fill or elevation on pilings. These maps and reg- <br />ulations undercstimate actual risk on fans because they are based on depth of flooding <br />alone, They do not reflect potential damage due to high velocity, debris, erosion or the <br /> <br />C-I <br />