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<br />Characteristics Identified by Counties <br /> <br />Some characteristics associated with alluvial fans observed and briefly described by the <br />county engineers include the following: fan-shaped or cone-shaped landform with visible contour <br />line formation, incised or v-shaped channels, braided channel with channel depth decreasing <br />toward downstream, overall or local sediment deposition, decreasing slope downstream, low <br />channel capacity, frequent clogged or blockage of channels by debris, frequent channel <br />movement, weak soil development on fan surfaces, channel flow change to sheetflow during fan <br />flooding, abandoned or discontinuous channels on fan, sediment bulking, stream avulsion, and <br />channellbank erosions. <br /> <br />A-3 <br /> <br /> <br />Observed Flooding Problems by Counties <br /> <br />Based on information and data from the survey, alluvial fan flooding generally consists of <br />mass erosion, debris and mud flow, and/or landslide, high sediment deposition, high velocity <br />flow, meandering flow, shifting or migration of flow paths, overbank flow, sheetflow, and mass <br />deposition in urbanized areas, The contributing factors to these problems are associated with the <br />watershed geology, fan topography and geomorphology, and other factors such as fire, bad <br />logging practices, seismic activity, influence by man's activities, and related land use <br />management. <br /> <br />A majority of the counties that have been affected by alluvial fan flooding recognize <br />alluvial fan flood hazards, and they were able to describe the most common problems and some <br />general characteristics of those fans. However, many engineers and flood plain administrators <br />lack the thorough knowledge and a standard procedure to identify and classify alluvial fans based <br />on their geomorphic, weathering, and geologic characteristics. Contributing to the problem are <br />limited resources and unavailable funding to initiate any detailed study on those alluvial fans. <br /> <br />COUNTY NEEDS AND INTEREST <br /> <br />While FEMA's current FIRM maps and NFIP's regulations provide some assistance, most <br />ofthe counties need assistance (a guidance manual) to identify, evaluate, classify, and delineate <br />alluvial fans in their areas. Many counties have also shown interest and support in this study to <br />establish a simple procedure to identify and classify alluvial fans in California. <br /> <br />CONCLUSIONS <br /> <br />The survey results show that there is no defined definition or classification system that <br />any county has adopted or is using to evaluate or classify alluvial fans. Local agencies are using <br />FEMA's current definition and general guidelines which govern the National Flood Insurance <br />Program for land use management on alluvial fans. Counties that are affected by alluvial fan <br />flooding need guidance and mitigation measures to help resolve problems associate with alluvial <br />fan flood hazards. Due to the lack of an accepted procedure to identify, evaluate, and classify <br />alluvial fans, a county may not even be aware of a fan that exists in its jurisdiction. <br />