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FLOOD10336
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Last modified
1/26/2010 10:13:03 AM
Creation date
10/19/2007 11:55:18 AM
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Template:
Floodplain Documents
County
Statewide
Community
State of California
Basin
Statewide
Title
Alluvial Fans in California - Identification, Evaluation and Classification
Date
5/1/2000
Floodplain - Doc Type
Educational/Technical/Reference Information
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<br />. <br /> <br />RESULTS OF THE SURVEY OF CALIFORNIA COUNTIES <br /> <br />. <br /> <br />Each of the 58 counties in California was contacted for information determined to be <br />useful in the alluvial fan study. All contacts were made by telephone, The cities within each <br />county were not contacted for this study. <br /> <br />RESULTS OF SURVEY <br /> <br />After discussions with the DWR and other Federal, State, and local agencies, the Corps <br />developed a questionnaire for use in soliciting information from the counties (see Attachment I). <br />The information included geologic and geomorphic characteristics of the alluvial fan landform, <br />watershed information, and flooding and flow deposit characteristics. Following are the basic <br />information subjects and findings from the survey (see Attachment 2). The completeness of the <br />responses often depended on the knowledge of the contact person that provided answers to the <br />questions. <br /> <br />Definition of Alluvial Fan <br /> <br />The only regulatory definition of an alluvial fan, which has been adopted by California <br />counties, has been FEMA's definition. FEMA's current definition of an alluvial fan is "a <br />sedimentary deposit located at a topographic break such as the base of a mountain front, <br />escarpment, or valley side, that is composed of streamflow and/or debris flow sediments and has <br />the shape of a fan, either fully or partially extended', based on the February 23, 2000, Guidelines <br />for Determining Flood Hazards on Alluvial Fan. However, a few counties have adopted <br />ordinances to conform to their communities' requirements in addition to the NFIP's regulations as <br />outlined in FEMA's Manual 37 Guidelines and Specifications for Study Contractors (1995), and <br />section 65.13 ofFEMA's National Flood Insurance Program regulations for managing alluvial <br />fan flood hazards. <br /> <br />A-I <br /> <br /> <br />Classification System for Alluvial Fans <br /> <br />No county provided a classification system for alluvial fans, <br /> <br />Watershed Information <br /> <br />Based on information from the survey, extensive geologic and geomorphic characteristics <br />of alluvial fan watersheds are not being used to assess alluvial fan flooding by California <br />counties. However, most respondents concurred that understanding a fan's watershed is very <br />important. It is vital that engineers and flood plain administrators investigate these watershed <br />characteristics. This investigation should focus on the erosive formations, sediment sources, and <br />geologic processes contributing to the growth and evolution of the fan. The watershed size can <br />be used to estimate the magnitude of the I OO-year runoff. The condition of the watershed will <br />yield clues to the nature of the flow events. Fires, logging, grazing, avalanches, vegetation, slope <br />stability, and glacial deposits are all factors that should be assessed since they contribute to the <br />flow magnitude, Alluvial fan flooding and the possibility of mud or debris flow is a direct <br />
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