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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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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 />(3) developing fan identification criteria, identifying analytical procedures for alluvial fans, and <br />determining a base classification system; and (4) proposing land use planning, management, and <br />regulation measures for alluvial fan areas. <br /> <br />SURVEYS AND RESEARCH <br /> <br />Survey of California Counties <br /> <br />An important part of this research was directed at the local level (county government). <br />A questionnaire was developed which focused on the technical aspects of evaluating and <br />delineating alluvial fans. Each of the 58 counties in California was contacted by telephone, and <br />the questionnaire was used to request information and data determined to be useful in the study. <br />The cities within each county were not contacted separately for this study. Basically, the surveys <br />showed that most California counties lack the knowledge, procedures, and/or data necessary to <br />identify and evaluate alluvial fans in their areas. Appendix A includes the questionnaire and <br />discusses the results from the survey of all of the California counties. <br /> <br />3 <br /> <br /> <br />Survey of Other Agencies <br /> <br />As part of this study, many other Federal, state, and local agencies in the arid western <br />states were also contacted for alluvial fan information and data, These agencies included FEMA, <br />the Association of State Flood Plain Managers, the Flood Control District of Maricopa County, <br />Arizona, and the Committee on Alluvial Fan Flooding of the National Research Council. These <br />agencies provided several publications containing background information for the Corps of <br />Engineers report. These publications included Alluvial Fan Flooding by the National Research <br />Council (1996) and Piedmont Flood Hazard Assessment for Flood Plain Management for <br />Maricopa County, Arizona, by Hjalmarson (1998), <br /> <br />Other Investigations and Research <br /> <br />Other sources of information included books, journals, conference proceedings, the <br />Internet, and personal communications with several geologists, civil engineers, National Flood <br />Insurance Program coordinators, and other water resources specialists with a particular interest in <br />alluvial fans. The review of this published literature and other information confirmed that the <br />identification and evaluation of alluvial fans and alluvial fan flooding are complex and require <br />detailed hydraulic, geologic, and geomorphic data specific to the area. A background description <br />of alluvial fan formation and flooding is included in Appendix B. <br /> <br />DEVELOPMENT OF CLASSIFICATION SYSTEM <br /> <br />Definition of an Alluvial Fan <br /> <br />An alluvial fan can be defined as "a sedimentary deposit at a topographic break, such as <br />the base of a mountain front, escarpment, or valley side, that is composed of fluvial and/or <br />debris flow sediment and which has the shape of a fan either fully or partly extended" (NRC, <br />1996). FEMA has adopted the new alluvial fan definition and recently issued new "Guidelines <br />
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