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Last modified
1/26/2010 10:12:48 AM
Creation date
10/5/2006 5:05:15 AM
Metadata
Fields
Template:
Floodplain Documents
County
Statewide
Basin
Statewide
Title
Guidelines for Determining Flood Hazards on Alluvial Fans
Date
2/23/2000
Prepared By
FEMA
Floodplain - Doc Type
Educational/Technical/Reference Information
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<br />- <br />!. <br />- <br />. <br />. <br />. <br />- <br />- <br />. <br />. <br />. <br />- <br />. <br />. <br />. <br />. <br />'. <br />. <br />. <br />. <br />. <br />I. <br />I. <br />- <br />. <br />I- <br />. <br />. <br />I- <br />- <br />. <br />- <br />. <br />. <br />- <br />I- <br />. <br />- <br />. <br />. <br />. <br />. <br /> <br />2 <br /> <br />02/23/00 <br /> <br />assessments of flood risk or in the reliable mitigation of the hazard. Counter to active alluvial fan <br />flooding hazards, an inactive alluvial fan flooding hazard is characterized by relatively stable flow <br />paths. However, areas of inactive alluvial fan flooding, as with active alluvial fan flooding, may be <br />subject to sediment deposition and erosion, but to a degree that does not cause flow path instability <br />and uncertainty. <br /> <br />An alluvial fan may exhibit both active alluvial fan flooding and inactive alluvial fan flooding <br />hazards. The hazards may vary spatially or vary at the same location, contingent on the level of flow <br />discharge. Spatially, for example, upstream inactive portions of the alluvial fan may distribute flood <br />flow to active areas at the distal part of the alluvial fan. Hazards may vary at the same location, for <br />example, with a flow path that may be stable for lower flows, but become unstable at higher flows. <br /> <br />An example of an alluvial fan that exhibits both active and inactive alluvial fan flooding is depicted <br />in Figure I. In this example, the area between the topographic apex and the hydrographic apex (apex <br />definitions will be discussed below) would be considered inactive alluvialfanflooding because this <br />reach is characterized by a stable, entrenched channel which can convey the 100-year flood discharge <br />without overbank flooding. The area below the hydrographic apex would be considered active <br />alluvial fan flooding because this area is characterized by flow path uncertainty, abrupt deposition, <br />and ensuing erosion of sediment as the channel loses its competence to carry material eroded from <br />a steeper, entrenched upstream source area. <br />
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