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<br />CONTENTS
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<br />4
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<br />APPLYING THE INDICATORS TO EXAMPLE FANS
<br />Henderson Canyon, California, 83
<br />Thousand Palms Wash, California, 93
<br />Lytle Creek, California, 96
<br />Tortolita Mountains, Arizona, 100
<br />Carefree, Arizona, 102
<br />Rudd Creek, Utah, I II
<br />Humid Region Alluvial Fans, lIS
<br />Summary, 125
<br />BOX
<br />When It Is Not a Fan, But It Acts Like One, 128
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<br />2
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<br />ALLUVIAL FAN FLOODING
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<br />agreement on the definition of alluvial fan flooding and clear guidelines that can help planners,
<br />regulators, and citizens reach a common understanding of what an alluvial fan is and when it
<br />presents a flood hazard, To help FEMA with this problem, the Committee on Alluvial Fan
<br />Flooding was established and charged to develop a revised definition of alluvial fan flooding, to
<br />specifY criteria that can be assessed to determine if an area is subject to alluvial fan flooding, and
<br />to provide examples that illustrate the definition and criteria.
<br />To begin, the committee needed a clear definition of "alluvial fan," Working from
<br />standard geologic definitions, the committee defines an alluvial fan to be " a sedimentary deposit
<br />located at a topographic break, such as the base of a mountain, escarpment, or valley side, that is
<br />composed of streamflow and/or debris flow sediments and that has the shape of a fan either fully
<br />or partially extended," This deposit is convex in cross-profile, On a smooth cone-shaped fan,
<br />floodwater can spread widely across the surface in the same way that marbles will follow random
<br />paths down a gently sloped surface. Alluvial fans evolve through geologic time, and their
<br />evolution is affected by climate change and tectonics, and therefore a wide variety of fan
<br />morphologies can be observed, from the ideal smooth surface on which flow paths can be
<br />predicted only with great uncertainty to deeply incised fans with flow confined to a single channel.
<br />In the latter case, the flow path can be predictable, and the fan surface is not susceptible to major
<br />flooding. As a result, neither the automatic assumption of uniform flood risk on an alluvial fan nor
<br />the acceptance of complete uncertainty of flooding across an alluvial fan is reasonable.
<br />The committee decided that the first step necessary to reduce the confusion was to define
<br />alluvial fan flooding as a flood hazard that occurs only and specifically on alluvial fans. According
<br />to the committee, alluvial fan flooding is characterized by flow path uncertainty so great that this
<br />uncertainty cannot be set aside in realistic assessments of flood risk or in the reliable mitigation of
<br />the hazard, The committee has determined that an alluvial fan flooding hazard is indicated by three
<br />related criteria: (1) flow path uncertainty below the hydrographic apex, (2) abrupt deposition and
<br />ensuing erosion of sediment as a stream or debris flow loses its competence to carry material
<br />eroded from a steeper, upstream source area, and (3) an environment where the combination of
<br />sediment availability, slope, and topography creates an ultrahazardous condition for which
<br />elevation on fill will not reliably mitigate the risk. .'
<br />The committee notes that alluvial fan flooding typically begins to occur at the
<br />hydrographic apex, which is the highest point where flow is last confined, and then spreads out as
<br />~heetflood, debris slurries. or in multiple channels along paths that are uncertain. The
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