Alluvial Fan Flooding (1996) / Chapter Skim
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Because such a designation can affect development opportunities, it can be controversial. NFTP regulations define alluvial fan flooding to be " flooding occurring on the surface of an alluvial fan or similar landform which originates at the apex and is characterized by high velocity flows; active processes of erosion, sediment transport ant} deposition; and unpredictable flow paths." In addition, although aiinviai fan pooping is a general term that can involve flooding over an entire surface, the FEMA mandate is to determine the extent of hazard associated with a flood with a 100-vear recurrence interval (i e a 1 percent nrohahilitv in a riven -- -a -- - rid rim ~ -- - ~ c7- -\ IT_ _ _ _ ^1~ _ _ 7!
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Alluvial fans evolve through geologic time, and their evolution is affected by climate change and tectonics, and therefore a wide variety of fan morphologies can be observed, from the ideal smooth surface on which flow paths can be predicted only with great uncertainty to deeply incised fans with flow confined to a single channel. In the latter case, the flow path can be predictable, and the fan surface is not susceptible to major flooding.
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It stands to reason that some of the same rules should apply to this more inclusive type of flood hazard, which the committee calls uncertain flow path,,flooa~ing, as apply to alluvial fan flooding, which is, in fact, a type of uncertain flow path flooding. In the simplest case, a fan is shaped like a simple cone emanating from a single, welldefined apex.
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For this reason, it is essential that any investigation of alluvial fan flooding include careful examination of the specific fan for which information is needed by specialists experienced in the study of alluvial fan processes and recognition of geomorphic indications of past and present flooding. The committee recognizes that the extent of site-specific examination may be constrained by factors such as the amount of time and money allocated to the project, the tools available to the investigator, and the investigator's experience.
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From page 5... ...
· Regulatory flexibility is necessary to realistically depict flood hazards given the variability in flood processes on alluvial fans. · The existing regulatory framework, which divides all flooding sources into either riverine or alluvial fan flooding, leads to inconsistency when imposed on specific sites.
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