Question:
| What is the definition of Alluvial Fan Flooding? |
Answer:
| As defined in 44 CFR 59.1, the current (2000) definition of "Alluvial Fan Flooding" means 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, and deposition; and, unpredictable flowpaths. The current definition under 44 CFR 59.1 will be revised to be consistent with the approach described in this appendix and specifically to eliminate reference to "similar landforms". The process described in this appendix is intended for flooding only on alluvial fans as described below. As interim guidance in the determination of "similar landform", unless the landform under investigation meets the three criteria under Stage 1 for composition, morphology, and location, the landform is not considered to be "similar." As established by the Guidelines, the following definitions are used; Alluvial Fan -- An alluvial fan is a sedimentary deposit located a topographic break such as the base of a mountain front, escarpment, or valley side, that is composed of streamflow and/or debris flow sediments and which has the shape of a fan, either fully or partially extended. Alluvial Fan Flooding -- Flooding occurring on the surface of an alluvial. Active alluvial fan flooding is a type of flood-hazard that occurs only on alluvial fans. It is characterized by flow path uncertainty so great that this uncertainty cannot be set aside in realistic assessments of flood risk or in the reliable mitigation of the hazard. An active alluvial fan flooding hazard is indicated by three related criteria:(a) flow path uncertainty below the hydrographic apex, (b) abrupt deposition and ensuing erosion of sediment as a stream or debris flow loses its competence to carry material eroded from a steeper, upstream source area, and (c) an environment where the combination of sediment availability, slope, and topography creates an ultrahazardous condition for which elevation on fill will not reliably mitigate the risk. Inactive alluvial fan flooding is similar to traditional riverine flood-hazards, but occurs only on alluvial fans. It is characterized by flow paths with a higher degree of certainty in realistic assessments of flood risk or in the reliable mitigation of the hazard. Counter to active alluvial fan flooding hazards, an inactive alluvial fan flooding hazard is characterized by relatively stable flow paths. However, areas of inactive alluvial fan flooding, as with active alluvial fan flooding, may be subject to sediment deposition and erosion, but to a degree that does not cause flow path instability and uncertainty. An alluvial fan may exhibit both active and inactive alluvial fan flood hazards. |
Last Updated: 2007-01-08 10:50:42.0
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