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<br />. <br />. <br />. <br />. <br />. <br />I. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />'. <br />- <br />- <br />I- <br />- <br />. <br />e <br />Ie <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br /> <br />1 <br /> <br />02/23/00 <br /> <br />GUIDELINES FOR DETERMINING FLOOD HAZARDS ON ALLUVIAL FANS <br /> <br />1 INTRODUCTION <br /> <br />Alluvial fans, and flooding on alluvial fans, show great diversity because of variations in climate, <br />fan history, rates and styles of tectonism, source area lithology, vegetation, and land use. <br />Acknowledging this diversity, this document provides an approach that considers site-specific <br />conditions in the identification and mapping of flood hazards on alluvial fans. Investigation and <br />analysis of the site-specific conditions may require knowledge in various disciplines such as <br />geomorphology, soil science, hydrology, and hydraulic engineering. Although the scope of study <br />may constrain the degree of site-specific consideration undertaken, it is essential that field <br />inspections of the alluvial fan be conducted. <br /> <br />As defined in 44 CFR 59.1, the current (1999) definition of "Alluvial Fan Flooding" means <br />flooding that occurs on the surface of an alluvial fan or similar landform, originates at the apex, <br />and is characterized by high-velocity flows; active processes of erosion, sediment transport, and <br />deposition; and unpredictable flow paths. <br /> <br />The current definition under 44 CFR 59.1 will be revised to be consistent with the approach <br />described in this document and specifically to eliminate reference to "similar landforms." The <br />process described in this document is intended for flooding only on alluvial fans as described <br />below. <br /> <br />As interim guidance in the determination of "similar landform, " unless the landform under <br />investigation meets the three criteria under Stage 1 for composition, morphology, and location, <br />the landform is not considered to be "similar. " <br /> <br />This document provides guidance for the identification and mapping of flood hazards occurring on <br />alluvial fans, irrespective of the level of fan forming activity. The term alluvial fan flooding <br />encompasses what will be described as active alluvial fan flooding and inactive alluvial fan flooding. <br /> <br />Active alluvial fan flooding is a type of flood-hazard that occurs only on alluvial fans. It is <br />characterized by flow path uncertainty so great that this uncertainty cannot be set aside in realistic <br />assessments of flood risk or in the reliable mitigation of the hazard. An active alluvialfanflooding <br />hazard is indicated by three related criteria: (a) flow path uncertainty below the hydrographic apex, <br />(b) abrupt deposition and ensuing erosion of sediment as a stream or debris flow loses its <br />competence to carry material eroded from a steeper, upstream source area; and (c) an environment <br />where the combination of sediment availability, slope, and topography creates an ultrahazardous <br />condition for which elevation on fill will not reliably mitigate the risk. <br /> <br />Inactive alluvial fan flooding is similar to traditional riverine flood hazards, but occurs only on <br />alluvial fans. It is characterized by flow paths with a higher degree of certainty in realistic <br />