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I I I I I I I I I I I I I I I I I I I 12 07/16/99 because of the likelihood of fine-grained sediments and shallow groundwater; during prolonged rainfall, the ground can become saturated, resulting in extensive sheetflooding as runoff lI1Tives from upslope. Fine-grained sediments can aggravate the likelihood of sheetflow because some clay mincrals swell when wct, fonning an impenneable surface at the beginning of a rainstonn. 4D (3) Debris Flow Some parts of alluvial fans are characterized by debris flows, flows with a very high concentration of sediment in relation to water. Debris flows pose hazards that are very different from those of sheetflows or water flows in channels. Identifying those parts of alluvial fans where debris flow deposition might occur requires the eX!lIllination of deposits from past flows. Debris flow deposits can be distinguished from fluvial deposits by differences in mOlphology, depositional relief, stratigraphy, and clast fabric. Exposures in channel banks can be examined and can be supplemented with shallow trenches in different deposits. 4D (4) Unstable Flow Path Flooding Active areas of an alluvial fan will generally be characterized by unstable and uncertain flow path flooding. This type of flooding usually creates a single channel just below the apex, but splits into multiple channels as it proceeds down the alluvial fan. These channels are subject to deposition and bank or bottom erosion that cause channel migration, avulsion and the formation of new channels. Areas subject to this type of flooding are characterized by shallow, braided or distributary, sand to gravel bed channels. Recently fonned channels may have less established vegetation, such as trees, than older channels in the same general area. 5 STAGE 3: DEFINING THE IOO.YEAR FLOOD WITHIN THE DEFINED AREAS The National Flood Insurance Program (NFIP) uses the lOO-year flood, the flood having a I-percent chance of being exceeded in any given year, to delineate Special Flood Hazard Areas (SFHA). Stages I and 2 describe methods of identifying alluvial fan landforms and areas of active and inactive deposition, erosion, and unstable flow path flooding. Stage 3 determines the severity and delineates the extent of the IOO-year flood within any floodprone area identified in Stage 2. The broad spectrum of alluvial fan landfonns and types of flooding as previously discussed illustrates the futility of developing a "cookbook" method to apply to all funs in all geographic areas. The analysis ofthe flood hazards on alluvial fans therefore requires a flexible approach that is based on site-specific evaluations. Several methods for quantifying the lOO-year flood are presented in the following sections and are summarized in Table I. Not all methods are appropriate for all situations. The assumptions and limitations of each should be carefully considered in deciding which methods to apply to particular areas of an alluvial fan.