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" ~ /8 ALLUVIAL FAN FLOODING distant areas by consideration of the scenano where all or part of the flow leaves the main channel. 4. The sandbar issue. For a wide, shallow wash, a base flood elevation determined using backwater conveyance calculations such as those performed by computer program HEC-2 (Hydrologic Engineering Center, 1990) might indicate local areas within the wash that are higher than the computed water surface and could therefore be shown outside the SFHA. This, according to FEMA, could allow someone to build a house on a sandbar in the middle of a braided alluvial wash. The aI/uvial fal/ flooding designation discourages this by including the sandbar within the area subject to flooding and disallowing the removal of the designation until some type of reliable mitigation is implemented. 5. The split flow issue. Because of variations in channel cross section shape during an event or because of blockage by sediment, a network of channels that progressively splits into smaller channels may not reliably distribute the flow of water and sediment in such a way that all of the channels remain stable and protect the higher interchannel areas from flooding. For the case where the channel network fails, the alluvial fal/ flooding designation enables one to identifY on the FIRM that the interchannel areas are at risk of being flooded even though they might be a meter or more higher than an adjacent channel. IMPLICATIONS OF ALLUVIAL FAN FLOODING AS A DISTINCT TYPE OF FLOODING Because these regulatory difficulties exemplity specific weaknesses in the riverine flooding paradigm, they portray some of the essential elements that make alluvial fan flooding a distinct type of flooding and they will help formulate the committee's revised definition. Because of its character, alluvial fan flooding offers particular challenges to floodplain managers and regulators. Figure I -4 compares the current paradigm that governs the analysis of ordinary riverine flooding, as viewed by the NFIP, to alluvial fan flooding. It contrasts the flood risk on two surfaces, labeled 1 and 2 in each case. The regulations require the identification of areas that have a I percent chance of being flooded in any given year. This process starts by the development of a graph showing peak flood discharge from the watershed plotted versus its recurrence interval. For most alluvial fan source areas, this step is tenuous at best and may be completely illusory in the cases where the events of significance are debris flows or where recent or frequent disturbance by fire makes it futile to view the watershed merely as a generator of independent rainfall-runoff events. However, this aspect of the uncertainty is not as controversial, perhaps because the methodology is widely used. After discharge is specified, the riverine approach and alluvial fan flooding approach to analysis and risk prediction diverge. In the riverine case (Figure 1-4a), location of the flood within the spatial domain is assumed to be along the perceived or historical flow path, which is recognized as the main channel (Figure 1-4a(ii)). Much work is then necessary to identity the flood hazard by determining the relationship between depth and discharge (Figure 1-4 a(iii)). This is usually done using Manning's equation or a similar technique embedded within a step- backwater computer model. Finally, the flood hazard is delineated via the water surface elevation, :~, ?~ ';"j