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IV. FEMA METHODOLOGY FOR DELINEATING FLOOD HAZARD AREAS ON FANS 4.1 The Need for Flood Hazard Delineation on Fans Alluvial fans are estimated to cover 15 t:o 25 percent of the semiarid areas in the southwestern United states and they are becoming increasingly important areas for urban expansion (Kusler, 1986; DMA, 1985). Not only are thEl fans considered aesthetically desirable areas for their view and drainage assets, but their development is becoming inevitable as space on t.he valley bottomlands is exhausted. As development progresses up the alluvial fan, the flood hazard must: be reassessed. The diversity of possible alluvial fan flooding scenarios creates problems for floodplain manaqers delineating hazard zones. Shallow water flooding may cover a large area of the fan or mudflow deposition may inundate only a few buildings. Urban development must be prepared for both. The design of levees, debris basins, and drainage systems should include provisions for heavy sediment loading, high impact pressures, hydrostatic and hydrodynamic forces, and a wandering flow path. 4.2 Description of FEMA I S Met:hodology In 1968, Congress passed the Nat:ional Flood Insurance Program (NFIP) Act to protect investments and developments in flood prone areas. 'From the outset of the flood insurance program there was concern regarding the delineation of flood hazards on alluvial fans. Initial attempts to predict: alluvial fan flow hydraulics were based on step-backwater or shallow flooding analysis (Dawdy, et.al., 1989). Guidelines for evaluating alluvial fan flooding were eventually prepared to assess flood hazards with a probabilistic determination of flood hydraulics. The NFIP guidelines promulgated by FEMA outlined a procedure to delineate the 100-year flood hazard. The FEMA methodology was a pioneering approach to the practical evaluation of flood hazard delineation on alluvial fans. It determines flood hydraulics using a single channel probabilistic approach, the basis for which was ,a publication by Dawdy (1979). This procedure relates the probability of a discharge defined at the fan apex to the probability o:E preassigned velocities and depths occurring on the fan downstream. The depth and velocities shown on a Flood Insurance Rate, Map (FIRM) represent those associated with the one percent annual flood risk at the determined boundaries. The methodology assum'as that the probability of the flood hazard is the same for points radially equidistant from the fan apex; thus the boundaries separating areas of different preassigned depths and velocities generally follow topographic contour lines. The method determin<las 1:he zones for which the 100- year return period flood depth and velocities vary by less than one foot and one foot per second, respectively. 10