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2 Flooding Processes and Environments on Alluvial Fans FORMATION AND NATURE OF ALLUVIAL FANS Alluvial fans develop where streams or debris flows emerge from steep reaches in which they are confined to relatively straight and narrow channels and flow into zones where sediment transport capacity decreases because of increases in channel width, reductions in channel gradient, or other influences. The channels on fans range from decimeters to several meters deep, These conditions develop at mountain fronts, in intermontane basins, and at valley junctions where there are major breaks in gradient or channel confinement, allowing both deposition of sediment and the lateral movement of channels to spread the sediment into a fan-shaped landform (Figure 2-1). Fan formation is particularly favored where sediment loads are high, for example, in arid and semiarid mountain environments, wet and mechanically weak mountains, and environments that are near glaciers or active volcanoes. Deposition is particularly rapid where there is a reduction in the transport capacity of a heavily loaded stream. Alluvial fans occur in a wide range of environments, including the western and eastern mountains of the United States, western Canada, and various montane, arid, and volcanic regions around the world. In North America, most fans that have been subject to development are in the western mountainous regions. Fans occur in the Appalachian Mountains, but flooding on them has not yet been analyzed by FEMA because development pressure is not intense. However, minor local damage has occurred on some of these fans (Jacobson, 1993) and will no doubt increase as development pressure increases. In the simplest cases of widely spaced stream or valley sources, fan geometry may be a sector of a simple cone emanating from a single, well-defined apex. In this simple case, a stream follows more-or-Iess a radial path down the cone, and the contours on the map of such a simple fan are convex downslope (Figure 2-1). Overall radial profiles are usually concave or virtually straight, and cross-fan profiles are convex. Where the sedimentary accumulations from several source areas encroach on one another, or where the deposition is forced by gradual widening or slope reduction along a valley, the simple conical fan shape may not be easy to identity (Figure 2- 2). Coalescence may lead to a general accumulation of overlapping fans along a mountain front, called a bajada (Figure 2-3). At their downstream margins, fans merge with the smoother depositional topography of valley floors, river terraces, and lake and coastal deposits, and the :29