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<br />30 <br /> <br />ALLUVIAL FAN FLOODING <br /> <br /> <br />FIGURE 2- I Sketch of a simple fan with single source and no incision. Contours are <br />convex downslope and closer together near Ihe apex. The dashed lines represent channels <br />that have not recently been invaded by water or debris flows. The solid, sinuous lines <br />emanating from the apex indicale channels that have conveyed flows recently. <br /> <br />channels may be small, shallow, and diffuse. Fans and bajadas are different from pediments, some <br />of which are cone-shaped, in that a fan forms through deposition, whereas a pediment is a <br />bedrock surface that is usually covered by a thin veneer of alluvium and colluvium. <br />Sediment may be transported to and across the fan by streamflow or debris flows. The <br />latter are slurries with such high sediment-water ratios and concentrations of fine sediment that <br />water cannot drain from them quickly enough to allow the sediment to settle out as traction load <br />on the channel bed. Instead, the slurries travel at speeds of several to more than 10 meters per <br />second (mls) as dense viscous mixtures involving particle sizes from clay to boulders several <br />meters in diameter. <br />Because the frequency, triggering mechanisms, size, and sedimentation processes of debris <br />flows are so different from those of water floods, and the morphology and other clues about the <br />nature of the flooding hazard on the respective types of fans are so radically different, it is <br />necessary to distinguish between streamflow fans (Bull, 1977) and debris flow fans (Whipple and <br />Dunne, 1992). Also, many fans are composites of stream and debris flow sediment. This chapter <br />