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SUMMARY 3 t "'. :i<Z' I<:,.~ '. , , .' ." " f . " it , ; (< r 1 t., " , ., ;~ rfi:r, :i~ , ,t+..'.' ;rJ;:~~~1-y 'kv~>~?: ~4\:~~1~ z~ ,'" ,;, v An alluvial fan is a sedimentary deposit located at a topographic break that is composed of fluvial and/or debris flow sediments and that has the shape of a fan either fully or partially extended, as illustrated by the Hanaupah Canyon alluvial fan in Death Valley, California. Courtesy of H. W. Hjalmarson. I , The committee also notes that the potential for erosion and deposition, the related uncertainty in flow path behavior, and the imprudence of elevation on fill as a mitigation measure are joint and separate characteristics shared among many flood hazards on depositional environments other than alluvial fans, although not usually with the same intensity. It stands.to reason that some of the same rules should apply to this more inclusive type of flood hazard, which the committee calls uncertain flow path flooding, as apply to alluvial fan flooding, which is, in fact, a type of uncertain flow path flooding. In the simplest case, a fan is shaped like a simple cone emanating from a single, well- defined apex. In such a case, a stream follows more-or-less radial paths down the cone, and the contours on the map of such a fan are convex downslope. However, the fan shape may not always be so apparent; for instance, it is obscured where the sedimentary accumulations from several source areas encroach on one another. At their downstream margins, fans merge with smoother depositional topography of the valley floor, river terraces, and lake and coastal deposits, and the channels may be small, shallow, and diffuse. Fans differ from pediments, some of which are cone. shaped, in that fans are formed by the accumulation of sediment, while pediments are erosional surfaces that are usually covered by a thin veneer of alluvium and colluvium.