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54 ALLUVIAL FAN FLOODING ~~ _ ~ . """ r II' j ',;1. fr '*~'::~ o ill <andl>;,t. WiIFjU~;te~:' ~oil ~oclety .!Jf~~ric ~~u,sbac~ ~ ii, d;P! !.r,~djng~~l~S'pa!.i: ~ . zSoc,rtyf.6r ca'~pe~al ~~li~ati~it, ~nO'lii .'?},. .il!~iads;B, 1;: .~.o<lf.ft\lPrdJJlsSe~,sment~orAaffi uS'e~14!mi ~. . ;;":}S~v~o. tal~~a~eni.itJO(l)j:l~lOjll";PI...~' m, }l:f!l;; a~II:R~ .f5a~4sli 1 ~94 ~he 7 !l~lIIfic!lP~ 9f~~. ~assessment. Z,,;Geomo""h. N~F..'3l!:487-500.'" !L- h~' ..~ ....t''''' -' .... . . ~" ''''' ...__ .0;, ~;'-';.__ .._ ~-_~--~__ "4<-.;;n --.. _~.pq\1II< '7Ji,lf$i4'l;'; STAGE 1: RECOGNIZING AND CHARACTERIZING ALLUVIAL FAN LANDFORMS Determining Whether or Not a Landform Is an Alluvial Fan The committee's definition of alluvial fan flooding specifically states that it occurs on alluvial fans. As a consequence, the first step in application of the definition is analysis of the area being considered for possible alluvial fan flooding. If this area does not meet the criteria for the definition of an alluvial fan, then it does not quality for consideration of alluvial fan flooding. The committee defines an alluvial fan as "a sedimentary deposit located at a topographic break sllch as the base of a mountaill front, escarpmel/t, or valley side, that is composed of streamflow al/dJor debris flow sedimel/ts and which has the shape of a fan, either fully or partially extel/ded. " These characteristics can be categorized as composition, morphology, and location, as follows. Composition Alluvial fal/s are lal/dforms COl/structed from deposits of alluvial sediments or debris flow materials, To meet the criteria in the committee's definition of an alluvial fan, the landform of interest must be a sedimentary deposit, an accumulation of loose, unconsolidated to weakly consolidated sediments. In the following text, we use the term "alluvium" to refer to sediments transported by both streams and debris flows, but we emphasize that this is a grammatical convenience. On a particular fan, the distinction between these two forms of sediment transport is critical to a correct interpretation of the flood hazard. Most sediments deposited during Quaternary time (2 million years ago to the present) still are loose and unconsolidated, as the processes of diagenesis that result in compaction, cementation, and lithification require millions of years to transform sediment to sedimentary rock. As a consequence, geologic maps commonly have a unit labeled" Qa!" that conventionally is mapped in yellow and represents Quaternary alluvium. Determining whether or not a landform is an alluvial sedimentary deposit might be as simple as checking a published geologic map to see if the underlying material is mapped as alluvium. If a geologic map is not available, the user can