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<br />3.2 Fan and Wat.ershed Characteristics <br /> <br />A number of characteristics of upstream watersheds and fan <br />surfaces can be readily identified in field surveys and then <br />related to flood dynamics on fans. The following <br />characteristics are thought to be key factors in alluvial <br />fan flooding. The effect of these characteristics on flood <br />dynamics will be discussed in Section 4. Figure 3.3 <br />summarizes these characteristics. <br /> <br />watershed Slo~e and Veqetation <br /> <br />The watersheds associated with alluvial fans in the semi- <br />arid west tend to exhibit steep slopes and relatively sparse <br />vegetation. The range of slopes includes values of lOX or <br />less typical of Boise, Idaho and values of SOX and greater <br />in tWe southwest. Sparsely vegetated watersheds are common <br />in the LOS Angeles area where fires repeatedly clear large <br />areas of grass and brush. Watersheds in Boise tend to have <br />substantial grass cover with some trees. <br /> <br />Sediment Size and ~ <br /> <br />Typical sediment sizes on fans vary from less than 0.1 mm <br />(very fine sands, silts, and clays) on mud flow fans such as <br />those in the Los Angeles area to 4 mm (fine gravel) and <br />larger on fans with very rocky watersheds. Mud flow fans <br />also exhibit discontinuous deposits of very coarse gravels <br />and even boulders which are transported by the highly <br />viscous mud mixture. A gradation of sizes occurs naturally <br />on fans, with the coarsest sizes being deposited near the <br />apex and fine sizes being transported to the toe. A <br />predominance of cohesive soils (high clay content) in the <br />watershed and on the fan surface is typical of mud flow <br />fans. <br /> <br />30 <br />