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<br />IXX. PHYSICAL PROCESSES OF FLOOD IUI,ZARDS ON ALLUVIAL li'ANS <br /> <br />The physical processes of flooding on alluvial fans are <br />briefly described to familiarize the reader with 1:he unique <br />problems experienced when analyzing fan flows. This discussion <br />will be helpful in understanding SCJllIe of .the features designed in <br />the two-dimensional model for the analysis of mudflows and channel <br />avulsion. <br /> <br />3.1 Alluvial Fan Evolution <br /> <br />The average rate of vertical accretion of alluvial fans in the <br />semiarid western United states <::an be measured in terms of <br />centimeters per thousands of years (French, 1987). Most fan <br />material, however, is deposited by a limited number of extreme flow <br />events. As alluvial fans grow, the!y may coalesce into alluvial <br />aprons. If the sediment supply from the upstream watershed <br />decreases, a channel or wash may incise in the fan. <br /> <br />The growth of an alluvial fan is episodic in nature, <br />alternating between periods of deposit and ent,renchment. <br />Identifying the depositional or erosional sequence is crucial to <br />the correct analysis of the flocding potential and design of <br />hydraulic structures. For example, an improperly designed flood <br />channel may fill with sediment and debris or conversely, a flood <br />containment wall may be undercut by local scour. <br /> <br />Most alluvial fans in the semiarid West are ephemeral and <br />precipitation dependent. The majori.ty of fan flows that: occur are <br />the result of landslides, prolonged or intense rainfall, snowmelt, <br />or a combination thereof. Flows can also be triggered by changes <br />in landuse or by catastrophic event:s such as volcanic eruption, <br />earthquake, fire, rockfall and dam failure. The vast majority of <br />flows, however, originate as rainfall-induced floods in areas of <br />erodible geologic formations. The propensity for a watershed to <br />generate mudflows depends on rainfall or snowmelt, geology, soils, <br />topography, vegetation, and landuse,. <br /> <br />The growth of an alluvial fan requires a source of sediment. <br />Watersheds with noncohesive sediments will generally produce flow <br />events which are essentially water flows. Those watersheds with <br />cohesive sediments and large percentages of fine sediment will <br />generate mudflows. Alluvial fans constructed with noncohesive <br />sediments can extend several miles onto the valley floor and often <br />have a braided channel zone in the lower portion of the fan. These <br />fans are relatively uniform in topo~rraphy and shape and have a mild <br />slope compared with alluvial fans created by mudflows. Mudflow fans <br />tend to be shorter, steeper and more irregularly shaped with <br />natural levees and boulder deposits. <br /> <br />4 <br />