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<br /> <br />FLOODING PROCESSES AND ENVIRONMENTS ON ALLUVIAL FANS <br /> <br />39 <br /> <br />deposition produces a gradient that is steeper than that required by intervening floods to transport <br />the sediment load supplied by the source basin or produces sufficient channel bank strength to <br />confine water flows to depths sufficient to transport the sediment supply. In this case, the water <br />floods will scour away some of the debris flow sediment, establishing a lower-gradient channel <br />incised within the debris flow deposits. At some distance down the fan, where the gradient of the <br />debris flow sediment surface has diminished, the required stream gradient intersects the fan <br />surface and a single-thread or braided channel or a swath of sheet flooding emerges from the fan- <br />head trench at what Hooke (1967) called an intersection point, that is, a transition between flow <br />and sediment transport process regimes. <br />In other cases, trenching at the fan head or even over the entire fan may occur as a result <br />of channel incision of older fan deposits, either because the sediment supply has diminished or <br />because the transport capacity has increased owing to climate or vegetation changes within the <br />source basin, or to tectonism. The roles of climatic change and tectonism in trenching the heads of . <br />fans are reviewed thoroughly by Bull (199 I). <br /> <br />FLOODING PROCESSES ON ALLUVIAL FANS <br /> <br />Flooding on Streamflow Fans <br /> <br />Since streamflow alluvial fans typically occur in arid and mountainous environments, one <br />of the first difficulties encountered in the quantification of alluvial fan flooding processes is the <br />magnitude-frequency relationship for flows supplied to the apex. The sparseness of hydrologic <br />monitoring stations in such regions and the shortness of most records render most estimates of <br />probable flood discharges highly uncertain. Fans receive high water discharges from hurricanes or <br />typhoons on the subtropical eastern sides of continents, and from more localized rainstorms or <br />from intense, persistent snowmelt in mountainous western North America. In southern Europe, <br />particularly in southeastern Spain, the most destructive discharges are again generated by <br />rainstorms. In each of these regions, the history of hydrologic analysis and prediction has been <br />one of surprises. <br />Stream flooding on alluvial fans differs from most riverine flooding in that the hazard not <br />only derives from the inundation itself, but also is intimately connected with sedimentation <br />processes. These latter have immediate impact during the flood itself, and they have long-term <br />geomorphic influence through the rearrangement of sediment on the fan. High flood stages in <br />channels are accompanied by high flow velocities, and by heavy loads of floating wood, and other <br />debris in some environments. High velocities are promoted by the relatively steep, hydraulically <br />simple nature of the channels. The flood hazard is markedly increased, however, by the potential <br />for channel change during the flood itself The loose bed material may be scoured several meters <br />deep. On some fans the loose, unconsolidated nature of the sediments allows rapid channel <br />widening by bank collapse if the flood persists for several hours or days. On others, the deposition <br />of bars along a channel margin causes the channel to shift against the opposite, concave bank at <br />rates of up to tens of meters per flood. Thus, rapid scour and filling of the channel cause changes <br />in the channel conveyance capacity between and during floods. <br />The largest and most widespread threat arises, however, through the process of avulsion <br />("tearing away") in which water escapes from a channel by scouring a new path through the bank. <br />