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<br />FLOODING PROCESSESAND ENVIRONMENTS ON ALLUVIAL FANS <br /> <br />47 <br /> <br /> <br />topography at and below sites of structural failure. This suggests that (1) major flood control <br />works are necessary to mitigate flood hazards on active alluvial fans, (2) predevelopment fan <br />topography influences the location of major flooding even after fans are urbanized and minor <br />flood control structures are in place, and (3) flood control works must be designed to address <br />specific types of hazards and special design consideration should be given for areas where water <br />can still reach after flood control structures are installed, <br />The small Glendora alluvial fan is highly urbanized, and the paths of floodwater for a flood <br />in January 1969 were significantly controlled by the network of streets that cross the fan parallel <br />and at right angles to the general direction of fan slope (Figure 2-7). The alluvial fans of <br />Cottonwood Creek and Stuart and Crane Gulches are also highly urbanized, and the paths of <br />floodwater are significantly controlled by the conveyance capacity and location of many streets. A <br />FIRM showing alluvial fan flooding hazards based on the assumption of uniform risk, thereby <br />ignoring the channels created by the streets, would be inappropriate for these urban areas. <br />The Horseshoe Park and Wadi Mikeimin alluvial fans each were formed during a single <br />flood. The Horseshoe Park alluvial fan was formed by a catastrophic flood from a dam failure. <br />The fan formed at a topographic break at the mouth of the Roaring River. Wadi Mikeimin also <br />formed at a break in floodflow confinement at the mouth of a river. The Roaring and Mikeimin <br />River fans were significantly modified by erosion following the fan-building floods. <br />Abnormally large volumes of sediment were produced by storm runoff from recently <br />burned basins of the Montrose, Cottonwood Creek, and Wasatch front alluvial fans. The close <br />proximity of these fan basins to urbanizing areas may result in a greater incidence of wildfires with <br />associated debris flows and alluviation on urbanized fan surfaces. <br />Reported flood characteristics of the sample of fans include high-velocity water flows, <br />debris flows, translatory waves, sheetflood, distributary flow, unstable and stable channel <br />boundaries, movement of flow paths, stable flow paths, and alluviation on the unchanneled fan <br />surface. The composite of these accounts of flooding shows a wide variability of processes and <br />flood hazard in time and space, which places a premium on field inspection and interpretation of <br />concrete evidence from each alluvial fan before a determination is made of flood risk. <br /> <br />REFERENCES <br /> <br />Beaty, C. B. 1963. Origin of alluvial fans, White Mountains, California and Nevada. Annals of the <br />Association of American Geographers 53:516-535. <br />Benda, L., and T. Dunne. 1987. Sediment Routing by Debris Flow. Publication. 165, Oxford, <br />England: International Association of Hydrological Sciences. <br />Brunner, G. W. 1992. Numerical simulation of mudflows from hypothetical failures of the Castle <br />Lake debris blockage near Mount St. Helens, Washington, Appendix A: Derivation of the <br />equation to calculated the ultimate bulking factor. In Program for Steep Streams <br />Workshop, Seattle, Washington, October 27-29, 1992. Portland, Ore.: U.S. Army Corps <br />of Engineers. <br />Bull, W. B. 1977, The alluvial fan environment. Progress in Physical Geography 1(2):222-270, <br />Bull, W. B. 1991. Geomorphic Responses to Climate Change, New York: Oxford University <br />Press. <br />