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<br /> <br />FLOODING PROCESSES AND ENVIRONMENTS ON ALLUVIAL FANS <br /> <br />41 <br /> <br />and then increasing the volume of that predicted flood peak to account for the observed sediment- <br />water ratio in a recent debris flow (which usually triggered the concern by surprising the agency in <br />the first place) (Brunner, 1992). Although such a procedure might give a reasonable answer for <br />those hyperconcentrated flows generated by runoff processes, it is wrong to mix in a probability <br />analysis the results of runoff processes (gauging station records of floods) with debris flows, <br />which are the usually triggered by some form of mass failure. <br />A particularly misleading situation arises when the assumption of interannual independence <br />that has been found to be a useful approximation for rain-generated and snowmelt floods is <br />applied to debris flow occurrence. This is because the occurrence of a debris flow in one year <br />substantially reduces the probability of future debris flows by removing the sedimentary <br />accumulations required for their generation and growth (Benda and Dunne, 1987; Keaton, 1988; <br />Keaton et aI., 1988). Fortunately, it is often possible to identifY through field observations those <br />conditions that favor the generation and growth of debris flows. For example, deep accumulations <br />of colluvium on bedrock indicate a relatively high probability of debris flow occurrence in <br />comparison to that in a basin in which most of the colluvium was evacuated in a relatively recent <br />meteorologic event, after a forest fire, or after a climatic change. Thick accumulations of sediment <br />along channels upstream of a fan indicate that no debris flow has passed for a considerable <br />amount oftime and therefore that the conditions are evolving toward a failure that could convey <br />large quantities of sediment from canyon floors to the fan. Such observations combined with a <br />probability analysis of rainfall or snowmelt required to trigger a mass failure are required for <br />estimating the debris flow risk at the apex. Estimating the probable magnitude is more time- <br />consuming, since it requires documenting volumes of sediment in old debris flow deposits or in <br />the valleys above the fan. <br />Avulsions of debris flows occur on boulder-rich fans and are particularly difficult to <br />forecast because of the uncertainty about the magnitude and rheology of the next debris flow. <br />However, some clues to the likelihood of an avulsion occurring can be obtained from field <br />inspection of the morphology and sedimentology of the fan itself In particular, useful indications <br />of the avulsion potential might be provided by (I) the volumes of sediment susceptible to <br />liquefaction in the source area (Keaton, 1988; Keaton et aI., 1988) and therefore the likelihood of <br />a peak discharge great enough to overtax the conveyance capacity of the channel for such a debris <br />flow; (2) previous blockage of the main channel by bouldery berms; (3) relatively low channel <br />banks near the apex or in the vicinity of any blockage in the main channel; and (4) the topography <br />of the fan surface at these locations. Calculations of the channel conveyance capacity for debris <br />flows with a range of rheology can be made for various channel cross sections down the fan to <br />judge the potential for overbank flow and spreading (Whipple, 1992; Whipple and Dunne, 1992). <br />At the distal margins of debris flow fans, low-strength flows often spread widely in a manner <br />similar to sheetflooding on streamflow alluvial fans (Figure 2-6). <br />A particularly hazardous situation arises on debris flow fans around active volcanoes <br />because of the huge volumes of sediment that can be liquefied and the persistence of the <br />liquefaction. For example, the October 1994 lahar (volcanic debris flow) generated by a typhoon <br />from the slopes of Mt. Pinatubo in the Philippines deposited approximately 50 million cubic <br />meters (1.8 billion cubic feet) on an urbanized and cultivated alluvial fan. The North Fork Toutle <br />River lahar generated by the Mt. St. Helens eruption of May 18, 1980, deposited approximately <br />100 million cubic meters (3.5 billion cubic feet) of debris (Fairchild, 1985). Such volumes <br />