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<br />38 <br /> <br />ALLUVIAL FAN FLOODING <br /> <br />debris flows in the modern climate. Descriptions of large debris flow fans in Owens Valley, <br />California, are provided by Whipple and Dunne (1992), and smaller debris flow fans in a wetter <br />environment are described by Kellerhals and Church (1990). <br />One approach to flood risk on debris flow fans concludes that even on active fans the <br />probability of a debris flow is less than I percent in anyone year, and therefore the "IOO-year <br />flood" is not a debris flow but a runoff event. This is a generalization that fails to appreciate an <br />important aspect of debris flow initiation, namely, that it is not an independent, random event in <br />the same way that runoff floods are assumed to be. Debris accumulates in source localities and <br />along stream channels over timescales from decades to centuries between failures that evacuate <br />the debris (Benda and Dunne, 1987; Dunne, 1991; Reneau and Dietrich, 1991). Thus, a frequency <br />count of dated debris flows in a region might indicate that the average frequency of occurrence is, <br />say, 200 years per fan (with a probability of occurrence in anyone year of 0.5 percent). However, <br />if a geologist were to walk up anyone of the source basins, he or she might find many potential <br />failure sites and the channels below them to be occupied by thick layers of sediment that have <br />accumulated since the previous debris flow occurred centuries earlier. In a neighboring valley, <br />recent debris flows may have stripped such sediment from the valley and reset the clock so that <br />the probability of debris flow is virtually zero for the foreseeable future. Thus flood risk estimates <br />can be refined by first recognizing from field evidence that debris flows are the dominant sediment <br />transporting agent on a particular fan and then examining the source basin to determine whether <br />debris availability favors an enhanced risk of a debris flow in the event of a large rainstorm or <br />snowmelt. <br /> <br />Composite Fans <br /> <br />Many fans are fed by both water floods and debris flows. Others were formed by debris <br />flows under a different climatic regime and are now the sites of stream sedimentation and flooding <br />only. Thus, both streamflow sediments and debris flow sediments and their associated <br />morphologies attest to the nature of the flood risk on different parts of the same fan. The debris <br />flow sediments are usually concentrated on the upper, steeper parts of the fans, producing a <br />surface laced with berms, lobes, and channel plugs. The lower, streamflow part of the fan has the <br />characteristics of an alluvial fan described above, although there may also be a contribution of <br />dilute debris flow deposition on these distal areas. An indication of the relative contributions of <br />debris flows and water floods can be obtained through systematic identification and mapping of <br />the distribution of the two types of sediments on the fan surface and in vertical sections along the <br />sides of channels. <br /> <br />Incised Channels on Fans <br /> <br />At the heads of some alluvial fans, channels are strongly incised in a fan-head trench, from <br />which they emerge at some distance downfan to take on the character of a diverging braided <br />channel network or a linear, boulder-leveed channel characteristic of debris flow fans, as described <br />above. This report calls this point the hydrographic apex. Several reasons for the transition are <br />identifiable in the field. The simplest case arises on a composite fan where episodic debris flow <br />