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<br /> <br />INDICATORS FOR CHARACTERIZING ALLUVIAL FANS <br /> <br />63 <br /> <br /> <br />.. <br />"':."'.:"$~; <br /> <br />..' <br />.-..~.._, <br /> <br />. ~. . '-","-', . <br /> <br />~. - -~ ...... <br /> <br />:'.", <br /> <br />--,',;" <br /> <br />..,(\ <br />......---....- . <br /> <br />.-;"C.' <br /> <br />'- <br /> <br />'1., <br /> <br />".-'.... \ . <br /> <br />~. . .-;i . <br /> <br />-~;; <br /> <br />:: / <br />." ;' ~... . <br /> <br />:f.;~' <br />. .....-<.. <br /> <br />~. ,. <br />"'::::' <br /> <br />.~ .' \.' <br /> <br />'i/:'l~ <br /> <br />FIGURE 3-4 Entrenched alluvial fan with deposition occurring atlhe distal part <br />oflhe fan. SOURCE: Reprinted with permission from Bull (1977). <br /> <br />than 1,000 years really is active, that is, that there is some likelihood of flooding and <br />sedimentation under the present climate. A systematic approach to the problem of estimating <br />whether a place may be inundated after a long period of inactivity is suggested in Chapter 2, the <br />section" Change Over Time," which deals with processes by which flooding and deposition can <br />migrate across an alluvial fan to invade places that have long been outside the zone of active <br />deposition, even in the current climate, Since there is no clear analytical technique for making <br />such projections, estimates of the probable spatial extent of inundation involve systematically <br />applied judgment, and the combination of hydraulic computations and qualitative interpretations <br />of geologic evidence concerning the recent history and probable future evolution of channel <br />forms, as well as flooding and sedimentation processes. The problem becomes even more difficult <br />when one considers the likelihood that the environmental conditions affecting the generation of <br />floods or debris flows in the source area may not have remained constant in time. <br />Judging where flooding and deposition might occur is particularly important in cases <br />where land use patterns are substantially altered by human activity (e.g., the Wasatch Range, <br />Utah) or where recent decades have been marked by more intense storm patterns. As an example <br />