FLOOD02285 - Laserfiche WebLink

Home Browse Search

8 ALLUVIAL FAN FLOODING ": and local public officials (see Box I-I). The committee's approach involved examining the hydrologic and geomorphologic processes that characterize flooding on alluvial fans in a range of varied environments. Understanding these processes from a natural science perspective provides the committee's basis for evaluating how current NFIP practice might be improved to better characterize flood hazards and to more accurately delineate zones of flood risk. The committee's revised definition of alluvial fan flooding is presented in this chapter. Later chapters provide an overview of flood and sedimentation processes on alluvial fans (Chapter 2), describe field indicators and methods to delineate hazard boundaries based on the revised definition (Chapter 3), and give examples of applying the field indicators to specific sites (Chapter 4). Chapter 5 presents the committee's conclusions and recommendations. Because the establishment of this committee was requested by FEMA, that agency and its consultants are the primary audience for this report. However, the committee hopes that communities participating in the NFIP, other agencies, and floodplain management professionals in general will also appreciate this effort to better manage natural hazards in alluvial fan environments. ORIGIN OF THE PROBLEM Following a series of damaging floods in the southwestern part of the United States during the 1970s, FEMA sought a new approach to flood risk assessment in areas where flow paths are difficult to predict. Pictures of these floods are shown in FEMA Document 165 (FEMA, 1989), and the images are memorable (see Figure I-I): water and debris flows along new paths not anticipated by planners and residents of the normally dry landscape, automobiles crushed by boulders, a house full of sand. These pictures depict a type of natural hazard different from ordinary riverine flooding. The hazard on active alluvial fans is less foreseeable, more difficult to control or resist, and more dangerous. Many of the pictures in FEMA Document 165 are of flooding on alluvial fans. The term applied by the NFIP to this image was alluvialfal/flooding, and the purpose of Document 165 was to explain how such floods occur. As our understanding changes of how floods occur, regulators develop new policies that eventually become formalized by the writing of regulations. Special rules were thus promulgated to regulate development and to set insurance premiums in areas subject to alluvial fan flooding. Conflict arose when Flood Insurance Rate Maps (FIRMs) prepared by contractors to FEMA were criticized by some participating communities. Most criticism focused on the underlying assumption (or "default assumption") in the FEMA procedure that flooding on alluvial fans is completely unpredictable, an assumption that is not always appropriate (French et aI., 1993). I The Problem of Delineating Flood Hazards on Alluvial Fans Faced with an increased need to map flood risk on alluvial fan areas in the I970s and 1980s, and needing a method that can be applied at reasonable expense, FEMA adopted an analytical technique proposed by Dawdy (1979). The procedure uses a general mathematical formula (known as the conditional or total probability equation) to describe the probability of an