Laserfiche WebLink
<br />14 <br /> <br />ALLUVIAL FAN FLOODING <br /> <br />risk assessment and provides guidance about how to deal with uncertainty in methods such as <br />those of Dawdy (1979) and Mifflin (1990) and the procedures discussed at the end of Chapter 3. <br />3. A description of indicators that are used to assess whether criteria indicative of alluvial fan <br />flooding are present and which allow an investigator to discriminate between those zones of a fan <br />where flow paths are uncertain and other zones where flow path uncertainty is unimaginable in the <br />current range of environmental conditions (Chapter 3). This type of evidence suggests ways of <br />dealing directly with uncertainty in flood risk assessment by (a) indicating realistically those areas <br />that are truly subject to flooding, and (b) showing the nature of the flooding phenomenon that is <br />being assessed. This chapter also illustrates how the alluvial fan flooding problem can be broken <br />down into a set of questions for which there is already an established body of scientific material <br />(e.g., channel bank stability) (US ACE, 1992). <br />4. A demonstration of the use of the committee's definition of alluvial fan flooding for <br />specific locations (Chapter 4), either field sites that were visited by committee members or sites <br />described in detail in the scientific literature. The examples illustrate that various levels of effort <br />yield answers of varying detail, but that even a one-day field examination can yield valuable <br />insights for assessing flood hazard zones. <br />5. A framework that suggests the appropriate direction to advance our ability to delineate <br />more accurately those parts of an alluvial fan that are subject to flooding by the 100-year flood by <br />dealing directly with flood process uncertainty (Chapter 3). <br /> <br /> <br />~: <br /> <br />.; <br /> <br />THE NFIP DEFINITION OF ALLUVIAL FAN FLOODING <br /> <br />As noted earlier, the NFIP defines alluvial fan flooding as "flooding occurring on the <br />surface of an alluvial fan or similar landform which originates at the apex and is characterized by <br />high-velocity flows; active processes of erosion, sediment transport, and deposition; and <br />unpredictable flow paths." <br />This definition emphasizes a type of flooding, not a landform, and thus is inherently <br />difficult to translate into the regulatory setting. Defining the hazard more explicitly in process <br />terms emphasizes that a variety of flooding processes with varying distributions and levels of <br />intensity occur on alluvial fans; because of the range of environmental conditions in which such <br />floods occur, a degree of flexibility is needed in defining and quanti tying them, For emphasis and <br />elaboration, the primary elements of the current NFIP definition are paraphrased here: <br /> <br />. Alluvial fan flooding has an unpredictable flow path. The perceived channel, if there is <br />one, may not be the actual conveyance route for water during a flood. <br />. Alluvial fan flooding occurs on the surface of an alluvial fan or similar landform for which <br />the spatial domain that is subject to flooding may extend over a larger area than the floodplain as <br />determined by the traditional hydrologic paradigm. <br />. Alluvial fan flooding has velocities high enough to erode new channels for floodwaters. <br />Similarly, such erosion may undermine adjacent buildings and destroy them even though the water <br />does not get deep enough to cause inundation. <br />