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INTRODUCTION 27 regulators use to apply the definition to a given situation. Erosion and deposition processes are essential criteria in judging alluvial fan flooding because they may affect hydraulic conditions. Where localized sediment deposition, bed form translation, and erosion produce changes in streambed elevation during an event that approaches typical depths of flow, the uncertainty introduced by these processes is significant. Consequently, floodwater surface elevations computed using preflood topography are not a comprehensive indicator of the true hazard for alluvial fan flooding situations as they are for riverine flooding. Flow path uncertainty, which means that the perceived, historical channel or network of channels cannot be relied on to convey the base flood, affects the spatial domain subject to flooding through the creation of new flow paths and/or the subdivision of flows into multiple distinct paths as shown in Figure 1-5b, For a network of channels that exist prior to a flood event, uncertainty in the distribution of flows through the network must be considered in order to rule it out as a factor that might cause flow path uncertainty, The criteria above frequently create a situation where the traditional formulas for mitigation, such as elevating a structure on fill, do not actually eliminate the hazardous condition. The unsuitability of fill as a hazard reduction strategy, however, is perhaps the most important characteristic distinguishing between riverine flooding and alluvial fan flooding. In summary, the committee's revised definition limits alluvial fan flooding to flood hazard on alluvial fans. The committee recognizes that alluvial fan flooding is one type of flood hazard under the wider category of UI/certail/ flaw path floodil/g. Such hazards may have considerable uncertainty associated with their behavior and require means other than fill for reliable mitigation. Chapter 4 presents examples that illustrate how the definition applies to specific cases. REFERENCES American Geological Institute. 1987. Glossary of Geology. 3rd Ed. R. L. Bates and 1. A. Jackson, eds. Alexandria, Va.: American Geological Institute. Bedient, P. B., and W, C. Huber. 1992. Hydrology and Floodplain Analysis, 2nd Ed. Reading, Mass.: Addison Wesley. Dawdy, D. R. 1979, Flood frequency estimates on alluvial fans. American Society of Civil Engineers Journal of Hydraulics Division, 105(HY1l): 1407-1413. Federal Emergency Management Agency (FEMA). 1985. Appendix 4: Alluvial fan studies. Guidelines and Specifications for Study Contractors. Doc. no. 37. Washington, D.C.: FEMA. Federal Emergency Management Agency (FEMA). 1987. Flood Insurance Study, City of Palmdale, California, Los Angeles County. Washington, D.C.: FEMA, Federal Emergency Management Agency (FEMA). 1989. Alluvial Fans: Hazards and Management. Doc. no. 165. Washington, D.C.: FEMA. Federal Emergency Management Agency (FEMA). 1990. FAN: An Alluvial Fan Flooding Computer Program, User's Manual and Program Disk. Washington, D.C.: FEMA. Federal Emergency Management Agency (FEMA). 1995. Appendix 5: Studies of alluvial fan flooding. Guidelines and Specifications for Study Contractors. Doc. no. 37. Washington, D.C,: FEMA.