Alluvial fans are gently sloping, fan-shaped landforms created over time by deposition of eroded sediment. They are commonly located at the base of mountain ranges in arid and semiarid regions such as the western United States. Since alluvial fans are found in apparently dry conditions, homeowners are often shocked to find that they can be the sites of destructive floods. Floods on alluvial fans caused by rainfall or snowmelt, although relatively shallow, strike with little if any warning, can travel at extremely high speeds, and can carry tremendous amounts of sediments and debris. Such flooding presents unique problems for federal and state planners in terms of quantifying the flood hazards, predicting the magnitude at which those hazards can be expected at a particular location, and devising reliable mitigation strategies.

During the past several decades, controversies have grown among the Federal Emergency Management Agency (FEMA), land owners and developers, and floodplain managers over regulating flood hazards in certain types of mountainous environments where water flow paths are especially difficult to predict. As one its responsibilities under the National Flood Insurance Program (NFIP), FEMA determines which areas face alluvial fan flooding hazards and then sets insurance premiums and restricts new construction based on the degree of hazard assessed. However, the current regulatory definition of alluvial fan flooding is very broad and often is applied to other landforms, which causes confusion.

The report recommends that the term “alluvial fan flooding” should apply only to “uncertain flowpath flooding” that occurs on alluvial fans, and not to such flooding on other landforms, such as alluvial plains, pediments, deltas, and braided streams. Alluvial Fan Flooding stresses that not all parts of alluvial fans are subject to alluvial fan flooding. Rather hazardous flood processes, as determined by a set of criteria suggested by the committee, occur only on “active” portions of fans The report offers an updated regulatory definition of alluvial fan flooding and presents criteria that can be assessed to determine if an area is or is not subjected to alluvial fan flooding, and provides examples of applying the definition and criteria to real situations in Arizona, California, Utah, and other areas. The report contains recommendations for FEMA, which is primarily responsible for floodplain mapping, and for state and local decision makers involved in flood hazard reduction.