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INTRODUCTION 7 fluvial and/or debris flow sediments alld which has the shape of a fall either fully or partially extended" Despite the fact that to geologists alluvial refers strictly to features and materials deposited by streams of water (American Geological Institute, 1976), in the case of fans the term has been used more loosely in the scientific, engineering, and planning literature to refer to the products of streams or debris flows. This usage is too well established for this committee to . reverse, However, to a person actually analyzing flood risk on the ground, the distinction is important, and so the committee will recognize the needs of these people in Chapter 2, which is concerned with the physical processes associated with flooding rather than policy and regulatory aspects. Chapter 2 explains that alluvial fans can be deposited by streams or by debris flows or by some combination of the two, and that recognition of the difference between these situations can be crucial for correctly identifYing the hazard potential in certain areas, The chapter thus distinguishes between stream-flow fans, debris flow fans, and composite fans. However, the committee has elected to follow common usage and use aliI/vial fan as the generic term for any of these categories. In NFIP Regulations, CRF 44, ~59, I, when the form and position of the flow paths is so radically uncertain that the risk of flooding at a place cannot be estimated through traditional procedures, a characteristic that frequently is associated with alluvial fans, this type of flooding is called alluvial fan flooding. Deviation from the traditional flood paradigm is further compounded on alluvial fans subject to debris flow hazard, that is, where the base flood is not caused by runoff but by a debris flow with triggering mechanisms, flow characteristics, and probability of occurrence that are completely different from those assumed in hydrological models of flood behavior. Because the designation that an area is subject to alluvial fan flooding sets in motion specific, restrictive federal regulations, the determination by FEMA that an area is subject to alluvial fan flooding rather than ordinary flooding during the 100-year flood can be controversial. Thus, this report is in part an attempt to clarifY what aliI/vial fan flooding means by providing a more precise definition and by describing how to apply the definition through the use of field indicators. The current chapter discusses this essential attribute of alluvial fan flooding in the section entitled "Origin of the Problem." FEMA developed a procedure for estimating flood risk in environments subject to alluvial fan flooding (Dawdy, 1979; FEMA, 1990), In particular, the method predicts the extent of a fan- shaped area subject to a 1 percent chance of flooding in any year, as well as the average speed and depth of such a flood, Application of the aliI/vial fan flooding definition, the associated regulations, and the procedure for risk estimation together aroused considerable opposition from floodplain managers and other interested parties in some (but not all) communities that participate in the NFIP, This conflict led, eventually, to confusion and mutual suspicion. Recognizing the need to resolve the conflict, FEMA requested the appointment of a National Research Council committee to study the issue of alluvial fan flooding, In particular, the committee was asked (I) to clarifY, as necessary, the definition of alluvial fan flooding contained in section 59.1 of the NFIP regulations, (2) to specifY criteria that can be used to determine whether an area is subject to alluvial fan flooding; and (3) to provide examples of applying the revised definition and criteria to real situations. The Committee on Alluvial Fan Flooding met four times to study these issues and conduct a series of field visits during which it consulted with FEMA staff, floodplain management experts,