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Appendix 3-B: FEMA's Guidelines for Study Contractors: Alluvial Fan Studies From Guidelines and Specifications for Contractors. September 1982, FEMA - 37/July 83 printing, Federal Emergency Management Agency, Washington, D.C. I. INTRODUCTION The methodology outlined in this Appendix is based on procedures developed by Dawdy (Reference I) and later modified (Reference 2) to account for split flow condi- tions generally found in the lower reaches of active alluvial fans. It is recommended that the Study Contractor review these publications for a complete discussion of the theory, rationale and assumptions used to develop this methodology. In portions of alluvial fans in which natural alluvial fan processes may not occur, such as in areas of entrenched channels, areas protected by flood control works, and heavily developed areas, the Study Contractor should exercise good engineering judgment in determining the most appropri- ate methodology or combinations of methodologies, When it is determined that an area in a community is subject to alluvial fan flood- ing, a thorough reconnaissance of the alluvial fan should be made in order to determine the source of flooding, the apex of the fan, the boundaries the fan, the areas of coales- cence of contiguous fans, the limits of entrenched channels, single and multiple channel regions where evident, and the areas of active alluvial fan processes. The reconnaissance should make use of available topographic, geologic, and soil maps; aerial photographs; his- toric records; and site inspection. Prior to undertaking any computations, the Study Contractor should obtain ap- proval from the PO for the use of the methodology outlined in this Appendix. 2. ASSUMPTIONS AND OBSERVATIONS The approach outlined in this Appendix makes use of statistical analyses that re- late the probability of given discharges at the apex of a fan to the probability of certain depths and velocity of flow occurring at any point on the fan below the apex. The methodology presented in this Appendix is based on assumptions and observations regard- ing flood flows on active alluvial fans outlined in the following sections. a. Channel Pattern and Location At the time of maximum flow during a major flood event on an active fan, flow does not spread evenly over the fan but is confined to only a portion of the fan surface that carries the water from the apex to the toe of the fan. In upper region of the fan, flood flows are confined to a single channel which is formed by the flow itself through erosion of the loose material that makes up the fan, Because of the relatively steep slopes in the upper region, flood flows are at critical depth and critical velocity. Below the apex of fan (or the zone of entrenchment in the case of mature fans), the channel will Occur at random locations at any place on the fan surface; under natural conditions, it is no more likely to follow a pre-existing flowpath than it is to follow a new flowpath, This channel has an approximately rectangular cross section for which depth, width, and velocity of flow can be expressed as functions of discharge at the apex of the fan. C-ll