Some geomorphic models of flood hazards on distributary flow areas in southern Arizona

Abstract

The Federal Emergency Management Agency (FEMA) uses a method for evaluating flood hazards on alluvial fans that assumes an equal chance of flooding along a radial arc across the fan surface. In southern Arizona there are distributary flow areas (alluvial fans) that do not conform with FEMA's assumption. Thirty-nine sample sites were chosen from the Basin and Range physiographic province in southern Arizona. These sites were classified into five categories of flood hazard; A, B, C, D, and E. The classification scheme is based on the potential randomness of flooding across each site. A method is proposed for locating the primary diffluence (apex) of a distributary flow area. Texture curve analysis is used to locate distributary flow areas on the piedmont plain. Two alternative methods, topologic analysis and a multiple regression model, are presented for evaluating flood hazards on distributary flow areas in southern Arizona. Only eight of the sample sites studied strictly conformed with FEMA's assumption of an equal probability of flooding along a radial arc across the fan surface. The topologic analysis may be used to determine if the FEMA method is appropriate for a given site. A multiple regression model provides rough predictions of the degree of flood hazard based on morphometric and hydrologic variables.