Geomorphic Assessment of Flood-Prone Areas on the Southern Piedmont of the Tortolita Mountains, Pima County, Arizona

Abstract

The character of flooding and the extent of flood-prone areas on the southern piedmont of the Tortolita Mountains have been disputed by local and federal flood plain management officials since at least 1987. Flood insurance rate maps (FIRMs) promulgated by the Federal Emergency Management Agency (FEMA) depict much of the Tortolita piedmont as being subject to alluvial-fan flooding. Officials of the Pima County Flood Control District have argued that 100-year floodplains delineated on these FIRMs include extensive areas that are not flood prone, and that the alluvial-fan methodology used to develop these maps is not appropriate for the Tortolita piedmont. We conducted a geomorphic analysis of the southern piedmont of the Tortolita Mountains in northern Pima County to critically evaluate the floodplain designations derived using the FEMA alluvial-fan methodology (AFM) (Dawdy, 1979; FEMA, 1985; 1989). The geomorphic character of a piedmont provides a long record of flooding events that is independent of specific hydrologic models because it reflects the cumulative effects of thousands of years of flooding. Drainage patterns, surface topography, soil development, vegetation, and stratigraphic studies were used to assess the character of flooding and the extent of flood-prone areas on the piedmont. We differentiated and mapped alluvial surfaces of the following ages: <100 years; < 5,000 years; 5,000 to 20,000 years; 20,000 to 100,000 years; and >100,000 years.