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

Pearthree, P.A.; Demsey, K.A.; Onken, J.A.; Vincent, K.R.; House, P.K.

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Citation

Pearthree, P.A., Demsey, K.A., Onken, J.A., Vincent, K.R., and House, P.K., 1991, Geomorphic Assessment of Flood-Prone Areas on the Southern Piedmont of the Tortolita Mountains, Pima County, Arizona. Arizona Geological Survey Open File Report, OFR-91-11, 3 map sheets, map scales 1:12,000 and 1:24,000, 31 p.

Publisher

Arizona Geological Survey (Tucson, AZ)

Description

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.

URI

http://hdl.handle.net/10150/630981

Additional Links

http://repository.azgs.az.gov/uri_gin/azgs/dlio/908

Language

Series/Report no.

OFR-91-11

Rights

Collection Information

Documents in the AZGS Document Repository collection are made available by the Arizona Geological Survey (AZGS) and the University Libraries at the University of Arizona. For more information about items in this collection, please contact azgs-info@email.arizona.edu.

North Bounding Coordinate

32.509

South Bounding Coordinate

32.3432

West Bounding Coordinate

-111.168

East Bounding Coordinate

-110.984

Collections

AZGS Document Repository