Alluvial Fan Flood Hazard Assessment: A Comparison of FEMA, NRCS, and Surficial Geological Maps in Ivanpah Valley, Nevada.
Amanda Williams1, Brenda J. Buck2, Douglas Merkler3, P. Kyle House4, Michael Howell5, Janice Morton1, Colin Robins5, and Maureen Yonovitz5. (1) Department of Geoscience, University of Nevada, Las Vegas, 1515 E. Reno, E101, Las Vegas, NV 89119, (2) Dept. Geoscience, Univ. Nevada - Las Vegas, 4505 Maryland Pkwy, Las Vegas, NV 89154, (3) 5820 South Pecos Road, USDA, USDA, Park Place Professional Complex, B A, Ste 4, Las Vegas, NV 89120, (4) Nevada Bureau of Mines and Geology, University of Nevada , Reno, UNR, Reno, NV 89557, (5) University of Nevada, 961 Medina De Leon Ave, Henderson, NV 89015
Alluvial fan flooding is increasingly important given the exponential urbanization of arid regions. However, flood behavior on alluvial fans in arid/semi-arid climates differs significantly from that of perennial rivers. Additionally, flood hazard maps differ widely depending on the methods, terminology, and spatial and temporal scales used. To improve understanding of arid system floods, and to ameliorate confusion for map users, we compared Federal Emergency Management Agency (FEMA) (CCRFCD, 2002), Natural Resource Conservation Service (NRCS) (Soil Survey Staff, 2007b), surficial geology (House et al., 2006a; House et al., 2006b) and flood hazard (House 2007) maps for the Ivanpah Valley, NV, the location of a planned new international airport. The FEMA map depicts areas that lie within the 100-yr flood hazard (Zone A) and those that do not (Zone X). The Order 3 NRCS survey shows 40 associations, each containing up to 5 distinct soil components and flood classes. The geologic map (1:50,000) contains 21 unique geomorphologic units. Of the 1,001.5 km2 study area, 3.4 percent lies within FEMA Zone A, compared to 21.9 percent within the House (2007) Very High flood class alone. Soil components within the NRCS map are correlative to the surficial geology units and if both are combined the result is nearly an Order 2 soil survey. The House (2007) map provides the most useful information on flood hazards for land-use planning. This map depicts active drainage systems, their flowpaths, and the locations of specific drainage divides, indicating that geomorphology is the most important and unifying criterion for alluvial fan flood hazard determination.