Mapping Flash Flood Severity in the United States

Abstract:

Flash floods have been a major natural hazard in terms of both fatalities and property damage. In the United States, flash floods have only been characterized on a case study basis due to the lack of a comprehensive database matching flood characteristics with geospatial and geomorphologic information. To characterize the ability of a basin to produce flash floods, a new variable called “Flashiness” is derived from the slope of the rising limb in hydrograph time series. It is the basis to document and predict the flash flood potential and severity over the U.S. First a representative and long archive of flood events spanning 78 years is used to analyze the spatial and temporal variability of observed flashiness. The areas and seasons prone to flash floods are documented, highlighting the flash flood alley in Texas, Appalachians, West Coast, and North American monsoon in Arizona etc. Then the flashiness is linked to geomorphologic and climatologic attributes to identify the basin characteristics driving the ability to produce flash floods. The significant impact of characteristics such as slope, precipitation, and basin area are quantified. Next the model is used to predict flashiness all over the continental U.S., specifically over regions poorly covered by hydrological observations. It highlights ungauged areas prone to flash floods such as parts of Florida, Southern Wisconsin, Montana and South Dakota etc. Finally these findings are validated using the National Weather Service storm reports and a historical flood fatalities database. This analysis framework will serve as a baseline for evaluating distributed hydrologic model simulations such as the Flooded Locations And Simulated Hydrographs Project (FLASH) (http://flash.ou.edu).