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USGS2016

A software tool for rapid flood inundation mapping

The GIS Flood Tool (GFT) was developed by the U.S. Geological Survey with support from the U.S. Agency for International Development’s Office of U.S. Foreign Disaster Assistance to provide a means for production of reconnaissance-level flood inundation mapping for data-sparse and resource-limited areas of the world. The GFT has also attracted interest as a tool for rapid assessment flood inundation mapping for the Flood Inundation Mapping Program of the U.S. Geological Survey. The GFT can fill an important gap for communities that lack flood inundation mapping by providing a first-estimate of inundation zones, pending availability of resources to complete an engineering study. The tool can also help identify priority areas for application of scarce flood inundation mapping resources. The technical basis of the GFT is an application of the Manning equation for steady flow in an open channel, operating on specially processed digital elevation data. The GFT is implemented as a software extension in ArcGIS. Output maps from the GFT were validated at 11 sites with inundation maps produced previously by the Flood Inundation Mapping Program using standard one-dimensional hydraulic modeling techniques. In 80 percent of the cases, the GFT inundation patterns matched 75 percent or more of the one-dimensional hydraulic model inundation patterns. Lower rates of pattern agreement were seen at sites with low relief and subtle surface water divides. Although the GFT is simple to use, it should be applied with the oversight or review of a qualified hydraulic engineer who understands the simplifying assumptions of the approach.

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USGS2010

Probability and volume of potential postwildfire debris flows in the 2010 Fourmile burn area, Boulder County, Colorado

This report presents a preliminary emergency assessment of the debris-flow hazards from drainage basins burned by the Fourmile Creek fire in Boulder County, Colorado, in 2010. Empirical models derived from statistical evaluation of data collected from recently burned basins throughout the intermountain western United States were used to estimate the probability of debris-flow occurrence and volumes of debris flows for selected drainage basins. Data for the models include burn severity, rainfall total and intensity for a 25-year-recurrence, 1-hour-duration rainstorm, and topographic and soil property characteristics. Several of the selected drainage basins in Fourmile Creek and Gold Run were identified as having probabilities of debris-flow occurrence greater than 60 percent, and many more with probabilities greater than 45 percent, in response to the 25-year recurrence, 1-hour rainfall. None of the Fourmile Canyon Creek drainage basins selected had probabilities greater than 45 percent. Throughout the Gold Run area and the Fourmile Creek area upstream from Gold Run, the higher probabilities tend to be in the basins with southerly aspects (southeast, south, and southwest slopes). Many basins along the perimeter of the fire area were identified as having low probability of occurrence of debris flow. Volume of debris flows predicted from drainage basins with probabilities of occurrence greater than 60 percent ranged from 1,200 to 9,400 m3. The predicted moderately high probabilities and some of the larger volumes responses predicted for the modeled storm indicate a potential for substantial debris-flow effects to buildings, roads, bridges, culverts, and reservoirs located both within these drainages and immediately downstream from the burned area. However, even small debris flows that affect structures at the basin outlets could cause considerable damage.

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