Flash Flood Screening Tool

Abstract

Flash floods have been a very unpredictive natural disaster. Previous attempts at zonation of flash floods based on predictive severity have been very data-intensive and exhaustive on focusing on a certain area. There are, however, simple models that utilize publicly available data to determine flash flood risks at specific locations. The Geographic Information System (GIS) –based tool, presented here, takes a Digital Elevation Model (DEM) and a Land-Use/Land-Cover map to compute basin characteristics. Flow velocity is modeled according to TR-55, which classifies it as sheet, shallow-concentrated and open-channel flow. With the help of various GIS tools, the drainage area of a user-given outlet point, as well as the flow lengths upstream to the headwaters and downstream to the outlet, can be calculated. These flow lengths are then used to classify the areas of sheet flow, while the areas of open-channel flow are user-defined. Velocities for sheet and shallow-concentrated flow are determined according to TR-55, while the velocities for open-channel flow are user-defined. Once the velocities are known, isochrones are determined and area vs. flow-time plots are developed. Intensity-duration-frequency (IDF) curves are used to define the precipitation for different concentration times and return periods. The rational method is then used to determine the expected peak flows. A sub-watershed of Little Walnut Creek, in Austin, will be valuated and shown as an example. This screening tool will help policymakers identify locations of potential high flash flood risk in which more-detailed hydrologic analysis would be needed