A Method to Account for Surface Impermeability In Gridded Flash Flood Guidance (204)
Brian Barjenbruch, National Weather Service Valley, NE, Elkhorn, NE
David Pearson, National Weather Service Valley, NE
Eugene Derner, National Weather Service Missouri Basin River Forecast Center
Gridded flash flood guidance (FFG) is produced by River Forecast Centers multiple times per day to provide an indication of the amount of rainfall it will take to cause small streams to overflow their natural banks. The FFG is produced on a 4 km by 4 km grid based upon streamflow and soil moisture conditions. FFG has proven to be a useful resource for forecasters in flash flood prediction, but can underperform in certain scenarios. One such scenario is when heavy rain falls upon a grid with an abundance of impervious surfaces, contributing to greater runoff than would otherwise be suggested based on soil moisture and streamflow characteristics alone. This is a frequent occurrence in urbanized areas where flash flood impacts can often be magnified in highly-vulnerable densely-populated locations.
A method was devised in an effort to account for enhanced runoff due to impermeability within the FFG. The first element of this method is a geospatial analysis of the USGS surface impermeability data set to determine the mean impermeability within each individual FFG grid space. The second step is a manual subjective evaluation of the NRCS runoff curve number based on satellite imagery. It is impractical to perform this manual evaluation for every grid space, so a representative sample was evaluated, with an interpolation applied to all grids based on the relation of the manual analysis to the geospatial analysis. The result is an adjusted initial NRCS runoff curve number to account for the surface impermeability in each FFG grid. This number directly influences the FFG values by suggesting greater runoff in areas of greater impermeability, effectively reducing FFG in areas with a high percentage of impermeable surfaces typical of urban development.