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
Abstract:
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.