The U.S. Geological Survey is evaluating a new map
product designed to give disaster officials a rapid estimate of the regional
impact of a hurricane along the coast. The map plots how frequently overwash,
a characteristic features of severe storms, occurs along the coast. Storm
surge is where high winds and low atmospheric pressure of a hurricane form
a bulge of high water that washes over the barrier beaches. Overwash is
the deposit left after such a high water pulse overtops or breaches the
dune line of a barrier beach. Much of the worst coastal damage from a hurricane
is in areas of extensive overwash.
The mapping is done from oblique aerial video taken
immediately after a storm. Every 50 meters, an observation is made whether
overwash is present, i.e. whether the dune line has been breached or topped
by the storm surge. Ten of these readings are combined into a percent measure
for every half kilometer of shoreline. A 50% value means that five of the
ten readings in that 500 meter interval indicate overwash. An intact dune
line will have low values; a dune breached or topped many times will have
high values. The percent readings can either be mapped
along the coast or plotted like a histogram.
The storm response overwash map can be used as a guide
to where storm surge is having the greatest impact along the coast. By
comparisons with pre-storm photography, the map can show how well pre-existing
dunes survived. The map is part of a national USGS effort for severe storms
and tsunamis that includes probabilistic estimates of storm landfalls and
detailed analysis of beach processes to supplement the storm response efforts.