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Interpreting the Storm Response Overwash Map

A map to rapidly assess the geologic impact of a major storm to the coastline
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.

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U.S. Department of the Interior, U.S. Geological Survey, St. Petersburg Coastal and Marine Science Center
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Updated: 28 July, 1998@01:16:02 PM (THF)