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Storm-Impact Scale
First line of defense (Dhigh)
The vulnerability of a barrier island to storm overwash and inundation is determined, in part, by the elevation of the 'first line of defense,' or Dhigh. On a natural beach system, this is either the first dune ridge or, in the absence of a dune, the beach berm. For areas that have been heavily engineered with shore-parallel coastal defense structures (e.g. seawalls), the top of the structure becomes the 'first line of defense.' These features, both natural and engineered, act as a first line of defense for inland areas during storms by protecting them from waves and surge.
The spatial variability of the dune crest plays an important role in making some areas of the coast more vulnerable to hurricane-induced coastal change than other areas. From lidar topographic surveys, detailed measurements of beach topography have been collected along the Nation's coastlines. From these high-resolution surveys, the locations and elevations of Dhigh are measured.
return to Storm Impact Scale Overview | continue to Measuring Dhigh
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