Coastal Change Hazards: Hurricanes and Extreme Storms

Storm-Impact Scale

Definition sketch showing R_high R_low, D_high and D_low. The dashed lines represent the swash excursion about wave setup (solid line). [larger version]

Overview

The impact of a storm on a barrier island is dependent not only on the magnitude of storm characteristics, such as storm surge and waves, but also on the elevation of the barrier island at landfall, such as the line of dunes paralleling the shore that act as the coast's first line of defense. Stretches of coast with very low dunes are potentially more susceptible during storms to extreme coastal change than those with higher dunes.

The potential vulnerability of a particular stretch of coast can be assessed using a conceptual model that scales the impacts of storms on barrier islands (Sallenger, 2000).   Within the model, the elevation of storm-induced water levels (R_high and R_low), including storm surge, astronomical tide, and wave runup, are compared to measurements of local dune morphology such as the elevation of the dune crest and toe, (D_high and D_low).

The hurricane-induced water levels (R_high and R_low) are the highest reaches of the waves on the beach during a the storm. By considering these water levels relative to coastal elevations D_high and D_low, the crest and base of the dune, four storm impact regimes can be defined for a specific area of the coast:

Swash: Total water levels are lower than the dune toe (R_high < D_low)
Collision: Total water levels exceed the dune toe (D_low < R_high < D_high)
Overwash: Total water levels exceed the dune crest (R_high > D_high)
Inundation: Storm surge and tide exceed the dune crest (R_low > D_high)

Within each of these regimes, the nature and magnitude of coastal change are expected to be unique.

Swash Regime No net change to the system

Collision Regime Net dune Erosion

Overwash Regime Net onshore transport on the order of 100 meters

Inundation Regime Net onshore transport on the order of 1,000 meters

continue to Swash Regime