Figure 1. Low elevations along Dauphin Island, AL made portions of this island susceptible to inundation during Hurricane Ivan (2004). [larger version]
Storm-Induced Coastal Change
Inundation and Island Breaching
Inundation occurs when the beach system, or the sandy profile located between the most seaward (primary) dune and the shoreline, is completely submerged under the rising storm surge. Inundation may occur over large stretches of the coasts, with surge covering the beach system along an entire island, or it may be localized to a small region of the coastline. Inundation is most likely to occur on an island that is naturally low in elevation (Fig. 1), although it may also be triggered by the severe erosion and overwash of the coastal dune system. During the period of inundation, currents and waves carry large volumes of sand from the seaward to the landward side of the island. When water levels subside, currents may reverse direction carrying sediment from sounds and bays back to the open ocean. These strong currents may carve a channel in the island, causing the island to be bisected in a process known as island breaching (Fig. 2). Breaches can vary in width from several kilometers to tens of meters (Fig. 3). Smaller breaches often fill with sand in the months following the storm, while larger breaches may become permanent inlets.
Figure 2: Lidar topography of Pine Beach, AL collected in November 1998 (top) and on September 19, 2004 (bottom), three days following the landfall of Hurricane Ivan to the east in Gulf Shores, AL. Large waves and surge during the storm inundated the beach system leading to barrier island beaching. [larger version]
Figure 3. Island breaching on Dauphin Island, AL during Hurricane Ivan in 2004 (middle panel) was intermittent, extending over 1.35 km along the island. One year later, during Hurricane Katrina's 2005 landfall, the breach widened and became continuous for over 2 km. [larger version]
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