Moving Islands

Not all islands are stationary. Barrier islands can creep slowly toward land in response to oceans waves and currents. Large storms contribute significantly to a barrier island's landward migration, a process also known as 'barrier island rollover.' Increased water levels, large waves and strong flows remove sand from the beach and dump it on the bay-side of an island.

Dramatic examples of 'barrier island rollover' occurred at Dauphin Island, Alabama during Hurricane Katrina [images above].

Scientists measured coastal changes that occurred after Katrina made landfall using a technique called laser altimetry (LIDAR). Still LIDAR shots taken at different times reveal how the coastline changed in response to Hurricanes Ivan and Katrina.

 
The first image shows Dauphin Island in May 2004 before Hurricane Ivan. The second image shows the same area post-Ivan on September 16, 2004. The shoreline has receded and in some places is located landward of the first row of homes. Overwash deposits have buried the road and some houses are missing.

The third image shows the devastating impact of Hurricane Katrina. As storm surge flooded the island, waves transported sand across the width of the island and deposited it in Mississippi Sound. This essentially shifted the entire island landward.

After Katrina most of the houses have been completely destroyed. The yellow arrow in each frame points to one of the few houses that remains standing after both storms. Each red square is a destroyed house.

 --Ker Than

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Credit:  USGS/NASA