Subtask 3.4: Evolution of Northern Gulf Coast Barrier Islands

Subtask Leader: Robert Morton - USGS Florida Integrated Science Center, St. Petersburg

Subtask 3.4 Aim:

Impacts of A. Hurricanes Camille (1969), B. Ivan (2004), and C. Katrina (2005) on Petit Bois Island. Photographs taken after landfall by the National Oceanic and Atmospheric Administration (NOAA) [enlargement]

Barrier island chains in the northern Gulf of Mexico extending from Mobile Bay, Alabama to Atchafalaya Bay, Louisiana are disintegrating rapidly as a result of combined physical processes involving sediment availability, sediment transport, and sea level. The cumulative loss of land area and rates of land loss from these ephemeral features are astonishing, but are to some extent expected, because present physical conditions are different from those that existed when the islands first formed.

For example, it appears that sediment supply has diminished, rates of relative sea-level rise have increased, and hurricanes and winter storms have been frequent events that generated extremely energetic waves capable of permanently removing sediment from the islands. These processes have repeatedly acted in concert to increase rates of beach erosion and to continuously reduce the area of coastal land.

At greatest risk of further degradation are the barrier islands associated with the Mississippi delta that include the Chandeleur-Breton Island, Timbalier Island, and Isle Dernier chains in Louisiana. These chains of individual transgressive barrier island segments have progressively diminished in size while they migrated landward. In contrast, the Mississippi-Alabama barrier islands are not migrating landward as they decrease in size. Instead, the centroids of most of the Mississippi-Alabama barrier islands are migrating westward in the direction of predominant littoral drift through processes of updrift erosion and downdrift deposition. Although the sand spits and shoals of the Mississippi-Alabama barriers are being transferred westward, the vegetated, ridge and swale complexes that comprise the interior cores of the islands remain fixed in space.

Substantial data collection efforts have already focused on the Louisiana barrier islands, but little geological research has been done on the Mississippi-Alabama barriers. Both Ship Island, Mississippi and Dauphin Island, Alabama have been permanently breached by hurricanes that created new tidal inlets that bisect the islands. Subsequent to Hurricane Katrina in 2005, the U.S. Army Corps of Engineers (USACE) has proposed several major coastal restoration activities, including closing the wide breach on Dauphin Island and restoring the Mississippi barriers to their pre-Hurricane Camille configuration.

The Mississippi barriers are part of the Gulf Islands National Seashore, which is managed by the National Park Service (NPS). Interest has been expressed by both the USACE and the NPS for information regarding historical changes in the Mississippi-Alabama barrier islands and the physical processes that cause those changes. Toward that end, the following research tasks are proposed that would provide insight into the rates of morphological change, the likely causes of those changes, and the most likely future conditions if no action is taken.

Subtask 3.4 Activities:

The aim of Subtask 3.4 is to examine historical changes in the land area of Mississippi-Alabama barrier islands between the mid-1800s and 2005 using historic maps and aerial photographs, and also to understand the processes by which severe storms act to abruptly modify these barriers.

The objectives of this subtask are to:

• Quantify long-term historical morphological changes of the Mississippi-Alabama barrier islands; and
• Analyze impacts of extreme storms on the Mississippi-Alabama barrier islands.

Subtask 3.4 Publications: