Historical changes in the Mississippi-Alabama barrier islands and the roles of extreme storms, sea level, and human activities

Abstract

An historical analysis of images and documents shows that the Mississippi-Alabama (MS-AL) barrier islands are undergoing rapid land loss and translocation. The barrier island chain formed and grew at a time when there was a surplus of sand in the alongshore sediment transport system, a condition that no longer prevails. The islands, except Cat, display alternating wide and marrow segments. Wide segments generally were products of low rates of inlet migration and spit elongation that resulted in well-defined ridges and swales formed by wave refraction along the inlet margins. In contrast, rapid rates of inlet migration and spit elongation under conditions of surplus sand produced low, narrow, straight barrier segments. Since the mid 1800s, average rates of land loss for all the MS islands accelerated systematically while maintaining consistency from island to island. In contrast, Dauphin Island, off the Alabama coast, gained land during the early 20th century and then began to lose land at rates comparable to those of the MS barriers. There is an inverse relationship between island size and percentage of land reduction for each barrier such that Horn Island lost 24% and Ship Island lost 64% of its area since the mid 1800s. Ship Island is particularly vulnerable to storm-driven land losses because topographic and bathymetric boundary conditions focus wave energy onto the island. The three predominant morphodynamic processes associated with land loss are: (1) unequal lateral transfer of sand related to greater updrift erosion compared to downdrift deposition, (2) barrier narrowing resulting from simultaneous erosion of the Gulf and Soundside shores, and (3) barrier segmentation related to storm breaching. The western three fourths of Dauphin Island are migrating landward as a result of storms that erode the Gulf shore, overwash the island, and deposit sand in Mississippi Sound. Petit Bois, Horn, and Ship Islands have migrated westward as a result of predominant westward sediment transport by alongshore currents, and Cat Island is being reshaped as it adjusts to post-formation changes in wave and current patterns associated with deposition of the St. Bernard lobe of the Mississippi delta. The principal causes of barrier island land loss are frequent intense storms, a relative rise in sea level, and a deficit in the sediment budget. The only factor that has a historical trend that coincides with the progressive increase in rates of land loss is the progressive reduction in sand supply associated with nearly simultaneous deepening of channels dredged across the outer bars of the three tidal inlets maintained for deep-draft shipping. Neither rates of relative sea level rise nor storm parameters have long-term historical rends that match the increased rates of land loss since the mid 1800s. The historical rates of relative sea level rise in the northern Gulf of Mexico have been relatively constant and storm frequencies and intensities occur in multidecal cycles. However, the most recent land loss accelerations likely related to the increased storm activity since 1995. Considering the predicted trends for storms and sea level related to global warming, it is clear that the barrier islands will continue to lose land area at a rapid rate without a reversal in trend of at least one of the causal factors. The reduction in sand supply related to disruption of the alongshore sediment transport system is the only factor contributing to land loss that can be managed directly. This can be accomplished by placing dredged material so that the adjacent barrier island shores revive it for island nourishment and rebuilding.