THE CHANDELEUR ISLANDS AS AN ANALOG FOR UNDERSTANDING BARRIER RESPONSE TO SEA-LEVEL RISE AND INCREASED STORMINESS

The acceleration in global sea-level rise (SLR) requires that we understand how coasts will respond to this forcing. Although mass balance, equilibrium relationships, and numerical models have been developed to predict sediment dispersal patterns and landward shoreline translation during SLR, observational data to test these models are sparse because of the relatively slow rates of global SLR during the past ~4.5 ka. Hence, the relationship between SLR and shoreline response remains poorly understood and controversial. Subsidence-driven, rapid SLR (~0.5 cm/a) along the eastern coast of Louisiana falls within the global range (0.18–0.82 cm/a) projected for the next century, and thus this region provides a natural laboratory for observing coastal response to SLR during historical time scales. Here, we present coastal evolutionary models for the Chandeleur Islands based on bathymetric and shoreline change data dating to the 1850s as well as shallow stratigraphic data. Our results capture a long-term trend of barrier thinning associated with shoreface erosion and lateral sediment distribution to the flanks of the island chain followed by island break-up and in-place disintegration. The destruction of backbarrier marshes results in loss of supratidal substrate—from which spits accrete and bars weld during recovery—and conversion of subaerial islands to ephemeral islands/shoals. It is not until this final stage of island evolution that cross-shore sand redistribution becomes an efficient enough process to translate the barrier sand body landward accompanying shoreface retreat. Our data show that the Chandeleur Islands have crossed an extrinsic threshold forced by increased hurricane frequency during the past decade. Existing models used to predict coastal response to SLR are based on geometric relationships that use time as a proxy for physical processes. They cannot account for shifting climatic trends such as increased storminess that, as shown here, can permanently alter sediment transport regimes leading to rapid island destruction. The Chandeleurs provide insights as to how more stable coasts might evolve during conditions of altered governing parameters, such as increased storminess as has been suggested for the Atlantic ~1 ka BP.