T221 The Barrier Island System: Evolution in Response to Changing Sea Levels

Tuesday, 2 November 2010
1:30 PM - 5:30 PM
Room 405

MAINLAND MARSH SHORELINE RESPONSE TO BARRIER ISLAND TRANSGRESSIVE SUBMERGENCE: CHANDELEUR SOUND, LOUISIANA, USA

Paper number: T221-11
Program: Technical Sessions
Time: 3:55 PM – 4:10 PM

Authors

Abstract

Many recent studies have sought to understand the response of barrier islands and their attending marshes to sea level rise. As barrier shorelines become less effective at protecting interior wetlands from wave attack and regulating estuarine processes, knowledge of the mode and rate of marsh response is critical for successful management and planning efforts. The Mississippi River delta plain (MRDP), specifically the Chandeleur Islands - Biloxi Marsh system in eastern Louisiana, serves as an excellent natural laboratory for studying these responses: this region is presently undergoing the highest rates of shoreline erosion (> 15 m/yr) in North America as wetlands are converted to open water in a regime of subsidence-driven rapid relative sea-level (RSL) rise (~1 cm/yr). The existing three-stage conceptual model for the evolution of barrier islands within the MRDP suggest that the disappearance of barrier islands could lead to the formation of a new barrier shoreline along the seaward periphery of mainland marshes, a situation that is likely to be influenced by shallow stratigraphy underlying the marsh surface and marsh response to RSL rise (i.e. accretion rates).

A series of integrated borings and vibracores were used to produce multiple three-dimensional stratigraphic and lithologic models and cross-sections of the subsurface of Biloxi Marsh. Chronology of the latest stage of the development was provided by 210Pb and 137Cs analyses from peat cores.. Together, these data have provided for the description of subsurface stratigraphy, the location of sand-rich deposits, and the determination of modern marsh stability. They suggest that much of the subsurface stratigraphy is rich in fine sediment such as clay and silt. Presently, sandy shorelines and islands in Chandeleur Sound are rare. Along some sections of fringing marsh, shell-lag shorelines and pocket beaches developed from winnowing of shell material. More resistant landmasses become isolated as marsh islands with shell-rimmed beaches and ultimately become submerged to form shell mounds within the Sound. It is likely that without adequate sand supply liberated during ravinement, this regional erosional behavior will continue and accelerate in the future under increased wave and tidal energy.