Regional Coastal Evolution along the Mississippi River Delta Plain Barrier Shorelines

Data from regional bathymetric surveys conducted in 2006-2007 along Louisiana's South-Central and Chandeleur barrier shorelines were combined with historical bathymetry from three time periods (dating to the 1880's) to provide a series of digital elevation models that show the 3-D morphological evolution. Both areas are undergoing pronounced shoreface retreat due to rapid relative sea level rise and a net loss of sediment. The two regions have contrasting evolutions: tidal inlet processes dominate the central barriers, whereas hurricane impacts and subsequent storm recovery processes dominate the Chandeleur Islands.

Along the South-Central barriers, conversion of interior wetlands to open water increases tidal prism, resulting in degradation of barriers due to increasing inlet cross-sectional area and sediment sequestration in ebb tidal deltas. The dominant trends during the past 125 years include erosion of ~9x10⁸m³ of sediment from the shoreface and an increase in combined tidal inlet cross-sectional area from ~41,400m² to ~139,500m². The rate at which tidal inlet cross-sectional area is increasing is directly proportional to the rate of interior wetland loss.

Along the Chandeleur Islands, long-term reduction in island area is event-driven and related to major storms. The islands do not fully recover from large magnitude hurricanes because storm waves transport sand away from the mid-barrier arc to the distal flanks of the system. Volume calculations indicate ~150x10⁶m³ of sediment has been deposited downdrift and seaward of the northern terminal spit during the past 125 years, whereas approximately half this volume has been deposited in the backbarrier.

This study is refining coastal evolutionary models and management schemes for these regions by: 1. quantifying erosional trends in degrading delta complexes, 2. identifying linkages between interior wetland loss and barrier response, 3. documenting sediment sinks, and 4. demonstrating the importance of implementing a system-wide approach to sediment management in the Mississippi River delta plain.