Regional Variations in Shore Response along Barrier Island Systems of the Mississippi River Delta Plain: Historical Change and Future Prediction

Abstract

Long-term changes in shoreline position along Louisiana's rapidly deteriorating barrier coastline were documented from 1855 to 1989 using National Ocean Service (NOS) topographic sheets and near-vertical aerial photography. An interactive computer mapping system was employed to compile and quantify shoreline data at approximately 880 shore-normal transects by magnitude, direction, and rate of change. The study area extends along the barrier coast of the Mississippi River delta plain from Raccoon Point (western Isles Dernieres) to Hewes Point (northern Chandeleur Island). Four barrier systems characterize the study area: (1) Isles Dernieres, (2) Bayou Lafourche, (3) Plaquemines, and (4) Chandeleur Islands. Long-term gulfside rates of change range from -23.1 to +0.9 m/yr, whereas bayside rates range from -5.0 to +24.0 m/yr. Louisiana barrier island systems have experienced landward migration, area loss, bayside erosion, and island narrowing as a result of complex interactions among subsidence, eustatic sea level rise, wave processes, storm impacts (cold fronts and tropical cyclones), inadequate sediment supply, and intense human disturbance (levees; oil, gas, and sulphur extraction activities; access canals; seawalls; jetties). Consequently, the structural continuity of Louisiana's barriers is weakening as the barrier shoreline continues to narrow, fragment, and finally disappear. Seven geomorphic response types characterize the barrier shoreline: 1) lateral movement, 2) advance, 3) dynamic equilibrium, 4) retreat, 5) landward rollover, 6) breakup, and 7) rotational instability. Although the Bayou Lafourche shoreline has the highest rates of erosion through landward rollover and retreat, the Isles Dernieres, Grand Terre Islands, and the eastern Plaquemines shoreline are experiencing the more devastating process of breakup and will probably disappear within the next 25 years. Consequently, these zones of breakup are the most critical coastal land loss areas along Louisiana's barrier shoreline and thus, further threaten productive estuarine habitats in Terrebonne/Timbalier and Barataria Bays.