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"The barrier islands of Louisiana are eroding at an extreme rate. In places up to 100 feet of shoreline are disappearing every year. Though it has long been assumed that this erosion was due to the area's rapid rate of relative sea level rise, recent studies by the U.S. Geological Survey show that other coastal processes, such as the longshore redistribution of sediments, are responsible for this erosion."Dr. Jeffrey H. List, U.S. Geological Survey
The environmental consequences of coastal erosion in Louisiana may be severe.
Louisiana's barrier islands are eroding so quickly that according to some estimates they will disappear by the end of this century. Although there is little human habitation on these islands, their erosion may have a severe impact on the environment landward of the barriers. As the islands disintegrate, the vast system of sheltered wetlands along Louisiana's delta plain are exposed to increasingly open Gulf conditions. Through the processes of increasing wave attack, salinity intrusion, storm surge, tidal range, and sediment transport, removal of the barrier islands may significantly accelerate deterioration of wetlands that have already experienced the greatest areal losses in the U.S. Because these wetlands are nurseries for many species of fish and shellfish, the loss of the barrier islands and the accelerated loss of the protected wetlands may have a profound impact in the billion dollar per year fishing industry supported by Louisiana's fragile coastal environment.
U.S. Geological Survey (USGS) studies collect information critical for improved predictions of long-term erosion rates .
The USGS, in cooperation with Louisiana State University, documented the long-term historical record of bathymetric and shoreline change along the Louisiana coast. For example, historical data over the past 100 years indicate that the shoreline at Bayou Lafourche has eroded back about 3 kilometers. The pattern of long-term, large-scale bathymetric change is key information in determining the processes of barrier island evolution and in formulating predictions of future changes. USGS scientists have assembled bathymetric surveys from data from the 1880's, the 1930's, and the late 1980's, and are in the process of assembling a similar survey in 1993 following the passage of Hurricane Andrew. This base of information will be used to evaluate the contribution of catastrophic events to the long-term evolution of this coastal area.
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Simulations of coastal waves quantify the wave-shielding properties of barrier islands. This information has been used by State agencies within Louisiana to evaluate the cost/ benefit ratio for barrier island restoration projects.
Recent USGS work indicates that rapid relative sea-level rise is not the primary cause of erosion of the barrier islands.
Until this USGS study was undertaken, environmental mana-gers thought that the principal cause of barrier island erosion was rising sea level. Now, we know that both the longshore movement of sediment and the general absence of sand-sized sediment is the principal cause of the islands' instability. The sediments underlying coastal Louisiana are made up mostly of silts and muds which do not contribute to the building of beaches, dunes, and spits-geomorphic features associated with healthy barrier islands. In addition, long-shore currents redistribute the available sand from headland areas to embayments, depriving shorelines of much needed sand.
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Seafloor change along the Louisiana barrier island coast from the 1930's to the 1980's shows historical patterns of seafloor erosion and accretion. This information, collected as part of the USGS's Louisiana Barrier Island Erosion Study, was instrumental in altering our understanding of the factors responsible for rapid shoreline retreat in the area.
USGS studies aid environmental managers in decisionmaking for erosion mitigation.
Experience suggests that the most cost-effective means for the preservation of Louisiana's barrier islands may be to renourish them while permitting their landward migration. Placement of hard, fixed structures appears to be a less effective strategy for preserving barrier islands in the face of historically high rates of shoreline and bathymetric change. On East Island of the Isles Dernieres chain, appropriate placement of sediment helped to preserve the island even as Hurricane Andrew passed over it. Any proposal to renourish barrier islands will need to consider where the necessary materials will be found and how naturally-occurring processes will affect the materials. Results of USGS studies are critical to managers charged with making optimal use of limited funds for erosion mitigation.
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The area shown in part of the Isles Dernieres barrier island chain in central Louisiana. The photograph at the top was taken in July 1992 before Hurricane Andrew while the photograph on the bottom was taken several days after the passage of Hurricane Andrew. Andrew caused severe erosion, except along the far end of the island where artificial renourishment took place in 1984.
Phone: (508) 457-2343
E-mail: jlist@usgs.gov