Fall 2006 | Munson Conservation Lecture Series

On September 20, 2006, Roy Dokka of Louisiana State University (LSU) delivered a lecture entitled “Losing Land: Underlying geologic and anthropomorphic vulnerabilities of the Louisiana Coast revealed by geodetic studies” as part of the Munson Conservation Lecture Series. Dr. Dokka is the Director of the Louisiana Spatial Reference Center, and the Center for GeoInformatics at LSU and specializes in the use of geographic datasets to examine the issue of changes in the coastal environment of Louisiana. The lecture focused on new data that suggests that the current paradigm used by coastal geologists and environmental engineers to examine coastal erosion is at best outdated and at worst was always incorrect.

Previous analysis of coastal erosion in Louisiana has focused on causal relationships regarding marsh salt water intrusion, channeling of river sediments, soil compaction due to removal of water, petrochemical extraction, and wave action erosion. However, new evidence suggests that in addition to these factors, tectonic processes warrant examination. In the age of postglacial rebound, the entire North American plate is again rebounding with the removal of continental glaciers. The melting glaciers contributed to rising sea levels which equilibrated along the current Gulf Coast. As sea levels rose, existing deltaic areas across the world were submerged and forced to retreat with the coast line. In the case of Louisiana, without the Mississippi River delta, the existing coastline would run from Mississippi near Bay St. Louis along the north shore of Lake Pontchartrain, and reemerge in the Gulf of Mexico along the shore of Sabine Lake near Port Arthur, Texas. However, the deposition of river sediments from the Mississippi and Red River has deposited large amounts of sediments onto the coastal plain and applied pressure to the underlying lithosphere. Movement of mantle material toward central and northeast North America due to glacial rebound combined with movement away from the heavy deltaic load has combined to create a net movement of mantle mass away from southern Louisiana. This mantle divergence alone would result in coastal subsidence, but the combined factors of salt dome extrusion, soil compaction, salt water intrusion and thermal expansion of the oceanic boundary layer as leading to rapid increases in relative sea levels across the northern Gulf Coast.

Subsidence rates over several centimeters per year along the Michaud Fault are deeply seated subsidence as is evident in deeply anchored well heads and other benchmarks. Such tectonic subsidence combined with other factors leading to increases in relative sea levels provide clear motivation for the construction of coastal flood control systems capable of resisting decades of rising sea levels. The possibility of increased hurricane activity provides a grave threat to the Gulf Coast. However, with anthropogenic climate change on the horizon the possibility of increased El Nino- Southern Oscillation (ENSO) activity, such as that seen in 1998, provides an equally grave threat. While increased Atlantic hurricane activity and increasingly positive ENSO cycles are likely mutually exclusive, whichever develops could be equally devastating.