Subsidence and Wetland Loss Related to Fluid Energy Production, Gulf Coast Basin Completed

Wetland losses in the northern Gulf Coast region of the United States are so extensive they represent critical concerns to government environmental agencies and natural resource managers. In coastal Louisiana, wetland loss has been such an important environmental and economic issue that decades of scientific research have been dedicated to the topic. Since the trend was first recognized in the late 1960s, billions of dollars in state and federal funding have been earmarked for coastal restoration projects intended to compensate for some of the historical wetland losses. In support of that effort, research (2000 - 2011) at the St. Petersburg Coastal and Marine Science Center (SPCMSC) focused on better understanding the physical processes and human activities that contributed to historical wetland loss in coastal Louisiana and the spatial and temporal trends of that loss.

The physical processes (land-surface subsidence and sediment erosion) responsible for historical wetland loss were quantified by comparing marsh-surface elevations, water depths, and vertical displacements of stratigraphic contacts that were correlated between short sediment cores at 10 study areas in the Mississippi River delta plain and six sites at Sabine National Wildlife Refuge (SNWR) in the western chenier plain. The sequential development and two-dimensional extent of land loss at the study areas were described by comparing historical maps, aerial photographs, and satellite imagery; and the temporal and spatial trends of historical wetland loss were compared with historical subsidence rates and hydrocarbon production trends for the same period. The total three-dimensional accommodation space that formed as the result of historical wetland loss was estimated by integrating the spatial data with emergent-marsh elevations and bathymetry from the study areas.

Study Area and Geologic Setting

Delta Plain

The 10 delta-plain study areas encompassed several different physiographic and geologic settings, including: (1) an upper delta-plain levee flank of the Mississippi River (Ironton), (2) a pre-existing upper delta-plain interior channel (Bayou Perot), (3) four upper delta-plain interdistributary areas (Bully Camp, Pointe au Chien, DeLarge, and Madison Bay), (4) three lower delta-plain interdistributary areas (Bay St. Elaine, Leeville, and Fourchon), and (5) a lower delta-plain beach-ridge margin area (Caminada).

Western Chenier Plain

The 6 western chenier-plain study areas were located in Sabine National Wildlife Refuge, which encompasses about 500 km² of coastal wetlands between Sabine and Calcasieu Lakes in the western chenier plain. The refuge occupies a broad, shore-parallel, topographically low area that formed between the topographically higher beach ridges along the gulf shoreline to the south and Pleistocene upland areas to the north.

Research and Results

Historical Wetland Loss

Analysis of historical satellite imagery and previously interpreted land-classification datasets identified more than 4,600 km² of low-lying wetlands in coastal Louisiana that converted to open water between 1932 and 2004.

Wetland Subsidence and Erosion

The two primary physical processes responsible for historical wetland loss in coastal Louisiana are land-surface subsidence and erosion.

Induced Subsidence Related to Hydrocarbon Production

Historical delta-plain subsidence rates were assessed  by analyzing tide-gauge records, elevation changes at benchmarks between repeat leveling surveys, and Global Positioning System (GPS) vertical velocities at Continuously Operating Reference Stations (CORS). 

Historical Accommodation Formation

Bathymetric data acquired at the Madison Bay, Point au Chien, Bully Camp, Leeville, Fourchon, Caminada, and Sabine National Wildlife Refuge study areas provided average one-dimensional (1D) (vertical) accommodation distances at those sites. These results were integrated with the two-dimensional (2D) (area) extent of historical wetland loss from 1956 to 2004 to estimate the total 3D (volume) accommodation space that formed historically on the western chenier and delta plains.

Photo Gallery

Photos of project fieldwork.