St. Petersburg Coastal and Marine Science Center
Historical Accommodation Formation
Accommodation is the space available for sediment accumulation as a result of a rise in sea level and/or land subsidence. Accommodation originally referred to the space that formed within sedimentary basins over millions of years as the result of eustatic sea-level rise and tectonic subsidence; however, accommodation also describes space that forms over smaller areas and shorter time scales. Historical conversion of coastal-plain wetlands of the northern Gulf of Mexico to open water is an example of accommodation formed at the decadal scale. Although several previous studies documented the temporal and spatial development of wetland loss in coastal Louisiana, these studies did not consider the three-dimensional (3D) aspects of regional 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.
Magnitudes of historical accommodation that formed locally at the western chenier plain study areas (about 20 x 106 m3) were significantly less than formed at the delta plain study areas (about 108 x 106 m3). These volumes provide estimates of the new sediment that would be needed just at the study areas to restore the coastal-plain wetlands to their pre-1956 areal extent and elevations. Similarly, estimates of historical accommodation formation at the surrounding subregional scale are also significantly less for the western chenier plain (about 53 x 106 m3) than for the delta plain (about 444 x 106 m3). In part, this reflects the much greater spatial extent of wetland loss and total area in the delta plain compared with the western chenier plain. However, when differences in analysis extents are considered, normalized magnitudes of accommodation that formed at SNWR are generally less than formed in the delta plain, and are significantly less than at some upper delta-plain study areas These differences are attributed to greater subsidence and, consequently, greater 1D accommodation in the delta plain.