MAJOR SUBSIDENCE OF THE MODERN LOUISIANA COAST

The coast of Louisiana is the site of America’s greatest wetland and energy heartland. This critical area is being increasingly threatened by progressive inundation by the relative rise of the Gulf of Mexico. This slow inundation was detected several decades ago and has been generally attributed to eustatic sea level rise, sediment starvation of the delta due to construction of flood control levees along the Mississippi River, and subsidence of the land relative to sea level. Although the former two effects are reasonably well understood, the lack of precise quantitative spatial data on the later related to a well defined, common datum has prevented the development of a satisfactory explanation of the role and causes of subsidence.

Analysis of ~2500 National Geodetic Survey (NGS) 1st order benchmarks and NOAA tide records show that south Louisiana has subsided at rates between a few mm/yr to over –30 mm/yr over the past 50 years; the mean rate is ~12 mm/yr in most coastal parishes. In the deltaic plain, the data show that subsidence in wetland areas was significantly higher than previous estimates based on long-term geologic measurements. The data also indicate that adjacent alluvial ridges where the population is concentrated have been similarly affected. The rate of subsidence at any particular benchmark depends on both local and regional geologic factors as well as anthropomorphic activities (hydrocarbon extraction and drainage). The geologic factors include sediment compaction, tectonic loading, and faulting. The highest rates occur adjacent to shallow oil fields and active normal faults. Rates of sinking in the Chenier plain in southwest Louisiana are similar to those of the deltaic plain, a region previously thought to be subsiding at low rates. Subsidence of areas lying outside Quaternary depocenters suggests that tectonic flexure due to sediment loads is also a major factor.

If subsidence continues at similar rates and efforts fail to: 1) restore the coast by returning sediment and fresh water back into the delta system from the now leveed Mississippi River, and 2) build protection levees at appropriate heights, substantial portions of Louisiana’s coast will lie below sea level and be inundated by end of this century.