Recent Reductions of Subsidence Rates In the Mississippi River Delta Plain

The Mississippi River delta plain has long been characterized as an area with high rates of relative sea-level rise. This concept was tested by integrating National Ocean Service tide gauge records with National Geodetic Survey benchmark releveling data and GPS elevations at Continuously Operating Reference Stations, providing a basis for understanding historical subsidence trends and most recent rates for southeastern coastal Louisiana. Tide gauge records indicate that rates of relative sea-level rise at Grand Isle accelerated from about -2.2 mm/yr between 1947 and 1964 to about -11.5 mm/yr between 1964 and 1991 and then decelerated to about -3.4 mm/yr between 1991 and 2006. These trends and rates are independently verified by repeat leveling surveys that yielded an average subsidence rate of -9.6 mm/yr from 1965 to 1993 at benchmarks between Leeville and Grand Isle and GPS-derived elevation changes at Cocodrie that yielded an average subsidence rate of -4.4 mm/yr from 2003 to 2005. The most recent slow rates of subsidence are similar to those averaged over geological time scales (e.g., radiocarbon-dated peats) that are attributed to natural sediment compaction and crustal loading.

The historical pattern of slow, then rapid, then slow subsidence may be caused by natural deep-basin processes (e.g., faulting, salt migration) but is more likely related to regional hydrocarbon production that followed the same general temporal trends. If accelerated subsidence was induced by reservoir compaction and fault reactivation associated with fluid withdrawal that also accelerated in the 1960s and 1970s, then the most recent reductions in subsidence rates likely reflect a balancing of subsurface stresses and a return to near-equilibrium conditions. Understanding historical and current trends in subsidence rates and their causes is critical for designing and successfully implementing coastal-restoration activities and for modeling and predicting expected impacts of relative sea-level rise on the delta plain.