RATES OF MODERN SUBSIDENCE ALONG THE LOUISIANA-MISSISSIPPI COAST

The staggering loss of Louisiana’s coast by submergence is one of America’s foremost environmental/economic problems. Since the 1900s, >4000 km2 of the coast has been lost. The causes of this loss has been controversial, but consensus is emerging that two main factors are at play: 1) sediment starvation due to construction of flood control levees and drainage systems in the 19th and 20th centuries; and 2) natural subsidence. In addition to affecting areas where people live, the lack of sediment input and sinking of the land relative to sea level have also made sensitive marshlands more vulnerable to saltwater intrusion and erosion by storms. Our ability to stem and mitigate the effects of submergence has been hampered by the lack of subsidence rates relative to a datum; heretofore all rates have been relative to local reference frames. The main effect of such a parochial view is that regional causes such as flexure of the edge of the North American plate due to sediment loading has been largely ignored in public policy debates. A state-wide analysis of National Geodetic Survey first-order leveling and National Ocean Service tide gauge data from the Mississippi and Louisiana coastal region is underway. The objectives are to recalibrate the vertical control network (NAVD88) and to establish accurate subsidence rates relative to a datum. Results based on over 1000 benchmarks indicate that coastal areas are moving downward with respect to sea level. From north to south, rates range a –10mm/yr near Jackson, MS to >-25mm/yr at the mouth of the Mississippi river. Coastwise increase in subsidence rates are affected locally by other conditions; e.g., rates are higher in areas: that lie adjacent to active normal faults, contain sediments/soils that have been recently deposited, that have high organic content, have high capacity for compaction, and/or have been modified by human activity (fluid withdrawal, drainage).