Rates of vertical displacement at benchmarks in the lower Mississippi valley and northern Gulf Coast
This report describes the methods and results of our research into the recent rates and spatial distribution of subsidence on benchmarks in the lower Mississippi Valley and northern Gulf Coast region. The analysis was accomplished using first-order leveling data and GPS observations from the National Geodetic Survey (NGS) and water level (tide gauge) data from the National Ocean Service. This study computed vertical velocities for over 2700 NGS benchmarks based on leveling data collected between 1920 and 1995. Subsidence affects coastal areas of Louisiana, Mississippi, Texas, and Alabama. The highest rates, over 25 mm per year, occur in the Mississippi river delta region and chenier plain of southwest Louisiana. Subsidence gradually slows toward the east, ending in western Florida. Subsidence continues toward the west along the Texas coast beyond the study area. These rates are substantially higher than rates reported in previous studies based on analysis of leveling or on geological investigations. Our analysis of the leveling data also indicates that subsidence rates increased in many areas during the later half of the 20th century. Another region of subsidence is centered on the Mississippi alluvial valley, extending northward from the coast to near Memphis, Tennessee. This subsiding region is flanked by regions of stability or uplift in northwest Louisiana and northeastern Mississippi. Our evaluation of independent subsidence measures validates the primary results. Displacement rates derived from the leveling network agree, on average, to within 1.5 mm/yr with rates derived from a number of coastal tide gauge stations. We also computed vertical displacement over six or seven years at three Continuously Operating Reference Station (CORS) sites. These values agreed to within about 2 mm/yr with the rates derived from the leveling data for nearby benchmarks. The CORS analysis also shows that subsidence is continuing today at comparable rates. We draw two primary conclusions from this study. First, subsidence is occurring at substantially higher rates than previously reported. These new rates provide insights into the causes of subsidence and should be integrated into plans to mitigate the effects of subsidence and the resultant inundation of coastal lands. Second, the results of this study prove the need for a Height Modernization program to provide the updated and sustainable elevation reference system that is essential to any efforts to mitigate the impending slow disaster threatened by subsidence.