Sea-Level Rise and Subsidence: Implications for Flooding in New Orleans, Louisiana

Global sea-level rise is projected to acceler­ ate twoto four-fold during the next century, increasing storm surge and shoreline retreat along low-lying, unconsolidated coastal margins. The Mississippi River Deltaic Plain in southeastern Louisiana is particularly vulnerable to erosion and inundation due to the rapid deterioration of coastal barriers combined with relatively high rates of land subsidence. Land-surface altitude data collected in the leveed areas of the New Orleans metropoli­ tan region during five survey epochs between 1951 and 1995 indicated mean annual subsidence of 5 millimeters per year. Preliminary results of other studies detecting the regional movement of the north-central Gulf Coast indicate that the rate may be as much as 1 centimeter per year. Con­ sidering the rate of subsidence and the mid-range estimate of sea-level rise during the next 100 years (480 millimeters), the areas of New Orleans and vicinity that are presently 1.5 to 3 meters below mean sea level will likely be 2.5 to 4.0 meters or more below mean sea level by 2100. Subsidence of the land surface in the New Orleans region is also attributed to the drainage and oxidation of organic soils, aquifer-system compaction related to ground-water withdrawals, natural compaction and dewatering of surficial sed­ iments, and tectonic activity (geosynclinal downwarping and movement along growth faults). The problem is aggravated owing to flood-protection measures and disruption of natural drainageways that reduce sediment deposition in the New Orleans area. 1 U.S. Geological Survey, Lafayette, La. 2 National Oceanic and Atmospheric Administration, National Geodetic Survey, Silver Spring, Md. 3 University of Wisconsin, Madison, Wis. Accelerated sea-level rise, the present altitude of the city, and high rates of land subsid­ ence portend serious losses in property in the New Orleans area unless flood-control levees and pumping stations are upgraded. The restoration and maintenance of barrier islands and wetlands that flank New Orleans to the south and east are other adaptations that have the potential to reduce the loss of life and property due to flooding. Accu­ rate monitoring of subsidence is needed to provide calibration data for modeling and predicting sub­ sidence in coastal Louisiana, as well as for support for constructing and maintaining infrastructure and levees. GPS technology is being tested in the New Orleans region as a means for more frequent, less expensive subsidence monitoring.