Subsidence

Current Understanding

The U.S. Gulf Coast from south Texas to the panhandle of Florida is slowly being drowned at varying rates by waters of the Gulf of Mexico. Whereas global sea level rise can account for only ~2 millimeters per year, the dominant factor is the sinking or subsidence of the land. Subsidence is defined as the lowering of the surface of the Earth with respect to a datum or point of reference.

Numerous studies have demonstrated that both natural and human-induced processes have played roles in the lowering of the land surface relative to sea level. Prior to human-induced changes in the amount of sediment carried by the Mississippi River and the construction of flood control levees, subsidence and much slower global sea level rise tended to be offset naturally by deposition of river sediments during floods and by organic sediment production in marshes. Following the great flood of 1927, local citizens wanted protection from floods and farmers and manufacturers in the Midwest wanted a stable Mississippi River in order to transport their crops and goods to market. If the U.S. Army Corps of Engineers (USACE) had not built the regional system of levees as requested by Congress, the Mississippi would have remained unreliable for commerce to and from the heartland and south Louisiana would have continued to be ravaged by floods.

An enormous volume of debris eroded from the Rocky Mountains and the Appalachians is carried by the waters of the Mississippi. Upon entering the Gulf of Mexico, the river slows to a stop and the sediments come to rest, forming the Mississippi River delta. Over time, the Gulf of Mexico basin has accumulated an aggregate thickness of sedimentary deposits of nearly 60,000 feet (more than 10 miles). This massive pile of sediments at the edge of the continent has two characteristics. First, its colossal weight has depressed and continues to depress the Earth’s crust. Second, the pile of sediment is weak and unable to support itself laterally. Over time, large tracts of the unstable pile have been displaced southward along sloping faults.

Geological and geophysical investigations have shown that subsidence is widespread, extending beyond the Mississippi River delta and coast, and is occurring more rapidly than previously thought. Several natural and human-related processes are known to be causing subsidence in the Gulf Coast today. Almost all previous studies have provided qualitative insights rather than quantitative measurements of how much subsidence has occurred. Modern-era subsidence is the integrated effect of multiple processes operating at several different spatial and temporal scales.

Some of these are natural processes:

Some are human-induced processes:

If major subsidence of the coast of Louisiana and surrounding states continues at rates such as those during the recent past, there will be major and likely catastrophic implications for the entire region. These could include:

  1. Growing vulnerability to tropical systems and associated storm surges,
  2. Destruction of the conventional vertical control network used to determine elevation for building levees, defining floodplains, and safe construction of communities, and
  3. Poorly developed strategies for future flood protection and ecosystem restoration.

All predictions of natural future impacts generally omit how humans will react. If nothing is done and the patterns of subsidence continue, sections of the northern Gulf Coast will be destroyed or rendered too dangerous in which to live. It is not a question of if inundation of unprotected areas will occur, but when. Since there is disagreement concerning the rates of subsidence, it is difficult to ascertain a specific time. Mitigation strategies can be developed to reduce short-term (100–200 years) risks to people and infrastructure, enhance the environment, and create economic development that could transform the state and region.

Near-Term Needs

Establishment and expansion of vertical control is mandatory if levees are to be rebuilt from accurate and up-to-date benchmarks. GPS with real-time capability coupled with an accurate and precise geoid model based on up-to-date airborne/satellite gravity data is the most cost-effective system to provide accurate elevations for recovery and rebuilding of homes and infrastructure. In applications where broad-based mapping is required, for example, in flood hazard delineation, airborne LIDAR (light detection and ranging) and satellite-based RADAR (radio detection and ranging) interferometric technologies offer unprecedented accuracy and economy.

FEMA and local governments need accurate elevations in order to determine how high citizens must raise or build homes and businesses to be above the 100-year flood elevation. Lack of local benchmarks will require surveyors to begin most of their surveys in outlying areas, resulting in greatly increased costs and completion times. Technology such as GPS linked to an updated geoid model can provide a highly cost effective solution to meet those needs.

Because almost all infrastructure in coastal Mississippi and Louisiana will need rehabilitation or reconstruction, accurate elevations are critical to the construction or repair of evacuation routes, waterways, sewerage and storm water drainage systems, communications, and other systems and services.

The height and geographical location of barriers tall enough to withstand storm surges will be guided by models such as those developed by NOAA and the Corps of Engineers. These barriers must be built high enough to account for future subsidence that will lower the land over the design life of the barrier. Future elevations can be estimated by taking today’s elevations and subtracting the effects of future sinking using subsidence velocity measurements.

Future engineered solutions to protect communities and infrastructure from flooding must include thoughtful consideration of the potential negative consequences to the ecosystem of human activities. Protection and restoration of ecosystems can be both successful and mutually supportive if designed properly. The behavior of coastal wetlands and barrier islands during storm surges needs to be investigated further to gauge their potential protective contributions.

Longer-Term Directions

A landscape model that can accurately predict future integrated effects of subsidence and sediment accretion needs to be developed.

A Gulf Coast-wide subsidence and accretion observing system that could be monitored by a real-time GPS, interferometric synthetic aperture RADAR (InSAR), and LIDAR would be useful in planning and developing mitigation programs.

Long-term planning for the Gulf Coast needs to incorporate the potential effects of climate change, both oceanic and terrestrial, on the future geomorphology and on storm surge dynamics.

Improved models of fault movement, petroleum extraction, and water pumping are required to predict the future extent of subsidence processes.

Planning for the use of water and sediment to nourish and establish wetlands should include patterns and rates of subsidence.