SUBSIDENCE AND SEA LEVEL RISE IN LOUISIANA:
A STUDY IN DISAPPEARING LAND

July 21, 2003 — Subsidence, or land sinking, is one of the main causes of coastal erosion and land loss in Louisiana and the surrounding Gulf states. Land loss rates have fluctuated over the years and recent studies show the rates have been reduced from 39 square miles per year between 1956 and 1978 to 24 square miles per year from 1990 to 2000.

Causes of Subsidence
Loading of the modern Mississippi River delta on the edge of the North American lithosphere (i.e., the crust and upper part of the Earth’s mantle), sediment compaction, faulting, and human activities are the main cause of subsidence in the Gulf states. The accumulation and compaction of several hundred feet of sediments since the last ice age has pushed the southern edge of Northern America downward, causing the Earth’s lithosphere to bend. Although the effect of this footprint (or load) of the delta is in southeast Louisiana, subsidence is also impacting northern Louisiana, eastern Texas, Mississippi and Alabama. Man-made causes, such as water and oil withdrawal from shallow wells, may also be contributing to subsidence in Louisiana.

Other direct and indirect causes of land loss and subsidence include:

• Levees: Land formed near the Mississippi River when it regularly overflowed its banks and deposited silt. When the area was settled, however, levees were built to contain the river. While levees stopped the flooding and protected the population, they also stopped the natural replenishing of the land.
• Invasive species: Animals not indigenous to the region are eating coastal vegetation and further reducing the strength of the land.
• Lack of bedrock: The land is naturally subsiding because it is built on Mississippi silt, instead of the firm bedrock that the rest of North America sits on.
• Coastal storms: Salt water intrusion from coastal storms erode barrier islands, killing native vegetation and accelerating erosion.

Subsidence Concerns in Louisiana
Due to subsidence, the state of Louisiana is becoming increasingly more vulnerable to destruction by coastal storms and erosion. The impacts of subsidence on wetlands, the population, and coastal roads and industries in Louisiana are of major concern for residents and officials alike.

• Louisiana Wetlands: Increased inundation from relative sea level rise causes wetlands loss. Indirectly, salt water intrusion kills salt intolerant vegetation, thus making barrier islands and wetlands vulnerable to increased wave action and erosion from coastal storms and hurricanes. Since most of coastal Louisiana is comprised of wetlands, the region is especially vulnerable to land loss.
• Coastal Populations: As much as 50 percent of the Louisiana’s population lives in coastal areas of elevations of three feet or less. As population increases in the region, vulnerability to coastal storms and hurricanes also grows.
• Public Safety Concerns: Recent studies by the NOAA National Geodetic Survey and the NOAA-funded Louisiana Spatial Reference Center at Louisiana State University have indicated that coastal Louisiana is sinking at a surprisingly fast rate. Last fall, NGS and LSRC measured the subsidence rates of Louisiana Highway 1, the major hurricane evacuation route for Grand Isle. There is evidence for approximately 1 foot of subsidence between 1982 and 2002 along LA-1 from Raceland to Grand Isle. From a public safety standpoint, evacuation roads along the coast like LA-1 have lowered to levels that concern citizens and emergency preparedness officials. Not only is LA-1 the main evacuation route for the area and the Port (supporting thousands of people working offshore), but also serves as a major service highway for moving materials and supplies connected with oil and gas operations in the Gulf of Mexico.

NOAA is Helping to Measure and Monitor the Effects of Subsidence, Sea Level Rise and Coastal Storms
In order to assess the rate of subsidence and measure relative sea level rise in Louisiana, up-to-date geodetic and water level data are needed. Unfortunately, most of this data are more than two decades old and data relationships between tide and geodetic datums are not well known. However, in cooperation with state and local agencies, the NOAA Ocean Service is implementing a two-stage (near-term and long-term) approach to address coastal subsidence issues.

Near-Term Plans

• New Water Level Stations: NOAA Center for Operational Oceanographic Products and Services recently installed new monitoring systems that will give more accurate forecasts of coastal water levels. On June 4, 2003, CO-OPS dedicated the St. Charles Parish Water Level Monitoring System. The system consists of two new real-time tide and water level stations — one in Lake Ponchartrain in Norco at Bayou LaBranche and one in Lake Salvador at Bayou Gauche — and two existing National Water Level Observation Network stations in Louisiana at Grand Isle and SW Pass. These two new stations will supplement the NWLON, which is the backbone of the National Water Level Program. NWLON consists of 175 long-term, continuously operating water level stations throughout the United States and its territories. "This joint effort between the Parish and NOAA to implement the St. Charles Parish Water Level Monitoring System will help determine the impact of storm surge, sea level rise and subsidence in Louisiana,” said Mike Szabados, director, CO-OPS. “This information will enable St. Charles Parish emergency managers to effectively plan and execute evacuations during coastal flooding events, thereby helping to save lives and property." Data from all four water level stations, including meteorological data, will be displayed on the local Data Acquisition System in the Parish Emergency Operations Center. Residents of St. Charles Parish and neighboring parishes will be able to access the data in real-time over the Internet and by telephone. Information from the water level monitoring system will also go directly to the NOAA National Weather Service to help improve storm surge and hurricane modeling efforts.

• GULFNET: In order to establish long-term records of rising water levels and the effects of storm surge, CO-OPS’ water level stations will be tied to GULFNET, a statewide network of GPS receivers operated by the LSRC. This collaboration between the geodetic reference system and the ongoing program for establishing updated tidal datums will provide the required baseline vertical datum information (Click NOAA photo to the right for a larger view of the GULFNET map. Please credit "NOAA.").
  
• Louisiana Spatial Reference Center: To address the lack of geodetic data and to create an accurate and long-term geodetic reference system for the state, the Louisiana Spatial Reference Center was created through a partnership between NGS and Louisiana State University. NOAA and LSRC, in partnership with the U.S. Army Corps of Engineers, New Orleans District and the Louisiana Society of Professional Surveyors, are building a system to provide up-to-date vertical and horizontal positioning data. There are currently three cooperative Continuously Operating Reference Stations in Louisiana that make up this system. LSRC and NGS will add several more CORS stations in 2003. This ongoing program will give the required vertical datum information for addressing Louisiana coastal subsidence concerns.

Long-Term Plans

• Comparing Land Elevation and Water Level Data: In order to find the true elevation of LA-1, NGS used the NWLON station at Grand Isle as the “point of departure.” The data from this station, which has been operating since 1947, allow for long-term sea level trends to be assessed and analyzed, acquiring the relative sea level rise. Indirectly, the relative sea level trends tell us how fast elevation moves relative to the water. By making survey ties between benchmarks at the Grand Isle tide station to the tide gauge and to the geodetic datum, and then subtracting out the estimated rates of global sea level rise, true changes in elevations can be found.
• NOAA National Weather Service: To increase personal and economic safety of population zones in coastal areas along the northern Gulf of Mexico shoreline, NOS and the NWS have joined forces to help mitigate the impact of storm surge and flooding from future hurricanes and coastal storms. Water level data from the CO-OPS’ stations are being fed directly to NOAA NWS to help improve storm surge and other modeling efforts. “Every year, we see new storms that impact a growing number of communities, roadways and infrastructure,” said Bill Proenza, director, NOAA Weather Service, Southern Region. “Our goal at NOAA is to provide the most timely, accurate information possible on the direction and severity of tropical storms and hurricanes. We recognize the changing landscape of coastal Louisiana (and other nearby coastal states) increases the threat from future storms.”

Other NOAA Efforts Related to Coastal Subsidence
This disappearing land changes shoreline and bathymetry negatively affecting coastal communities, industries, the environment and navigation. As a result, other NOAA offices have become actively involved in subsidence- related issues:

• NOAA Office of Response and Restoration working on vulnerability atlas: Eighteen percent of the nation’s oil is produced in coastal Louisiana. The loss of coastal land in Louisiana threatens 30,000 oil wells and their associated infrastructure. The NOAA Office of Response and Restoration is working to minimize the threat to oil development and transportation infrastructure due to coastal land loss (i.e., essentially infrastructure that was once on land is now under water, or once considered low risk is now considered high risk). One way that ORR is working to reduce the impacts from these risks is by developing a vulnerability atlas. Part of this atlas includes high resolution and precision shoreline and bathymetry data. By having accurate water levels connected to land elevations, NGS is able to collect aerial photography and conduct LIDAR surveys to determine shoreline boundaries.
• NOAA Office of Coast Survey updating coastal charts: The NOAA Office of Coast Survey has also determined that the Gulf Coast off of Louisiana is high priority. OCS is performing hydrographic surveys off of Louisiana, such as Atchafalaya Bay and Approaches, this field season to update outdated nautical charts for the area. The Gulf Intracoastal Waterway, one of the most heavily traveled commercial waterways in the United States, is also a part of OCS’ charting responsibility. OCS is now working to produce a suite of vector charts called Electronic Navigational Charts, including 25 different waterways on the GIWW between New Orleans and Galveston.
• NOAA Fisheries assisting with coastal restoration efforts: Data collected in recent surveys can be used in other projects. For instance, digital elevation models can be generated to assist with the design, construction and engineering of new marsh surfaces by tying in tidal datums to geodetic datums using the Global Positioning System and leveling techniques. Since marsh vegetation is sensitive to elevation and how often and how long it is inundated, frequency and duration of inundation analyses of the high waters from water level stations are conducted to assist biologists in determining where and when to plant various vegetative species. In addition to gathering this invaluable information, the NOAA Marine Fisheries' Restoration Center is also actively engaged in on-the-ground restoration projects in Louisiana through the Congressionally enacted Coastal Wetlands Planning, Protection and Restoration Act (CWPPRA or Breaux Act) of 1990. Through the CWPPRA program, NOAA Fisheries, other federal agencies and the Louisiana Department of Natural Resources develop and construct large-scale, multi-million dollar restoration projects in coastal Louisiana.

NOAA is dedicated to responding to coastal land loss and loss of coastal ecosystems issues in Louisiana and other Gulf States. These NOAA efforts will help to save lives, minimize negative economic impacts, and protect property and the environment for the State of Louisiana and the nation.

Relevant Web Sites
NOAA Ocean Service

NOAA Center for Operational Oceanographic Products and Services

NOAA Office of Response and Restoration

NOAA National Geodetic Survey

NOAA-LSU STUDY: PORTIONS OF GULF COAST SINKING AT SIGNIFICANT RATE: Increased Attention Given to Address Public’s Vulnerability to Severe Weather

NOAA PARTNERS WITH ST. CHARLES PARISH COMMUNITY TO DEDICATE NEW REAL-TIME WATER LEVEL MONITORING SYSTEM

NOAA National Weather Service

NOAA National Water Level Observation Network

NOAA Office of Coast Survey

Electronic Navigational Charts

NOAA UNVEILS NEW WEB SITE FOR MONITORING AND TRACKING SEA LEVEL CHANGES

NOAA Restoration Center

Media Contact:
Glenda Tyson, NOAA Ocean Service, (301) 713-3066 ext. 191 or Ben Sherman, NOAA Ocean Service, (301) 713-3066 ext. 178