AND SEA LEVEL RISE IN LOUISIANA:
A STUDY IN DISAPPEARING LAND
21, 2003 — Subsidence, or land sinking, is one of the main causes
of coastal erosion and land loss in Louisiana and the surrounding Gulf
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.
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
direct and indirect causes of land loss and subsidence include:
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.
species: Animals not indigenous to the region are eating coastal
vegetation and further reducing the strength of the land.
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.
Salt water intrusion from coastal storms erode barrier islands, killing
native vegetation and accelerating erosion.
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.
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.
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.
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.
is Helping to Measure and Monitor the Effects of Subsidence, Sea Level
Rise and Coastal Storms
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.
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
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.").
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
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.
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.”
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-
Office of Response and Restoration working on vulnerability
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
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.
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.
Center for Operational Oceanographic Products and Services
Office of Response and Restoration
National Geodetic Survey
STUDY: PORTIONS OF GULF COAST SINKING AT SIGNIFICANT RATE: Increased Attention
Given to Address Public’s Vulnerability to Severe Weather
PARTNERS WITH ST. CHARLES PARISH COMMUNITY TO DEDICATE NEW REAL-TIME WATER
LEVEL MONITORING SYSTEM
National Weather Service
National Water Level Observation Network
Office of Coast Survey
UNVEILS NEW WEB SITE FOR MONITORING AND TRACKING SEA LEVEL CHANGES
Tyson, NOAA Ocean Service,
(301) 713-3066 ext. 191 or Ben Sherman,
NOAA Ocean Service, (301) 713-3066