OCS Sand
and Gravel Mining
The MMS/GOMR is supporting the development of environmental assessments in
coming years that propose to use sand resources from the Federal Outer
Continental Shelf (OCS) for coastal restoration and levee construction.
Available sand resources are recognized on Ship Shoal, a submerged remnant of a
barrier island system from a now-abandoned Mississippi delta lobe located 12 mi
off the central Louisiana coast. Water depths from 3 to 9 m (9-30 ft) occur
across the crest of Ship Shoal. Estimates report that the crest and flanks of
Ship Shoal contain approximately 1.5 billion yd3 of sand (Figure 1).
Geological and geophysical studies of Ship Shoal have identified these sandy
sediments to be an ideal source material to restore and replenish the rapidly
eroding barrier islands protecting coastal Louisiana.
Figure 1. Isopach (thickness) map showing high-quality sand
resources on Ship Shoal over a bathymetric base (University of New Orleans,
Coastal Research Laboratory).
The deltaic and coastal wetlands of southern Louisiana continue to be lost at
rates estimated at between 25 and 40 mi2 per year. Subsidence of the
land is caused by natural processes including (1) compaction of deltaic
sediments, (2) faulting with downward displacement toward the Gulf of Mexico,
and (3) sea-level rise. These natural phenomena can be aggravated by human
activities such as groundwater and hydrocarbon extraction. Moreover, land loss
has been magnified by changes in the Mississippi River’s flow parameters and
sediment load distribution after decades-long construction of upstream
navigation and flood control systems.
The barrier islands along coastal Louisiana act to dampen wave energy and tidal
surge during tropical storms that erode the islands as well as back-barrier
wetlands and coastal lowlands. Eventually, fresh- and brackish-water wetlands
are converted to salt wetlands or finally open water. The barrier island systems
protecting the Barataria and Terrebonne basins, Isles Dernieres and Timbalier
Islands, are eroding at alarming rates and are projected to effectively
disappear by 2012 without intervention or mitigation. The recent back-to-back
Tropical Storm Isadore and Hurricane Lili in 2002 served to focus attention on
just how rapidly this process is occurring.
At risk are 3 million ac3 of coastal plain and wetlands that contain
(1) over 1.5 million people south of Interstate 10, (2) abundant animal and
plant life including many endangered species, (3) sensitive estuary ecosystems
that yield 1.1 billion pounds of finfish and shellfish landings per year for the
U.S. consumer, and (4) billions of dollars of infrastructure in the form of oil
and gas wells, platforms, pipelines, roads, refineries, bridges, cities, towns,
canals, and ports; through which perhaps 80% of the oil and gas produced in the U.S.
Gulf of Mexico or transported to the U.S. from foreign markets enters or is
transferred.
The 1990 Coastal Wetlands Planning, Protection, and Restoration Act (CWPPRA;
also known as the Breaux Act) funds wetland creation, island restoration, and
beach nourishment projects nationwide in a cost-sharing arrangement between
local sponsors and the Federal Government. Since 1997, all elements of
Louisiana’s coastal restoration planning were consolidated into a multiagency
Louisiana Coastal Area Ecosystem Study to assist in establishing priorities for
future projects. The MMS is a participant. Other Congressional actions authorize
flood control structures and levees for coastal Louisiana areas, usually through
the U.S. Army Corps of Engineers (COE).
The first CWPPRA projects proposing to use Ship Shoal sand are two barrier
island restoration projects sponsored by the State of Louisiana on Isles
Dernieres, an important barrier island arc that has undergone significant
erosion (Figure 2).
Figure 2. Map illustrating over 100 years of land loss on
Isles Dernieres (University of New Orleans, Coastal Research Laboratory).
Two polygons each 10 mi2 in size are proposed on the crest of Ship
Shoal as borrow sites for sand dredging (Figure 3). The first project is to fill
a tidal channel called New Cut between Trinity and East Islands using 1 million
yd3 of sand from South Pelto Blocks 12 and 19. The second project is
to restore the west flank of Whiskey Island using 2.5 million yd3 of
sand from Ship Shoal Block 88. A third project proposing to use Ship Shoal sand
is sponsored by the COE. In the period 2004-2012 the COE proposes to use 12
million yd3 of sand from South Pelto Block 13 as the foundation
material for a 72-mi flood control levee in Terrebonne Parish (the Morganza
levee).
The MMS is currently carrying out the environmental analyses required by the
National Environmental Policy Act for these proposals. The barrier island
restoration projects are scheduled to begin during the 2nd quarter of
2004 and last about 4 months each. The Morganza levee project is scheduled to
begin late in 2004 and would involve approximately 2 years of continuous
dredging and impoundment of sand in two temporary holding areas near the Houma
Navigation Canal until needed for levee construction (Figure 3).
Figure 3. Map showing (1) sand borrow polygons in the Ship
Shoal and South Pelto areas (2) OCS block numbers, (3) distances between sand
borrow polygons, Isles Dernieres restoration projects, the project’s shore base
at the Port of Terrebonne at Houma, Louisiana, and temporary sand storage areas
for the Morganza levee project.
Link to MMS
National Sand and Gravel Program