Mermentau Basin: Summary of the Basin Plan
STUDY AREA
The Mermentau Basin lies in the eastern portion of the Chenier Plain in
Cameron and Vermilion Parishes. The 734,000-acre basin is bounded on the east by
Freshwater Bayou Canal, on the South by the Gulf of Mexico, on the west by
Louisiana State Highway 27, and on the north by the coastal prairie. The Grand
Chenier and Pecan Island ridge systems are linked by Louisiana Highway 82 and
divide the basin into two distinct subbasins: the Lakes Subbasin north of the
highway and the Chenier Subbasin south of the highway (Figure ME-1). About 18
percent (128,200 acres) of the basin lands are publicly owned as Federal refuges
and State wildlife management areas.
EXISTING CONDITIONS AND PROBLEMS
The basin contains about 450,000 acres of wetlands, consisting of 190,000 acres of fresh marsh, 135,000 acres of intermediate marsh, and 101,000 acres of brackish marsh. A total of 104,380 acres of marsh has converted to open water since 1932, a loss of 19 percent of the historical wetlands in the basin.
Prior to human alterations, delta-building processes associated with the Mississippi River resulted in periodic building of marsh along the gulf coast of the Mermentau Basin. Construction of flood control and navigation projects on the Mississippi and Atchafalaya rivers restricted those natural processes to relatively small portions of the coast. Consequently, marsh-building now occurs on only the eastern-most portion of the Mermentau Basins coastline. This condition is further aggravated by continuing subsidence and sea level rise. In the Mermentau Basin, relative sea level rise results in an average water level rise of 0.25 inches per year. Although natural wetland building processes only occur along the eastern shore, natural marsh maintenance processes (e.g., plant deterioration and regeneration) can be fairly effective at keeping wetland loss rates low. However, these processes have been altered or interrupted and the ability of the system to maintain the marsh is jeopardized.
The two subbasins suffer from distinctly different hydrologic problems. The most critical wetland problem in the Lakes Subbasin is excessive flooding. A 5-mile-long segment of Louisiana Highway 27 almost totally blocks drainage from the western portion of the Lakes Subbasin into adjacent wetlands of the Calcasieu/Sabine Basin. Similarly, along the southern boundary of the Lakes Subbasin, Louisiana Highway 82 blocks drainage across 17 miles of marsh. The Freshwater Bayou navigation channel has altered the historic drainage pattern in the eastern portion of the Lakes Subbasin. These numerous blockages of drainage outlets significantly increase ponding in the subbasin.
The Catfish Point Control Structure, built to reduce saltwater intrusion into
Grand Lake via the Mermentau River, controls the major drainage outlet from the
Lakes Subbasin. High water levels in the gulf frequently prevent the drainage of
the subbasin through the structure. Farther upstream, development and
channelization of the Mermentau River watershed have increased the rate of
run-off into the Lakes Subbasin. These factors, in combination with the loss of
historic drainage outlets, result in periods of prolonged high water levels
following heavy basin-wide precipitation. Because upland drainage improvements
are continuing
Figure ME1. Mermentau Basin, Boundary throughout
the Mermentau River watershed, high water levels in the Lakes Subbasin will
remain a problem.
Natural freshwater inputs from the Lakes Subbasin into the marshes of the
Chenier Subbasin are reduced by the same highway embankments that impound water
in the northern subbasin. The loss of those freshwater inputs is compounded by
waterways and canals that create additional connections between the gulf and
area marshes, facilitating saltwater intrusion.
FUTURE WITHOUT-PROJECT CONDITIONS
If nothing is done to solve the problem of wetland loss in this basin, current estimates project a continuing loss rate of 1,980 ares per year. Table ME-1 shows projected losses for 20- and 50-year periods for each subbasin.
In absence of remedial action, about 18 percent, or 62,900 acres, of the land in the Lakes Subbasin would be lost over 50 years. This loss would occur in wetlands adjacent to the shorelines of White and Grand Lakes and the banks of the GIWW and Freshwater Bayou Canal. Interior losses would continue in the Deep Lake area, the Freshwater Bayou wetlands, and the vicinity of Little Pecan Bayou.
Chenier Subbasin wetland losses are projected to be 32 percent, or 36,100
acres, over the next 50 years. Interior wetland losses would continue to occur
south of Pecan Island and Grand Chenier. Erosion along the gulf shoreline would
continue at the present rate of 20 to 40 feet per year.
Table ME-1
Projected Marsh Loss
Projected Loss at 20 yrs. Projected Loss at 50 yrs.
Subbasin (Acres) (Percent) (Acres) (Percent)
Lakes 25,160 7.3 62,900 18.3
Chenier 14,440 12.6 36,100 31.5
Totals 39,600 8.6 99,000 21.4
BASIN PLAN
The short-term portion of the Mermentau Basin plan depends on modifying existing structures and creating additional outlets to reduce ponding in the Lakes Subbasin and reducing salinity intrusion in the Chenier Subbasin. In addition, the plan utilizes shoreline protection, hydrologic restoration, marsh creation with dredged material, marsh management, terracing, and vegetative plantings. The long-term portion of the plan relies on hydrologic restoration and vegetative plantings. Figure ME-2 indicates the strategy for the basin. A detailed discussion of the plan formulation and evaluation process is in the Mermentau Basin Plan, Appendix H.
In the Lakes Subbasin, the short-term critical projects use two methods to
move water out of the subbasin for the purpose of reducing flooding stress on
vegetated wetlands: modifying the Vermilion Lock (which is no longer
operational) and the
Figure ME2. Mermentau Basin, Strategy Map operation of the Schooner Bayou Control Structure and Freshwater Bayou Lock, and creating additional outlets such as a structure at Black Bayou.
The short-term supporting projects within the Lakes Subbasin protect interior wetlands by hydrologic restoration (Sawmill and Humble Canals), rebuild open water areas (Big Burn and Deep Lake), and protect shorelines and banks (White Lake, Freshwater Bayou, and the GIWW).
The long-term supporting projects within the Lakes Subbasin treat critical loss areas by hydrologic restoration ( Miami South Levee and Coteau Plateau Marsh) and vegetative plantings (Little Pecan Island and along the GIWW).
For the Chenier Subbasin, the short-term critical projects use water evacuated from the Lakes Subbasin to treat the saltwater intrusion problem (White Lake Diversion, Grand/White Lake Diversion, and Hog Bayou Freshwater Introduction).
The short-term supporting projects within the Chenier Subbasin protect the gulf shoreline from the Mermentau River to the eastern boundary of the Rockefeller Refuge, restore hydrology (Rollover Bayou Structure), create wetlands (Pecan Island Terracing), and plant vegetation along the gulf shoreline.
Table ME-2 lists all the projects in the selected plan. A detailed
description of all projects in the selected plan is contained in Appendix H.
COSTS AND BENEFITS
Lakes Subbasin.
Implementation of the 30 evaluated projects in the selected plan (critical
and supporting short-term projects) will protect, create, or restore 6,710 acres
of wetlands and decrease marsh losses over a period of twenty years by an
estimated 27 percent at a cost of approximately $53,358,000. Three critical
hydrologic restoration projects in the subbasin were not evaluated for cost or
habitat benefits and will require further study and evaluation. The benefits for
these projects will depend on their ability to reduce the water levels in the
subbasin. Additional projects will need to be evaluated for the subbasin for
protection of acreage not covered under the present plan.
Chenier Subbasin.
The selected plan is expected to create, protect, or restore 3,150 acres of
wetlands and reduce marsh loss over a period of twenty years by 22 percent
at a cost of approximately $19,571,000. One project was not evaluated for cost
or habitat benefits and will require further study and evaluation. There is a
need to develop and evaluate other projects to achieve no net loss of wetlands.
If dredging technology becomes more cost-effective, the option of pumping
sediments from the gulf into shallow open water or deteriorating marshes will
need to be investigated. This can only be used in the more saline subbasin
marshes. It should only be done during the spring floods when the gulf
salinities are the lowest in order to avoid placing sediments with higher
salinities into marsh environments.
Table ME2. Summary of Mermentau Basin Projects
Insert Table ME-2. Summary of Mermentau Basin Projects
Insert Table ME-2. Summary of Mermentau Basin Projects