Teche/Vermilion Basin: Summary of the Basin Plan
STUDY AREA
The Teche/Vermilion Basin contains roughly 243,000 acres of wetlands in Vermilion, Iberia, and St. Mary parishes. The basin extends westward from Point Chevreuil through East and West Cote Blanche Bays, and includes Marsh Island and Vermilion Bay. The basin is bordered on the east by the West Atchafalaya Basin Protection Levee, on the west by Freshwater Bayou Canal and Louisiana Highway 82, on the north by the Lafayette/Vermilion and St. Martin/Iberia parish lines, and on the south by the Gulf of Mexico (Figure TV-1).
EXISTING CONDITIONS AND PROBLEMS
Much of the basin is occupied by three large bays: East Cote Blanche Bay, West Cote Blanche Bay, and Vermilion Bay. Marsh Island is an important hydrologic feature because it separates these bays from saltier water in the Gulf of Mexico. Therefore, marshes in this basin are primarily fresh, intermediate, and brackish with relatively few salt marshes. The Teche/Vermilion Basin lost 42,293 acres (14.8 percent) of marsh since 1932, nearly half of which was lost between 1951 and 1974, which is a relatively low rate compared to rates in other basins. Marsh loss is relatively slow because the basin is in the later stages of the delta lobe cycle; the more delicate wetlands deteriorated centuries ago. In fact, the delta lobe cycle has proceeded to the point that the basin should be experiencing rapid wetland creation in association with the emerging Atchafalaya River delta, but wetlands are not being built at maximum rates because the flow of fresh water and sediments down the Atchafalaya River is controlled at the Old River Control Structure. Fresh water and sediments from the Atchafalaya River benefit the basin nonetheless. Furthermore, numerous live and relic oyster reefs southeast of Marsh Island buffer water exchange between the big bays and the Gulf of Mexico, which also contributes stability.
Although the basin is geologically stable and benefits from the emerging Atchafalaya River delta, geomorphologic and hydrologic conditions have been altered by the dredging of navigation and petroleum access canals and the construction of spoil banks and levees. The effects of these alterations vary greatly from place to place, but generally they have created artificial barriers between wetlands and wetland maintenance processes, or removed natural barriers between wetlands and wetland decay processes. Interior marshes, traditionally maintained by annual flooding with fresh water in the spring, may deteriorate when exposed to increasing marine conditions, particularly in marshes where the soils have low mineral content. However, marshes near the Gulf of Mexico benefit from linkage with the gulf because winter storms deliver sediments to those marshes. Many landowners have responded to changing conditions caused by large-scale alterations by managing hydrologic conditions on a small scale using marsh management techniques. It is possible that some of these management efforts may not preserve marsh, particularly older ones. However, marsh management is an actively evolving field.
Some wetland loss might also be related to herbivory. Moderate herbivory
alone is not believed to cause wetland loss, but it may be the final straw in
marshes experiencing additional stresses such as flooding or saltwater
intrusion.
Figure TV-1. Teche/Vermilion, Basin Boundaries.
Most wetland loss in the basin occurs either as shoreline erosion or in isolated hot spots. Areas are classified as hot spots when they experience rapid loss relative to other marshes within this basin. Hot spots in this basin are smaller than in other basins; they presumably originate from hydrologic changes that alter the balance between the marsh maintenance and deterioration processes, but the specific causes vary from place to place. Canals and spoil banks have impounded some areas and increased tidal energy in other areas. Thus, some areas have become isolated from sediment input, whereas water exchange removes more sediments than are introduced in other areas. Inadvertent impoundment also causes some areas to flood excessively.
Shoreline erosion on the large bays is caused primarily by natural wave energy. Wave energy has gradually increased over the centuries because the bays are naturally getting deeper due to the very slight but constant subsidence and global sea-level rise. Wave energy is also believed to have been increased because humans reduced the size of the oyster reefs between Marsh Island and Point Au Fer that shielded the large bays from wave and tidal energy in the Gulf of Mexico. Severe shoreline erosion occurs on Marone and Redfish Points, Shark Island, and the shore of Weeks Bay.
Shoreline erosion can dramatically affect wetland loss when it causes relatively isolated marsh drainage systems to become hydraulically connected with dynamic water bodies such as navigation canals and the large bays. In other areas, shoreline erosion is particularly rapid and causes the direct loss of significant wetland acreage. These may be classified as hot spots of erosion. Erosion caused by boat wakes and water surges associated with the passage of large vessels also causes wetland loss along the GIWW and other navigation canals.
FUTURE WITHOUT-PROJECT CONDITIONS
Over the next 20 years, 14,700 acres or 6.1 percent of the marsh (based on
1988 marsh acres) will be lost unless preventative measures are taken (Table
TV-1). Within the next 50 years, 36,750 acres or 15.1 percent percent of the
marsh will be lost. Cumulative losses since 1932 will approach 28 percent by
2040. In 50 years, shoreline erosion will reduce Marone Point, Redfish Point,
and Shark Island, and Weeks Bay will be larger. The interior marshes on Marone
Point, those north and south of the GIWW between the Vermilion River Cutoff and
Tigre Lagoon, the south central marshes on Marsh Island, and marshes on State
and Rainey refuges will become shallow ponds. This will reduce fisheries
available for harvest by commercial and recreational fishermen and wintering
habitat for millions of waterfowl. The growing ecotourism industry will be
negatively affected, and storm surge protection will be reduced.
Table TV-1
Wetland Loss in the Teche/Vermilion Basin.
Measured Loss Projected Loss in 20 years Projected Loss in 50 years
1932-1990 (Acres) (Acres) (Percent) (Acres) (Percent)
42,293 14,700 6.1 36,750 15.1
BASIN PLAN
Several objectives were developed to guide protection, restoration, and creation of wetlands within the Teche/Vermilion Basin. These objectives were based on prevailing conditions in the basin. A description of the plan formulation process is contained in Appendix G.
The short-term portion of the plan is dominated by projects that protect critical shorelines, restore more natural hydrological conditions, and determine the causes of marsh loss in hot spots so that site specific counter-measures can be designed. Locations of major areas of activity are noted in Figure TV-2. The long-term goal of the plan is to maximize spring flooding of wetlands, which will require feasibility studies and coordination with adjacent basins.
Shoreline erosion will ultimately slow because the bays are gradually filling with Atchafalaya River sediments. But this may take centuries without additional flow from the Mississippi River into the Atchafalaya River. Nonetheless, it may be possible to accelerate this process in some areas, and high priority is given to projects that speed this beneficial process, such as sediment trapping in Little Vermilion Bay.
There are substantial benefits to protecting some current shorelines that shield relatively isolated marsh ponds and bayous. It is preferred that these projects use beach nourishment, dredged material, and sediment trapping, but it may be necessary to use hard structures to protect some fragile but critical shorelines. Such projects are cost effective because they prevent rapid hydrological changes from occurring throughout large areas. This is the primary focus of critical short-term projects in many areas such as Lake Sand at Marsh Island.
Several critical projects restore more natural hydrological conditions on a small scale. For example, the Cote Blanche Hydrologic Restoration project slows shoreline erosion, restores hydrologic barriers between interior marshes and the bays, and controls water exchange between the GIWW and the project area, but does not include complete enclosure by levees. The net result is that this marsh is protected from artificial water exchange and shoreline erosion, but can still flood with fresh, sediment-rich water from the Atchafalaya River that is available in the adjacent GIWW and bays each spring.
Reducing loss in hot spots requires various measures such as sediment trapping, hydrologic restoration, and freshwater diversion. Addressing hot spots requires site-specific techniques in different areas because causes of wetland loss and the availability of counter measures vary throughout the basin. Restoring spring flooding with fresh, sediment-rich waters may someday stop marsh loss in hot spots, but it is important to protect these areas from loss now because if they convert to ponds, they will have to be restored--a much more expensive process.
Thus, these projects are also classified as critical short-term even though specific causes of wetland loss must first be determined in each hot spot. Once site specific causes of marsh loss have been determined, then appropriate techniques, e.g., sediment trapping, hydrologic restoration, and freshwater diversions, can be implemented.
Restoring spring flooding to interior marshes provides optimum salinity
levels and introduces mineral sediments, which promote plant growth. Restoring
spring flooding on a regional scale is an important long-term goal, but it
requires increased sediment delivery to the Wax Lake Delta; managing diversions
into the Vermilion
Figure TV-2. Teche/Vermilion Basin, Strategy Map.
River, Bayou Teche, and the GIWW during the spring flood; or increasing discharge of the Atchafalaya River. Increasing fresh water and sediments available from the Atchafalaya will also speed bay filling, which will slow shoreline erosion and initiate wetland creation in Vermilion Bay, West Cote Blanche Bay, and East Cote Blanche Bay. Detailed study and planning are necessary to determine if these concepts are feasible. Thus, no projects are proposed at this time even though restoring spring flooding on a regional scale is a critical long-term strategy.
Projects in the Teche/Vermilion Plan are listed in Table TV-2, which displays the project type and classification. A detailed description of all projects proposed in the Teche/Vermilion Basin can be found in Appendix G, Table 9.
COSTS AND BENEFITS
The short-term projects proposed in the selected plan will protect or create
4,770 acres of marsh and prevent 30 percent of the predicted loss at a cost of
$34,039,000 (Table TV-3). In addition, 5,010 acres of marsh and submerged
aquatic vegetation will be enhanced. Costs and benefits of the other three
short-term critical projects cannot be determined until the site-specific causes
of marsh loss can be determined in each hot spot.
Table TV-3
Costs and Benefits of the Selected Plan
Acres Created, Percent Total
Project Protected, or Loss Benefited Cost
Classification Restored Prevented Acres ($)
Critical Short-Term 3,840 26 8,720 22,149,000
Supporting Short-Term 930 4 1,060 11,890,000
Total 4,770 30 9,780 34,039,000
Less than half of the marsh loss predicted to occur in this basin can be countered with the projects listed in the plan. Additional efforts will therefore be needed to achieve no net loss of wetlands. Substantial gains may be possible by addressing marsh loss in the hot spots. However, the most beneficial action is likely to be maximizing spring flooding on a regional scale. In addition to slowing marsh loss processes of saltwater intrusion and sediment starvation, this would likely promote creation of new wetlands. This is one of the few basins with substantial potential for wetlands creation, and every avenue to maximize spring flooding should be explored.