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The Teche / Vermilion Basin

Location

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 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.

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.

Teche / Vermilion Basin Projects

Projects in the 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.

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

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.

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Dynamics of the Basin

The Teche/Vermilion Basin contains approximately 243,000 acres of wetlands in Vermilion, Iberia, and St. Mary parishes (figure 23). 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, on the north by the Lafayette/Vermilion and St. Martin/Iberia parish lines, and on the south by the Gulf of Mexico. Portions of the marshes are privately owned, and over 50% of the marshes are managed as wildlife refuges by the state of Louisiana and the Audubon Society. Collectively, the Louisiana State Wildlife Refuge, Marsh Island Wildlife Refuge, and the National Audubon Society's Paul J. Rainey Wildlife Refuge encompass over 139,000 acres of marsh and associated bayous, ponds, and lakes. The basin is composed of 5% cypress/tupelo swamp, 14% fresh marsh, 11% intermediate marsh, 36% brackish marsh, and 3% salt marsh (LCWCRTF 1993). The marsh soils are classified as highly organic peats, which are continually forming on the marsh surface through plant production and, more importantly, below ground through root growth (Nyman et al. 1992).

The principal hydrologic features of the basin include the Vermilion River, Charenton Canal, the Gulf Intracoastal Waterway (GIWW), the natural levee ridges of the Vermilion River and Bayou Teche, East and West Cote Blanche bays, and Vermilion Bay. Unlike other basins in the chenier plain, the Teche/Vermilion Basin has direct riverine inputs. This basin is experiencing an increase in riverine conditions because of sediment-laden freshwater flow from the Atchafalaya River (DeLaune et al. 1987). Water and sediment from the Atchafalaya River enter the basin from the east, flow westward, and dominate hydrological conditions in East and West Cote Blanche bays, which are gradually filling with sediment.

Wetlands in the Teche/Vermilion Basin are stable relative to many other Louisiana coastal marshes, with only 50,540 acres of land loss (9% of land area) since 1932 (Dunbar et al. 1992). The current basin land loss is approximately 650 (Barras et al. 1994) to 900 (Dunbar et al. 1992) acres per year and accounts for less than 3% of Louisiana's total coastal land loss. This lower relative loss rate is attributable to the older, and therefore more compacted, soils of the chenier plain. Wetland loss in the basin tends to be localized and occurs primarily as either shoreline erosion or in isolated hot spots of interior marsh deterioration (figure 23). Alterations resulting in hot spots of loss include the construction of navigation channels, oil and gas access canals, spoil banks, and levees. Some of the basin's wetland loss may also be related to heavy grazing by nutria and possibly muskrat. At current land loss rates, the Teche/Vermilion Basin will lose between 12,840 and 17,860 acres of land during the next 20 without restorative action.

Marsh loss in the Teche/Vermilion Basin also occurs as a result of shoreline erosion along large bodies of water and navigation canals. Shoreline erosion on the large bays is mainly caused by natural wave energy. In contrast, erosion along the GIWW and other navigation channels is caused by boat wakes and water surges associated with the passage of large vessels. Some shorelines form natural barriers between dynamic water bodies, such as bays and navigation channels, and the relatively isolated marsh ponds and bayous of the marsh interior. When these shorelines erode, the high energies associated with the dynamic water bodies begin to erode the fragile interior marsh (figure 24).

Although the bays are relatively sediment rich, spoil banks, levees, and access canals have isolated some areas from sediment input, which can result in a net loss of sediment. Areas that become impounded are subject to excessive flooding, while areas that have existing hydrologic barriers removed may be subject to saltwater intrusion.

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Discussion

In order to successfully protect, restore, and enhance the marshes within the Teche/Vermilion Basin, it is critical that projects be implemented to optimize use of freshwater and sediment inflows, address marsh loss hot spots, and preserve shorelines that serve as barriers between dynamic water bodies and interior marshes. Efforts to maximize flooding input from the Atchafalaya River are critical since, unlike other basins in the chenier plain, the Teche/Vermilion Basin has significant potential for fluvial inputs. The CWPPRA projects that have been or will be implemented address these objectives. Although it is too early to determine the success of the CWPPRA projects, preliminary indications are that the project goals are being met.

About This Site

The Louisiana Coastal Wetlands Planning Protection and Restoration Act Program web site contains information and links relating to coastal restoration projects in coastal Louisiana. This site is funded by CWPPRA and is maintained by the USGS National Wetlands Research Center.