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The Barataria Basin

Location

The Barataria Basin is an irregularly shaped area bounded on each side by a distributary ridge formed by the present and a former channel of the Mississippi River. A chain of barrier islands separates the basin from the Gulf of Mexico. In the northern half of the basin, which is segregated by the Gulf Intracoastal Waterway (GIWW), several large lakes occupy the sump position approximately half-way between the ridges. The southern half of the basin consists of tidally influenced marshes connected to a large bay system behind the barrier islands. The basin contains 152,120 acres of swamp, 173,320 acres of fresh marsh, 59,490 acres of intermediate marsh, 102,720 acres of brackish marsh, and 133,600 acres of saline marsh.

Within the Barataria Basin, wetland loss rates averaged nearly 5,700 acres per year between 1974 and 1990. During this period, the highest rates of loss occurred in the Grande Cheniere and Bay Regions. Wetland loss within the Barataria Basin is attributed to the combination of natural erosional processes of sea-level rise, subsidence, winds, tides, currents, and herbivory, and the human activities of channelization, levee construction, and development.

Freshwater and sediment input to the Barataria Basin was virtually eliminated by the erection of flood protection levees along the Mississippi River and the closure of Bayou Lafourche at Donaldsonville; therefore, the only significant source of fresh water for the basin is rainfall. Only a small amount of riverine input, designed to mimic a natural crevasse, is introduced into the basin's wetlands through the recently completed siphons at Naomi and West Pointe a la Hache. This lack of fresh water, and the loss of the accompanying sediments, nutrients, and hydrologic influence, forms the most critical problem of the Barataria Basin.

The second critical problem is the erosion of the barrier island chain. As individual islands are reshaped or breached, or succumb to the forces of the Gulf of Mexico, passes widen and deepen with the result that a greater volume of water is exchanged during each tide.

Four islands-West Grand Terre, East Grand Terre, Grand Pierre, and Cheniere Ronquille-had a combined area of just over 1,800 acres in 1990. By 2015, the islands will be reduced to a total of approximately 1,000 acres. East Grand Terre and Grand Pierre are predicted to disappear by 2045, and the remaining islands will consist of only 400 acres.

The result of the problems described above is an increase in tidal amplitude in the marshes in the central basin. This cumulative effect is exemplified by increased salinities in the lower half of the basin, increased land loss rates, and change in vegetation.

Site-specific problems of shoreline erosion, especially in areas with organic soils, poor drainage, salinity stress, and herbivory, are apparent throughout the basin. Solving these problems is important, but less urgent than solving the critical problems described above.

Barataria Basin Projects

Projects in the Barataria Basin

Summary of the Basin Plan

STUDY AREA

The Barataria Basin (Figure BA-1) is located immediately south and west of New Orleans, Louisiana. The basin is bounded on the north and east by the Mississippi River from Donaldsonville to Venice, on the south by the Gulf of Mexico, and on the west by Bayou Lafourche. The basin contains approximately 1,565,000 acres. Portions of nine parishes are found in the basin: Assumption, Ascension, St. James, Lafourche, St. John the Baptist, St. Charles, Jefferson, Plaquemines, and Orleans. The basin is divided into nine subbasins: Fastlands, Des Allemands, Salvador, Central Marsh, Grande Cheniere, L'Ours, North Bay, Bay, and Empire.

EXISTING CONDITIONS AND PROBLEMS

The Barataria Basin is an irregularly shaped area bounded on each side by a distributary ridge formed by the present and a former channel of the Mississippi River. A chain of barrier islands separates the basin from the Gulf of Mexico. In the northern half of the basin, which is segregated by the Gulf Intracoastal Waterway (GIWW), several large lakes occupy the sump position approximately half-way between the ridges. The southern half of the basin consists of tidally influenced marshes connected to a large bay system behind the barrier islands. The basin contains 152,120 acres of swamp, 173,320 acres of fresh marsh, 59,490 acres of intermediate marsh, 102,720 acres of brackish marsh, and 133,600 acres of saline marsh.

Within the Barataria Basin, wetland loss rates averaged nearly 5,700 acres per year between 1974 and 1990. During this period, the highest rates of loss occurred in the Grande Cheniere and Bay Regions. Wetland loss within the Barataria Basin is attributed to the combination of natural erosional processes of sea-level rise, subsidence, winds, tides, currents, and herbivory, and the human activities of channelization, levee construction, and development.

Freshwater and sediment input to the Barataria Basin was virtually eliminated by the erection of flood protection levees along the Mississippi River and the closure of Bayou Lafourche at Donaldsonville; therefore, the only significant source of fresh water for the basin is rainfall. Only a small amount of riverine input, designed to mimic a natural crevasse, is introduced into the basin's wetlands through the recently completed siphons at Naomi and West Pointe a la Hache. This lack of fresh water, and the loss of the accompanying sediments, nutrients, and hydrologic influence, forms the most critical problem of the Barataria Basin.

The second critical problem is the erosion of the barrier island chain. As individual islands are reshaped or breached, or succumb to the forces of the Gulf of Mexico, passes widen and deepen with the result that a greater volume of water is exchanged during each tide.

Four islands-West Grand Terre, East Grand Terre, Grand Pierre, and Cheniere Ronquille-had a combined area of just over 1,800 acres in 1990. By 2015, the islands will be reduced to a total of approximately 1,000 acres. East Grand Terre and Grand Pierre are predicted to disappear by 2045, and the remaining islands will consist of only 400 acres.

The result of the problems described above is an increase in tidal amplitude in the marshes in the central basin. This cumulative effect is exemplified by increased salinities in the lower half of the basin, increased land loss rates, and change in vegetation.

Site-specific problems of shoreline erosion, especially in areas with organic soils, poor drainage, salinity stress, and herbivory, are apparent throughout the basin. Solving these problems is important, but less urgent than solving the critical problems described above.

FUTURE WITHOUT-PROJECT CONDITIONS

Projected wetland loss over the next 20 and 50 years within Barataria Basin, by the subbasins, is shown in Table BA-1. Without actions to correct the problems mentioned above, another fifth of the basin's wetlands would be lost to open water by 2045. Roughly 65 percent of the projected wetland loss, or more than 100,000 acres, would occur in the North Bay, L'Ours, Bay, and Empire subbasins. As wetlands bordering Barataria Bay erode and as its connection with the gulf becomes substantially larger because of the disappearance of the barrier islands, the bay would enlarge, absorbing adjacent waterbodies. With no action, moderate wetland losses (about 20 percent) would occur in the middle of the basin (Central Marsh and Salvador subbasins), and relatively minor losses (about 8 percent) would occur in the upper basin (Des Allemands) over the next 50 years. The disappearance of wetlands throughout Barataria Basin would mean the loss of critical breeding, nesting, nursery, foraging, or overwintering habitat for economically important fish, shellfish, furbearers, migratory waterfowl, alligator, and several endangered species. Loss of wetland habitat and the accompanying trend toward higher salinities would lead to lower biodiversity and productivity.

Table BA-1

Projected Marsh Loss in the Barataria Basin.


Projected Loss in 20 years Projected Loss in 50 years

Subbasin (Acres) (Percent) (Acres) (Percent)

Des Allemands 1,010 3 2,520 7

Salvador 4,610 4 11,540 11

Central 7,380 10 18,440 26

L'Ours 6,240 21 15,590 53

North Bay 10,160 12 25,390 31

Grand Chenier 6,510 44 14,660 100

Empire 17,460 58 30,110 100

Bay 22,790 28 56,980 70

Total 76,160 17 175,230 38


Projected losses are based on Geographic Information System data compiled by the U.S. Army Corps of Engineers. Loss rates also are based on a projection of the 1974 to 1990 rates.

The disappearance of wetlands and the wildlife and fishery resources dependent on them would affect the economic structure of numerous communities in the lower and middle basin areas as supporting businesses (marinas, boat manufacturers, seafood processors, retailers, etc.) decline. In addition, the storm buffering benefits the barrier islands and lower basin wetlands provide these communities, would be reduced as wetland loss continues. This loss would force relocations or require the expansion of flood protection and drainage facilities for many basin communities, and maintenance costs would increase for existing facilities.

BASIN PLAN

The selected plan focuses on the key strategies of freshwater and sediment diversion, combined with outfall and hydrologic management to reduce tidal exchange. Two additional mutually exclusive strategies were considered to offset the increase in tidal amplitude: sediment replenishment of the existing barrier islands or construction of a set of interior barrier islands. The former has been included in the selected plan because it supports the natural system, and would maintain the marshes located between the proposed interior barrier and the existing barrier islands. Supporting strategies of marsh creation with dredged material and shoreline protection address localized areas of marsh loss. A detailed description of the plan formulation process is contained in Appendix D. Strategies of the selected basin plan are shown in Figure BA-2, and projects are listed in Table BA-2.

Restoration of riverine input into the basin via freshwater diversion from the Mississippi River through the authorized Davis Pond Freshwater Diversion project helps in solving the first critical problem of freshwater and sediment deprivation. This diversion is vital to the health of the upper part of the basin because fresh water and nutrients slow the loss of marsh and swamp. Additional diversions from the Mississippi River on the eastern side of the basin, and the reconnection of Bayou Lafourche and subsequent construction of small diversions on the western side, are long-term solutions to the first critical problem. However, a study of the sediment and water budget for the Mississippi River must be completed first.

Sediment replenishment and marsh creation on the bay side of the barrier islands will strengthen the buffering capabilities of the barrier chain. Longshore sediment drift studies will determine the efficacy of installing segmented breakwaters or jetties to trap sediments that are, at present, transported from the system. Studies are planned on methods to reduce the cost of construction and to better evaluate the benefits of barrier islands to interior marshes. However, sediment replenishment of critical barrier islands (located adjacent to major tidal passes) needs to be implemented in the short term.

Hydrologic management to decrease tidal flux through the critical area of the central marshes and LOurs Ridge will preserve the marshes in this area and slow the inland progression of the marine influence. Methods to reduce marsh loss rates and shoreline erosion, while providing access to the estuarine-dependent marine organisms so important to the economy of this basin, should be developed and implemented as soon as possible.

Several site-specific areas of loss are scattered throughout the basin. Small-scale measures to preserve, restore, and enhance these marshes and swamps are important. Implementation of these projects will maintain these areas until the critical long-term projects are in place.

The selected plan uses a mix of measures to achieve short-term basin objectives. Hydrologic restoration (77 percent), outfall management (8 percent), and barrier island nourishment (6 percent) account for the majority of the acres preserved, created, or enhanced. Marsh creation with dredged material, shoreline protection, and marsh management complete the short-term restoration process. The long-term portion of the plan, necessary to achieve no net loss of wetlands, consists of additional freshwater and sediment diversions, and continued barrier island replenishment.

COSTS AND BENEFITS

Table BA-3 summarizes the wetland benefits and costs over the next 20 years for the short-term projects proposed in the Barataria Basin selected plan and for the Davis Pond Freshwater Diversion project. The Davis Pond Freshwater Diversion project will preserve 83,000 acres over 50 years at a cost of $68.8 million. However, to be comparable to the CWPPRA projects, benefits and costs for 20 years (32,220 acres and $26,696,000) were used.

In the Des Allemands Subbasin, no direct benefits are achieved because there are no selected plan short-term projects and Davis Pond Freshwater Diversion is located south of the subbasin. However, this area will indirectly benefit from plan implementation because significant portions of the seaward subbasins will be restored or maintained, thus providing a continued barrier to the inland progression of marine influence.

Implementation of the short-term projects in the Salvador Subbasin would prevent 28 percent of the predicted loss. In the Central Marsh Subbasin, implementation of already funded projects BA-2, PBA-35, and XBA-65A, plus the deferred project BA-6, would result in predicted marsh enhancement of 177 percent. When estimated Davis Pond Freshwater Diversion benefits are added to the Salvador and Central Marsh Subbasins, marsh enhancement increases to 337 and 281 percent, respectively. The CWPPRA costs are $39,889,000.

Plan implementation would prevent 12, 13 and 55 percent of the predicted loss in the L'Ours, North Bay and Grande Cheniere Subbasins. The projects located in this mid-basin area are designed to protect wetlands against tidal and erosive forces. Adding the Davis Pond Freshwater Diversion benefits to the North Bay Subbasin prevents 75 percent of the predicted loss. The CWPPRA costs for this area are $8,344,000.

The lower basin marshes and barrier islands which make up the Empire and Bay Subbasins are projected to undergo the greatest losses. Plan implementation would only reduce the losses in these areas by 5 and 8 percent, respectively. The Davis Pond Freshwater Diversion project would prevent the loss of an additional 17 percent of wetlands in the Bay Subbasin. The CWPPRA costs are $66,425,000.

For a total expenditure of $114,658,000 on the selected plan projects, 23,050 acres of wetlands will be created, restored or protected. Over the next 20 years, 30 percent of predicted loss in the entire Barataria Basin would be prevented. Benefits from the Davis Pond Freshwater Diversion project increases the predicted amount of marsh saved to 73 percent, including gains in two subbasins.

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Dynamics of the Basin

Located south and west of New Orleans, the Barataria Basin is bounded on the north and east by the Mississippi River, on the west by Bayou Lafourche, and on the south by the Gulf of Mexico (figure 21). The basin is approximately 120 miles long, with a width ranging from 24 to 35 miles. The basin contains approximately 1,565,000 acres, of which 341,500 acres (22%) are leveed or developed areas. The region contains major corridors of developed areas along the Mississippi River and Bayou Lafourche. While most of the land is privately owned, two wildlife management areas and one national park with a total of 65,000 acres are located within the basin.

Several natural and constructed physiographic features in Barataria Basin influence habitat distribution, hydrology, land use, and wetland restoration opportunities. Major features include natural and artificial levees of the Mississippi River and Bayou Lafourche, the GIWW, U.S. Highway 90, the central marsh landmass, the chenier complex, and a chain of barrier islands. The island chain is eroding and will continue to deteriorate unless restorative measures are implemented. The USACE maintains major navigation channels in the basin. These include the Barataria Bay Waterway, which runs from Barataria Pass at Grand Isle to the GIWW south of Lake Salvador; the GIWW, which runs east-west through the central reaches of the basin; and the Empire-Gulf Waterway, which runs from the gulf to the Mississippi River in southeast Barataria. All are major navigation routes for the oil and gas industry, the sulphur industry, and commercial and recreational fishing. Sediment deposited by the Mississippi River in the former St. Bernard and Lafourche deltas filled the margins of the Gulf of Mexico and built these marshes over the last several thousand years (Frazier 1967). These marshes received periodic inputs of sediment and fresh water from the Mississippi River until the early 1900s but are now isolated from the river. Although there is currently no river discharge into these marshes, extensive non-saline marshes exist where water exchange with the gulf is restricted, primarily because average rainfall (162 centimeters/year) exceeds average evapotranspiration (102 centimeters/year) in southeast Louisiana (Newton 1972).

Water volumes and water levels in the basin are strongly influenced by tides, winds, and precipitation. Tides of the northern gulf have a relatively small range between high and low, measuring 1 foot in the gulf and 0.1 foot in the upper basin (LCWCRTF 1993). Storm tides can account for more than half of the daily water level fluctuations in the basin (Jarrett 1976, Levin 1990). Water exchange within the basin is highly variable (Richie 1985, Richie and Penland 1989). The dominant water exchange route between the upper and lower basin is through Little Lake, Bayou Perot, and Lake Salvador.

Since 1932, Bararataria Basin has lost almost 17% of its land area (Dunbar et al. 1992). Recent annual wetland loss estimates in Barataria Basin range between 5,200 (Dunbar et al. 1992) and 7,100 (Barras et al. 1994) acres per year (figure 21). At this rate, Barataria Basin will lose up to 142,340 acres of land during the next 20 years-a loss greater than that in any other basin in Louisiana's coastal zone. The subsidence rate in the Barataria Basin, based on USACE tide gage readings (1947-78) at Bayou Rigaud, Grand Isle, Louisiana, is 0.80 centimeters (0.03 inches)/year (Penland et al. 1989).

Without actions to correct the land loss and habitat degradation in the Barataria Basin, another fifth of the basin's wetlands may be converted to open water by the year 2045. Approximately 65% of wetland loss would be concentrated in the southern half of the basin. The Barataria Bay would enlarge, absorbing adjacent water bodies, and its connection with the Gulf would become substantially larger as barrier islands disappear. As a result of continued erosion of the barrier island chain, the tidal passes will enlarge and deepen, reducing the potential hydrologic benefits of the islands to the basin.

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Discussion

All of the restoration projects planned for the Barataria Basin support the objectives and strategies outlined for the basin. Both outfall management projects support the strategy of managing freshwater and sediment input and enhancing fringe marshes in the basin. Continued emphasis on fresh water diversion from the Mississippi River is also of prime importance. The potential seems to exist not only to stop wetland loss, but to build new marsh through sediment and fresh water reintroduction into the Barataria Basin.

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.