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Pontchartrain Basin: Summary of Basin Plan

STUDY AREA

The 1,700,000-acre Pontchartrain Basin is an abandoned delta generally bounded by the Pleistocene Terrace on the north and west, by Chandeleur Sound on the east, and by the Mississippi River and the disposal area of the Mississippi River Gulf Outlet (MRGO) on the south. Portions of nine parishes lie within the basin: Ascension, St. James, St. John the Baptist, St. Charles, Jefferson, Orleans, St. Bernard, St. Tammany, and Livingston. The basin is divided into six distinct areas: the upper, middle, lower, and Pearl basins, and the Lake Maurepas/Pontchartrain and Lake Pontchartrain/Borgne land bridges (Figure PO-1). Approximately 17 percent of the land in the basin is in public ownership.

EXISTING CONDITIONS AND PROBLEMS

The three large lakes, Maurepas, Pontchartrain, and Borgne cover 55 percent of the basin. Lakes Maurepas and Pontchartrain are separated by land bridges of cypress swamp and fresh/intermediate marsh. A brackish marsh land bridge separates Lake Pontchartrain from Lake Borgne.

The basin contains 483,390 acres of wetlands, consisting of nearly 38,500 acres of fresh marsh, 28,600 acres of intermediate marsh, 116,800 acres of brackish marsh, 83,900 acres of saline marsh, and 215,600 acres of cypress swamp. Since 1932, more than 66,000 acres of marsh have converted to water in the Pontchartrain Basin--over 22 percent of the marsh that existed in 1932. The primary causes of wetland loss in the basin are the interrelated effects of human activities and the estuarine processes that began to predominate many hundreds of years ago, as the delta was abandoned.

The Mississippi River levees significantly limit the input of fresh water, sediment, and nutrients into the basin. This reduction in riverine input plays a part in the major critical problem in the Pontchartrain Basin--increased salinity. Construction of the MRGO, which breaches the natural barrier of the Bayou La Loutre ridge and the Pontchartrain/Borgne land bridge, allowed saline waters to push farther into the basin. Relative sea level rise of up to 0.96 feet per century gives saltier waters greater access to basin wetlands. Mean monthly salinities have increased since the construction of the MRGO and other canals. However, these mean increases are less than the overall variability in salinity. In recent years, salinities have stabilized. The heightened salinity, caused mainly by subsidence, stresses wetlands, especially fresh marsh and swamp.

A second critical problem, occurring in the lower basin, is the erosion along the MRGO caused by ship-induced waves. The channel's north bank continues to eroding at a rate of 15 feet per year. This mechanism has resulted in the direct loss of over 1,700 acres of marsh since 1968.

The third critical problem is the potential loss of the Pontchartrain/Borgne and the Pontchartrain/Maurepas land bridges where wetland soils are especially vulnerable to erosion. Since 1932, approximately 24 percent of the Pontchartrain/Borgne Land Bridge has been lost to estuarine processes such as severe shoreline retreat and rapid tidal fluctuations, and the loss rate is increasing. During the same time, 17 percent of the Pontchartrain/Maurepas Land Bridge marshes disappeared due to subsidence and spikes in lake salinity. In addition, from 1968 to 1988, 32 percent of the cypress swamp on this land bridge either converted to marsh or became open water. These land bridges prevent estuarine processes, such as increased salinities and tidal scour, from pushing further into the middle and upper basins. If these buffers are not preserved, the land loss rates around Lakes Pontchartrain and Maurepas will increase dramatically.

The fourth critical problem is that several marshes in the basin are vulnerable to rapid loss if adequate protection is not provided soon. Examples of theses areas are: marshes adjacent to lakes and bays where if the narrow rim of shore is lost, interior erosion will increase dramatically; the perched fresh marsh on the MRGO disposal area which will drain and revegetate with shrub unless the back levee dikes are repaired; and near Bayou St. Malo, where unless canals are plugged, rapid water level fluctuations and salinity intrusion into adjacent marshes will continue.

Site specific problems of shoreline erosion, poor drainage, salinity stress, and herbivory are apparent throughout the basin. Solving these problems is important, but less urgent than solving the four critical problems described above.

FUTURE WITHOUT-PROJECT CONDITIONS

If nothing is done, and marsh loss continues at the pace set from 1974-1990, another 62,400 acres, or 23 percent of the basin's existing marshes, would be lost by the year 2040, as displayed in Table PO-1. If no action is taken, 69,400 acres of swamp, 32 percent of the basin's existing swamp, would be converted to marsh or open water by 2040. This does not include the possible loss of the upper basin swamps. As the land bridges are lost, estuarine processes would push farther into the basin and erosion rates would increase. The middle basin would be a lake surrounded by shallow ponds where marshes once existed. The lower basin marshes would be a tattered remnant of what exists today. Fewer fish and shellfish would be available for commercial or recreational fishermen. Vast marshes for wintering ducks would no longer exist. The emerging ecotourism industry would be hindered, and storm surge protection would be lost as lakes and bays inched closer to levees and roads.

Table PO-1

Projected Marsh and Swamp Loss

Projected Loss in 20 years Projected loss in 50 years

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

Upper Basin

Swamp 0 0 0 0

Pontchartrain/Maurepas Land Bridge

Swamp 23,200 38 58,000 95

Marsh 1,320 6 3,300 15

Middle Basin

Swamp 9,600 62 11,400 74

Marsh 3,800 12 9,500 30

Pontchartrain/Borgne Land Bridge

Marsh 4,560 10 11,400 30

Lower Basin

Marsh 14,580 9 36,450 24

Pearl River Basin

Marsh 700 4 1,750 10

Total Swamp Loss 32,800 15 69,400 32

Total Marsh Loss 24,960 9 62,400 23

Insert Figure PO-1. Pontchartrain Basin, Basin and Subbasin Boundaries.

Back of Figure PO-1. Pontchartrain Basin, Basin and Subbasin Boundaries.

BASIN PLAN

The main strategies of the basin plan are shown in Figure PO-2. Restoration of riverine input into the basin via freshwater diversion from the Mississippi River through the Bonnet Carr Spillway solves the first critical problem, salinity. This is preferred to the strategy of a navigable gate in the MRGO because the diversion has the added benefit of restoring fluvial input and is less costly overall and on a per-acre basis. The project is already authorized and need not be funded under the CWPPRA. An outfall management plan for the diversion is critical. Construction of a rock dike on the north bank of the MRGO and the beneficial use of all the material dredged for the MRGO would stop erosion, addressing the second critical problem, and create large amounts of marsh. The diversion at the Bonnet Carr Spillway and bank protection with marsh creation along the MRGO are critical projects.

Additional short-term projects include the following.

In the long term, getting more fresh water and nutrients into the basin is critical. Five small-scale freshwater diversions into swamps and marshes of the basin are proposed. First, however, a study on the sediment and water budget for the Mississippi River must be completed.

Going beyond these diversions to achieve no net loss of wetlands in the long-term depends on cost-effective importation of sediment either by diversions or by dedicated dredging with dispersal by barging or pipelines. This critical long-term strategy could significantly reduce wetland loss in the basin, but it is very costly at this time.

Creation of artificial barrier islands could preserve the outer saline marshes. Although expensive, it is defined as critical and retained in the selected plan for possible implementation in the long term. Studies are planned on methods to reduce the cost of construction and to better evaluate benefits to interior marshes. If costs can be reduced and benefits increased, priority for implementing this strategy will increase.

The selected plan uses a combination of measures to achieve basin objectives. Projects accounting for the majority of the acres preserved or created are distributed in the following manner: hydrologic restoration (27%), freshwater diversion/outfall management (28%), shoreline protection (24%), and marsh creation (18%).

In summary, the short-term portion of the basin plan consists of the freshwater diversion at the Bonnet Carr Spillway and bank protection and marsh creation along the MRGO complemented by the preservation of the land bridges, critical areas, and other wetlands using numerous hydrologic restoration, marsh creation, and shoreline protection projects. The long-term portion of the plan, necessary to achieve a no net loss of wetlands, consists of additional freshwater diversions, sediment import, and the creation of barrier islands.

Projects included in the Pontchartrain Basin Plan are listed in Table PO-2. The table provides the classification (e.g., critical, supportive, demonstration), estimated benefits and costs, and status of these projects. A complete listing of all the projects proposed for the Pontchartrain Basin can be found in Appendix A, Table 8. More detailed information on each project is also included in Appendix A.

COSTS AND BENEFITS

An expenditure of $132,738,000 on short-term projects and $72,000,000 on construction and 20 years of maintenance of the Bonnet Carr Freshwater Diversion will create or preserve 17,320 acres of marsh and 3,600 acres of swamp and thus prevent 69 percent of the marsh loss and 7 percent of the swamp loss in the Pontchartrain Basin (see Table PO-3).

As shown in the table, short-term projects prevent 83 to 92 percent of the future marsh loss on the land bridges and achieve no net loss of marsh in the middle basin. However the plan prevents only 44 percent of the marsh loss in the lower basin. Clearly, additional long-term efforts are needed to preserve these eroding marshes. Construction of the artificial barrier islands prevents the loss of an additional 33 percent of the lower basin. However, the cost of barrier island creation, using present technology, is an additional $600 million. Long-term sediment import projects are essential in achieving no net loss in the lower basin. Sediment import into the upper basin is necessary to begin to preserve its cypress swamps. The cost of these sediment import projects is unknown. Thus, complete restoration of the upper and lower basins requires investigation of cost effective techniques to build barrier islands and import sediment.

Insert Figure PO-2. Pontchartrain Basin, Strategy Map.

Back of Figure PO-2. Pontchartrain Basin, Strategy Map.

Insert Table PO-2. Summary of Pontchartrain Basin Projects.

Insert Table PO-2. Summary of Pontchartrain Basin Projects (Continued).

Insert Table PO-2. Summary of Pontchartrain Basin Projects (Continued).

Insert Table PO-3. Results of Short-Term Projects and Bonnet Carr Freshwater Diversion.

Pontchartrain Basin: Summary of Basin Plan

Pontchartrain Basin: Summary of Basin Plan

Pontchartrain Basin