Breton Sound Basin: Summary of Basin Plan
STUDY AREA
The Breton Sound Basin encompasses approximately 676,400 acres, of which
184,100 acres are wetlands. It is bounded on the west by the Mississippi River,
on the north by Bayou La Loutre, on the east by the south bank of the
Mississippi River Gulf Outlet (MRGO), and on the south by Baptiste Collette
Bayou and Breton Island (Figure BS-1). The basin includes portions of Plaquemine
and St. Bernard parishes. It consists of approximately 51,300 acres of public
land, equaling 28 percent of the total lands within the basin.
EXISTING CONDITIONS AND PROBLEMS
The Breton Sound Basin is the remnant of a Mississippi River delta lobe, the abandoned St. Bernard Delta. The principal hydrologic features of the Breton Sound Basin include the Mississippi River and its natural levee ridges; the flood protection levee; the MRGO south disposal bank; Bayou Terre aux Boeufs and River aux Chenes (abandoned delta distributaries); and the freshwater diversions at Caernarvon, Whites Ditch, Bohemia, and Bayou Lamoque.
The natural processes of subsidence, saltwater intrusion, and erosion of wetlands, and the human effects of river levee construction and the oil and gas industry, have caused major impacts to the Breton Sound Basin in recent decades. The two major wetland problems resulting from the natural processes and human intervention in this basin are sediment deprivation and saltwater intrusion.
Historically, the basin was flushed with large quantities of fresh water and sediments annually during the spring. Marine waters would then rise and enter the basin during the late summer and early fall months and would be flushed out the following spring. In the early 1930s, flood protection levees were raised along the Mississippi River as far south as Bohemia in the Breton Sound Basin. This prevented the annual input of fresh water, nutrients, and sediment that nourished the wetlands and combatted saltwater intrusion.
Between 1940 and 1970, 12.9 square miles (8,256 acres) of canals were dredged across and between the abandoned distributary ridges that run from the river to the outer fringes of the marsh (Gagliano et al., 1970). This has allowed channelized outflow of fresh water and increased tidal flux.
The combination of natural processes and human intervention has allowed salt water to enter close to the head of the basin. Much of the fresh and intermediate marsh that occurred in the upper basin earlier in this century has either converted to more saline habitats or has become open water as a result of sediment and nutrient deprivation brought about by the construction of flood protection levees and saltwater intrusion caused by the dredging of oil and gas access canals through and between the natural distributary ridges.
Subsidence combined with sediment and nutrient deprivation has contributed greatly to the marsh loss in the upper and middle basin and even more greatly in the Bohemia Subbasin. The subsidence rate ranges from 0.6 feet per century in the upper portion of the basin to 4 feet per century in the lower portion. The effect of subsidence is very apparent in the area south of Bohemia, which was created by alluvial deposits of the Mississippi River less than 1,000 years ago. Large areas of wetlands flanking the Mississippi River in this area have subsided and are continuing to subside and convert to open water. Periodic overbank flows from the FIGURE BS1. BASIN AND SUBBASIN BOUNDARIES
Mississippi River occur in this area, and some wetlands immediately adjacent to the river are being maintained by this input of sediments and fresh water.
A significant cause of wetland loss in the Breton Sound Basin is erosion of
shorelines by wind-wave action. Along the shoreline of the outer marshes and
around the perimeter of the larger bays, erosion rates of 5 to 10 feet per year
are common. These high rates occur in the fringe marshes because the Breton
barrier islands are so far offshore that they offer little protection to the
estuary behind them.
FUTURE WITHOUT-PROJECT CONDITIONS
Table BS-1 shows the losses estimated over the next 20 and 50 years based on
1974-1990 loss rates from Table 2.
Table BS-1
Projected Marsh Loss
Projected Loss in 20 years Projected Loss in 50 years
Subbasin (Acres) (Percent) (Acres) (Percent)
River aux Chenes 500 2 1,230 4
Caernarvon 5,100 7 12,760 16
St. Bernard 2,300 6 5,760 14
Bohemia 5,480 16 13,720 41
Total 13,380 7.3 33,470 18.2
The effects of the Caernarvon Freshwater Diversion Structure, which is
expected to preserve 320 acres per year for 50 years or 16,000 acres, are
reflected in the projected losses for the Breton Sound Basin.
Marsh loss will continue in the upper and middle parts of the basin where sediments from the Caernarvon structure are insufficient to offset impoundment and sediment deprivation. The marshes in the lower basin will continue to deteriorate from wind-generated wave action and tidal scour, following the general abandoned delta break-up process. Marshes south of Bohemia will continue to subside, erode, and convert to open water except for those areas nearest the river, which will be maintained by periodic overbank flow.
The economies of communities in the basin are largely based upon oil and gas
and renewable biological resources. Fishery harvests have increased, largely due
to increased numbers of harvesters, each of which is harvesting less per
man-hour than was harvested ten years ago.
BASIN PLAN
The selected plan (Figure BS-2) provides a balanced approach to create, restore, protect, and enhance wetlands through the optimization of the available resources afforded the basin. Management and restoration of fluvial input form the foundation of the selected plan. In the short term, management of the Caernarvon Freshwater Diversion Structures outfall along with outfall management of Whites Ditch, Bohemia, and Bayou Lamoque Freshwater Diversions is vital to the restoration of this basin because such projects will help to maintain and restore the hydrology of the basin. Also, in the short term, construction of a small-scale controlled sediment diversion at Grand Bay and the restoration of overbank flow at Olga will create and nourish marsh through sediment transport.
Restoration of fluvial input to the basin through the construction of a 20,000-cfs sediment diversion, tentatively at Bohemia, is the core of the long-term strategy to restore the basin. A feasibility study is necessary to determine the optimum location for such a diversion. In support of the long-term strategies, construction of interior barriers and the restoration of natural ridges will help to restore the natural compartmentalized hydrology within the basin.
Projects selected for inclusion in the Breton Sound Basin plan are listed in
Table BS-2. The table indicates project type; classification (i.e., critical,
supporting); project status; acres created, restored, or protected; net
benefited acres; cost per benefited acre; and the estimated project cost.
COSTS AND BENEFITS
The proposed projects, short- and long-term critical and short-term supporting, will create, restore, or protect approximately 5,200 acres, 39 percent of the predicted loss at an estimated cost of $11,367,000. Including submerged aquatic vegetation and enhancement of existing marsh, an additional 4,400 acres will benefit from plan implementation.
The selected plan provides a balanced approach to improving conditions in the basin. Hydrologic restoration measures such as outfall management and sediment diversion account for the majority of the acres created, restored, and protected.
If cost-effective construction techniques are developed, the Fiddler Point
Barrier Island project could be implemented. This project would protect an
additional 1,190 acres, preventing 10 percent of the projected loss. The cost of
constructing this barrier island system using present technology is estimated to
be $55,115,000. The cost per acre is $118,000 and is nearly 30 times the average
cost per acre of the other proposed projects. Thus, the recommendation is to
proceed with the rest of the plan and postpone barrier island construction until
techniques are developed to decrease their cost.
INSERT FIGURE BS-2. SELECTED PLAN MAP
INSERT TABLE BS-2. SUMMARY OF BRETON SOUND BASIN PROJECTS