Atchafalaya Basin: Summary of Basin Plan

STUDY AREA

The Atchafalaya Basin is located in the central part of the coastal zone, west of the Terrebonne Basin (Figure AT-1). It encompasses 58,400 acres of wetlands in St. Mary Parish. The basin boundaries are the Mississippi River and Tributaries (MR&T) system levees below Berwick and Calumet to the north, Bayou Shaffer southward along the bank of the Lower Atchafalaya River to its mouth then following the shoreline around Atchafalaya Bay to Point Au Fer to the east, and a north-south line extending through Point Chevreuil to the west.

EXISTING CONDITIONS AND PROBLEMS

Major features in the basin include the Lower Atchafalaya River, Wax Lake Outlet, Atchafalaya Bay, and the Atchafalaya River and Bayous Chene, Boeuf, and Black navigation channel. Features of the Mississippi River and Tributaries (MR&T) flood control system, including the Old River complex and the Atchafalaya Basin Floodway system, define the flow and sediment resources entering the basin and influence the basins evolution.

Previous Mississippi River delta complexes, including the Sale-Cypremort and the Teche deltas, formed the majority of the land within the Atchafalaya Basin. Delta growth in Atchafalaya Bay is a recent occurrence, with subaqueous delta, or land underwater, forming in the decade from 1952 to 1962 and subaerial delta, or land above the water, forming during the 1973 flood. About 16,000 acres of subaerial land exist today in the Lower Atchafalaya River and Wax Lake Outlet deltas in Atchafalaya Bay.

The Atchafalaya Basin is unique among the basins because it has a growing delta system with nearly stable wetlands. Wetland loss is minor in the areas north of Atchafalaya Bay when compared to the other basins. The total wetland loss in the area is approximately 3,760 acres between 1932 and 1990. The average loss from 1974 through 1990 is 87 acres per year. Wetland loss in this area is site dependent; loss is primarily due to erosion, human activities, and natural conversion. Storms and hurricanes cause shoreline erosion between Wax Lake Outlet and Point Chevreuil. Oil and gas pipelines disrupt the natural movement of flow and sediment within the wetlands. The development of the Lower Atchafalaya River, from a tidal to a riverine system, has created natural levees along the banks of the river, disrupting the movement of flow and sediment into the wetlands.

In Atchafalaya Bay, wetland gain, rather than loss, is taking place. However, natural processes and human activity are limiting the effectiveness of flow and sediment resources in creating new wetlands by affecting sediment delivery, deposition, and retention. Winter storm fronts, waves, and currents refine and reshape the deltas in the bay by eroding and reworking sediments. MR&T project features such as the Wax Lake Outlet Control Structure affect the location and quantity of flow and sediment entering the bay. Sediments available for delta building in the Lower Atchafalaya River delta deposit in the channel above Atchafalaya Bay. These sediments reach the delta only during significant high water events. The Chene, Boeuf, and Black navigation channel affects deposition and retention of sediments within the Lower Atchafalaya River delta. The majority of sediments conveyed by the Lower Atchafalaya River do not reach the delta; sands fall out in the navigation channel where they are dredged to maintain navigation;

Insert Figure AT-1. Atchafalaya Basin Boundaries

silts and clays are conveyed out of the bay. The lack of sediments available for delta growth in the Lower Atchafalaya River delta is evident when the growth rate of this delta is compared to that of the Wax Lake Outlet delta. The Wax Lake Outlet delta receives approximately one third the amount of flow and sediment of the Lower Atchafalaya River delta, and yet grows at a rate three times as great.

FUTURE WITHOUT-PROJECT CONDITIONS

Wetland loss in the area north of Atchafalaya Bay will generally continue at historical rates, resulting in 4,350 acres lost in this area in 50 years, or 8 percent of the existing acreage. Periodic overflow from the Atchafalaya system will continue to augment the wetlands, contributing to their overall stability. However, as the Lower Atchafalaya River and the Wax Lake Outlet evolve into riverine systems, natural levees will continue to form along the channel, disrupting the flow of sediment into the wetlands.

The deltas in Atchafalaya Bay will continue to grow. In 50 years, approximately 67,000 acres of subaerial delta will be present in both the Lower Atchafalaya River and the Wax Lake Outlet deltas. Of this subaerial land, approximately 27,550 acres will be vegetated wetlands--9,760 acres in the Lower Atchafalaya River delta and 17,790 acres in the Wax Lake Outlet delta, representing a gain in excess of 600 percent over the existing acreage.

As the deltas continue to grow, Atchafalaya Bay will change toward a riverine environment. Changes in salinity, water temperature, and turbidity will reduce shrimp, oyster, and marine fisheries production and increase furbearing, waterfowl and freshwater species production.

Table AT-1 shows projected wetland gain in the Atchafalaya Basin.

Table AT-1

Projected Wetlands in the Atchafalaya Basin

Measured Loss Projected Gain Projected Gain

1932-1990 in 20 years in 50 years

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

3,760 6.4 6,790 11.6 19,060 32.6

BASIN PLAN

Three strategies are available to increase the quantity of sediment delivered to Atchafalaya Bay: realign the entrance to Wax Lake Outlet, modify the Lower Atchafalaya River to increase its efficiency, and dredge sediments. Realigning the entrance to the Wax Lake Outlet is the preferred strategy. It creates more wetlands at a lower cost than the other two strategies.

Three strategies are available to reduce the quantity of sediment bypassing the Lower Atchafalaya River delta: relocate the navigation channel; relocate the flow and sediment to Wax Lake Outlet; and manage the growth of the Lower Atchafalaya River delta (delta management). Relocating the navigation channel is the preferred strategy because it solves a major problem of limited growth of the Lower Atchafalaya River delta without creating flood problems in the Teche/Vermilion Basin or significantly reducing flow and sediment to the Terrebonne Basin. However, it has the potential for significant environmental and engineering problems. Delta management, on the other hand, can be initiated now and continue over the long term until these issues are resolved.

Delta management, relocating the navigation channel, and realigning the entrance to Wax Lake Outlet are the selected large scale measures to reduce the impact of human activity on the growth and development of wetlands in the Atchafalaya Basin. Priority projects to reopen Natal Channel and Radcliffe Pass and reduce the height of the Big Island in Atchafalaya Bay also reduce the impact of human activity in the short-term. These projects work toward the long-term goal of overall delta management. Other short-term measures support the overall basin plan. Management in the established wetlands north of Atchafalaya Bay by closing oil and gas pipelines and reopening closed distributaries, restores fluvial input disrupted by human activity and natural processes. Shoreline protection reduces erosion. Dredging sediments creates wetlands that offset loss from human activity and natural processes.

Delta management is the critical component of the plan for the basin because of its significant impact on delta growth. Reopening Natal Channel and Radcliffe Pass and reducing the height of Big Island are critical to the success of the restoration plan because they will shape the direction of future delta management activities in the Lower Atchafalaya River delta. Results of delta management will be enhanced in the long term with the relocation of the navigation channel. This long-term effort will require engineering and environmental studies to ensure a feasible plan.

The short-term portion of the plan contains projects that can be implemented under the CWPPRA with minimum effort. Small scale projects such as shoreline protection measures are effective in solving small, site dependent problems of wetland loss and erosion and creating small areas of wetlands.

In summary, the selected plan uses sediment diversion, marsh creation, and shoreline protection measures to achieve the basin objectives. The predominant feature is sediment diversion. The selected plan emphasizes management of existing resources until these resources can be increased in the future.

Nine individual projects are part of the selected plan for the Atchafalaya Basin. Table AT-2 summarizes these projects, indicating project type, cost, acres created, whether the project is critical or supporting, and if it is to be implemented in the short term or long term. Appendix F contains a detailed description of each project.

Appendix F contains a description of the plan formulation process. Figure AT-2 shows the main elements of the plan.

COSTS AND BENEFITS

The selected plan creates, protects, and restores approximately 11,090 acres of wetlands over 20 years and a total of 28,150 acres in 50 years. The three critical projects create, protect, or restore 8,110 acres of wetlands over a 20 year period at a cost of $15,981,000. In addition, these projects benefit an additional 5,960 acres. The critical long-term project, delta management, creates an additional 4,070 acres of wetlands in 50 years. Short-term supporting projects create, protect, or restore 350 acres of wetlands in 20 years at a cost of $3,407,000 and benefit an additional 2,110 acres. Long-term supporting projects create 15,630 acres in 50 years at a cost of $110,590,000.

Insert Table AT-2. Summary of Atchafalaya Basin Projects

Insert Figure AT-2. Atchafalaya Basin. Strategy Map