Calcasieu / Sabine Basin: Summary of the Basin Plan
STUDY AREA
The Calcasieu/Sabine Basin is located in southwest Louisiana in Cameron and Calcasieu parishes and consists of approximately 630,000 acres. The northern boundary of the basin is defined by the Gulf Intracoastal Waterway (GIWW). The eastern boundary follows the eastern leg of State Highway 27; the western boundary is the Sabine River and Sabine Lake; and the southern boundary is the Gulf of Mexico (Figure CS-1). About 24 percent (148,600 acres) of the basin lands is publicly owned as Federal refuges.
EXISTING CONDITIONS AND PROBLEMS
The basin contains about 312,500 acres of wetlands, consisting of 32,800 acres of fresh marsh, 112,000 acres of intermediate marsh, 158,200 of brackish marsh, and 9,500 acres of saline marsh. A total of 122,000 acres have been lost since 1932, 28 percent of the marsh that existed in 1932.
Marshes within the Calcasieu/Sabine Basin began forming about 3,500 years ago. Whenever the Mississippi River established a westerly course, large quantities of reworked riverine sediment were deposited along the gulf shore, resulting in southerly growth of the shoreline. When the Mississippi River shifted to an easterly course, the sediment supply decreased and erosive forces were greater than sediment deposition due to littoral drift. As a result, the shoreline converted to a more typical beach-like nature and gradually retreated. The repetitive occurrence of these pulses of sediment due to change in the Mississippi Rivers course helped to build the systems of cheniers (oak ridges) in the basin.
The progradation process served to establish an undulating land form along the gulf coast. The areas between the cheniers were collecting points for water and, over time, built up by decomposition and regeneration of plant materials to form low salinity marshes. These interior marsh areas would occasionally receive pulses of mineral sediment input due to storm tides.
Calcasieu and Sabine lakes are the major water bodies within the basin. Freshwater inflow to the basin occurs primarily through these lakes via the Calcasieu and Sabine rivers. Marshes within the basin historically drained into these two large lakes. This process was altered by the construction of channels to enhance navigation and mineral extraction activities. Navigation channels now dominate the hydrology of the basin. The Calcasieu Ship Channel is maintained at 40 feet deep by 400 feet wide and extends from the Gulf of Mexico to Lake Charles, Louisiana. The GIWW is maintained at 12 feet deep by 125 feet wide. The reach of the GIWW between the Sabine River and the Calcasieu Ship Channel was dredged to a depth of 30 feet in 1927. The Sabine-Neches Waterway, between the Gulf of Mexico and Port Arthur, Texas, is 40 feet deep by 400 feet wide.
The hydrology of the marshes between Sabine and Calcasieu lakes has also been
altered by numerous relatively small access canals. The GIWW and this network of
canals have established a hydrologic connections between the Sabine and
Calcasieu Estuaries. Additionally, a number of bayous which once drained
adjacent marshes into either of the estuaries have been connected to one
another. Consequently, marshes between Sabine and Calcasieu Lakes have become a
large interlinked system with water draining and circulating to the northern,
eastern, and western portions of the basin.
Figure CS-1. Calcasieu/Sabine Basin, Basin Boundaries
The water circulation patterns allow for higher salinity water to enter the interior marshes (saltwater intrusion). The basin soils, which are 87 percent organic and support lower salinity marsh vegetation, are infiltrated by the more saline waters. This leads to increased stress and loss of the plant communities, and eventually erosion and sediment transport out of the inner marsh areas.
Subsidence and sea level rise are natural processes that contribute to wetland deterioration and loss. Under pristine conditions, natural marsh building and maintenance processes are effective in maintaining coastal marshes despite subsidence and sea level rise; however, human alterations have disrupted the hydrologic processes which contributed to wetland building and maintenance, while subsidence and sea level rise continues. In the Calcasieu/Sabine Basin, subsidence and sea level rise result in an average water level rise of 0.25 inches per year. Although natural wetland building processes no longer occur, natural marsh maintenance processes can be fairly effective at keeping wetland loss rates low.
Erosion is a problem along the shores of Calcasieu and Sabine lakes and the banks of the GIWW. Erosion related breaching of the lakes shores threatens adjacent marshes because of the vulnerability of their typically weaker soils to increased water exchange and saltwater intrusion. Along the Gulf of Mexico, shoreline retreat is causing the loss of back-beach marshes and is threatening to alter the hydrology of interior marshes. Flood control projects on the Mississippi and Atchafalaya rivers, and construction of jetties on the Mermentau River, Calcasieu Ship Channel, and at Sabine Pass, have altered long shore sediment transport and sediment availability.
In summary, wetland loss within the basin is largely the result of extensive hydrologic alterations to wetland building and maintenance processes. Recent observations regarding marsh recovery indicate that in some areas, reducing salinities may protect and restore wetlands.
FUTURE WITHOUT-PROJECT CONDITIONS.
Land loss data for the period 1933 to 1990 reveals that 122,000 acres of
wetlands have been lost in the basin. The current wetland loss rate of 1,100
acres per year is based on composite data for the period of 1974 to 1990. Table
CS-1 shows the projected wetland loss over 20- and 50-year periods under the no
action alternative.
Table CS-1
Projected Marsh Loss
Projected Loss at 20 yrs. Projected Loss at 50 yrs.
Subbasin (Acres) (Percent) (Acres) (Percent)
Calcasieu 9,400 9.5 23,400 23.7
Sabine 12,500 8.4 31,200 20.9
Totals 21,900 8.9 54,600 22.0
BASIN PLAN
The Calcasieu/Sabine Basin Plan (Figure CS-2) has two possible strategies to
reduce the effects of saltwater intrusion and tidal scour: locks in the major
waterways or structures in the many canals where saltwater enters interior
marshes.
Insert Figure CS-2. Calcasieu/Sabine Basin, Strategy Map.
The latter is more cost effective and can be completed in a shorter time. The short-term projects in the plan include shoreline and bank protection, hydrologic restoration, freshwater introduction, marsh management, marsh creation with dredged material, and terracing. An additional freshwater introduction project is a long-term project in the basin plan. A detailed description of the plan formulation and evaluation is contained in Appendix I.
The core of the plan is structures at points where saltwater enters smaller canals that lead to interior marshes: the perimeters of Calcasieu and Sabine lakes, the Gulf of Mexico, and major waterways. This treats the adverse effects of basin-wide hydrologic alterations. Hydrologic restoration projects at Black Lake, Rycade Canal and twelve other areas, and marsh management in the Cameron-Creole area and at Brown Lake, are critical in preserving marshes. Shoreline protection projects at Sweet and Willow Lakes, from Constance Beach to Ocean View, and at five others sites, are also critical in preserving marsh. Freshwater introduction from the Toledo Bend Reservoir and marsh creation with dredged material from the Calcasieu Ship Channel are other critical projects. All these projects meet the key objectives of preserving marsh by restoring hydrology and maintaining the geological framework of the basin.
The availability of suspended sediment is limited throughout most of the basin. Freshwater diversions have been incorporated into projects where nutrient and sediment introduction may benefit wetlands. To the degree possible, actively managed perimeter structures will be opened during periods when nutrients and sediments can be introduced into wetlands.
Supporting projects are located in interior large open water areas and other severely eroding areas where perimeter projects alone would not provide a sufficient degree of protection or restoration. Bank protection at Johnsons Bayou; hydrologic restoration at Oyster and Mud Bayous and other sites; marsh management in Tripod Bayou, East Mud Lake, and Black Lake; marsh creation at Hog Island Gulley; beach nourishment with dredged material; freshwater introduction from the GIWW; sediment and nutrient trapping in Deep Lake and Browns Lake-Starks Canal area; and terracing are all supporting projects. These short-term projects help preserve the wetlands of the basin
Table CS-2 lists all the projects in the selected plan. A detailed description of projects in the selected plan can be found in Appendix I.
COSTS AND BENEFITS
The selected plan projects will protect, restore, or preserve 24,810 acres of
wetlands at a cost of $136,460,000. The plan will prevent all of the marsh loss
expected to occur over the next twenty years, producing a net gain of 2,910
acres of wetlands over this same period.
Table CS-2. Summary of the C/S Projects.
Table CS-2. Summary of the C/S Projects.
Table CS-2. Summary of the C/S Projects.
Table CS-2. Summary of the C/S Projects.