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The Terrebonne Basin

Location

The Terrebonne Basin is an abandoned delta complex, characterized by a thick section of unconsolidated sediments that are undergoing dewatering and compaction, contributing to high subsidence, and a network of old distributary ridges extending southward from Houma. The southern end of the basin is defined by a series of narrow, low-lying barrier islands (the Isles Dernieres and Timbalier chains), separated from the mainland marshes by a series of wide, shallow lakes and bays (e.g., Lake Pelto, Terrebonne Bay, Timbalier Bay).

The Verret and Penchant Subbasins receive fresh water from the Atchafalaya River and Bay, while the Fields Subbasin gets fresh water primarily from rainfall. The Timbalier Subbasin gets fresh water from rainfall and from Atchafalaya River inflow to the GIWW via the Houma Navigation Canal (HNC) and Grand Bayou Canal; it has the most limited fresh water resources in the entire Deltaic Plain.

The Terrebonne Basin supports about 155,000 acres of swamp and almost 574,000 acres of marsh, grading from fresh marsh inland to brackish and saline marsh near the bays and the gulf. The Verret Subbasin contains most of the cypress swamp (118,000 acres) in the Terrebonne Basin. The northern Penchant Subbasin supports extensive fresh marsh (about 166,000 acres), including a predominance of flotant marsh, with 98,000 acres of intermediate and brackish marsh in the Lost Lake-Jug Lake area and about 17,000 acres of saline marsh to the south. Fresh marsh is also dominant in the Fields Subbasin (approximately 23,000 acres). The Timbalier Subbasin grades from fresh marsh in the northern part of the subbasin to saline marsh near the bays, but is dominated by brackish (71,000 acres) and saline (153,000 acres) marsh types.

Of the four subbasins, only the Fields Subbasin experiences problems which are local and relatively minor. The Timbalier Subbasin experiences substantial subsidence and is essentially isolated from major freshwater and sediment inputs. Marsh loss rates are high due to the resulting sediment deficit, saltwater intrusion along the Houma Navigation Canal and other canals, historic oil and gas activity, and natural deterioration of barrier islands, which contributes to the inland invasion of marine tidal processes (including erosion, scour, and saltwater intrusion). The subbasin is rapidly converting to an open estuary.

In recent years, the Penchant and Verret Subbasins have experienced significant freshwater impacts from the Atchafalaya River. Historic wetlands loss resulting from subsidence, saltwater intrusion, and oil and gas activity appears to have moderated, but areas of cypress swamp (Verret) and flotant marsh (Penchant) are experiencing stress from high water levels in the Penchant Subbasin, the use of freshwater and sediment resources is not being maximized.

Terrebonne Basin Projects

Projects in the Terrebonne Basin

Summary of the Basin Plan

STUDY AREA

The Terrebonne Basin is bordered by Bayou Lafourche on the east, the Atchafalaya Basin floodway on the west, and the Gulf of Mexico on the south. The Terrebonne Basin is divided into four subbasins--Timbalier, Penchant, Verret, and Fields, as shown in Figure TE-1. The basin includes all of Terrebonne Parish, and parts of Lafourche, Assumption, St. Martin, St. Mary, Iberville, and Ascension parishes.

EXISTING CONDITIONS AND PROBLEMS

The Terrebonne Basin is an abandoned delta complex, characterized by a thick section of unconsolidated sediments that are undergoing dewatering and compaction, contributing to high subsidence, and a network of old distributary ridges extending southward from Houma. The southern end of the basin is defined by a series of narrow, low-lying barrier islands (the Isles Dernieres and Timbalier chains), separated from the mainland marshes by a series of wide, shallow lakes and bays (e.g., Lake Pelto, Terrebonne Bay, Timbalier Bay).

The Verret and Penchant Subbasins receive fresh water from the Atchafalaya River and Bay, while the Fields Subbasin gets fresh water primarily from rainfall. The Timbalier Subbasin gets fresh water from rainfall and from Atchafalaya River inflow to the GIWW via the Houma Navigation Canal (HNC) and Grand Bayou Canal; it has the most limited fresh water resources in the entire Deltaic Plain.

The Terrebonne Basin supports about 155,000 acres of swamp and almost 574,000 acres of marsh, grading from fresh marsh inland to brackish and saline marsh near the bays and the gulf. The Verret Subbasin contains most of the cypress swamp (118,000 acres) in the Terrebonne Basin. The northern Penchant Subbasin supports extensive fresh marsh (about 166,000 acres), including a predominance of flotant marsh, with 98,000 acres of intermediate and brackish marsh in the Lost Lake-Jug Lake area and about 17,000 acres of saline marsh to the south. Fresh marsh is also dominant in the Fields Subbasin (approximately 23,000 acres). The Timbalier Subbasin grades from fresh marsh in the northern part of the subbasin to saline marsh near the bays, but is dominated by brackish (71,000 acres) and saline (153,000 acres) marsh types.

Of the four subbasins, only the Fields Subbasin experiences problems which are local and relatively minor. The Timbalier Subbasin experiences substantial subsidence and is essentially isolated from major freshwater and sediment inputs. Marsh loss rates are high due to the resulting sediment deficit, saltwater intrusion along the Houma Navigation Canal and other canals, historic oil and gas activity, and natural deterioration of barrier islands, which contributes to the inland invasion of marine tidal processes (including erosion, scour, and saltwater intrusion). The subbasin is rapidly converting to an open estuary.

In recent years, the Penchant and Verret Subbasins have experienced significant freshwater impacts from the Atchafalaya River. Historic wetlands loss resulting from subsidence, saltwater intrusion, and oil and gas activity appears to have moderated, but areas of cypress swamp (Verret) and flotant marsh (Penchant) are experiencing stress from high water levels in the Penchant Subbasin, the use of freshwater and sediment resources is not being maximized.

Figure TE-1. Basin and Subbasin Boundaries, Terrebonne Basin.

FUTURE WITHOUT-PROJECT CONDITIONS

Under a no action alternative, and assuming continued losses at the 1974-1990 rate, existing wetlands would be lost in the magnitude outlined in Table TE-1. The projected loss of more than half the Timbalier marshes in 50 years could be exceeded, because of the expectation that protection by existing barrier islands will cease within a few years to a few decades. The actual loss of Penchant marshes may be less than shown, because of benefits from Atchafalaya fresh water and sediment that have been increasing.

With no action, the Timbalier Subbasin will become 75 percent (or more) open water, with the shore reaching as far north as the suburbs of Houma. In the Penchant Subbasin, losses will likely be concentrated in the northern and central sectors, further exposing areas of open water and broken marsh. The inefficient use of Atchafalaya fresh water and sediments will continue to squander this significant resource. With continued high marsh losses, biological productivity and diversity will decrease. With loss of critical habitat for commercially and recreationally important fish, shellfish, and furbearers, as well as for endangered species, fish and wildlife dependent economic activities will decline. Flooding problems will increasingly impact economic activities throughout the Terrebonne Basin, leading to grave consequences for the oil and gas industry and for other human infrastructure.

Table TE-1.

Projected Marsh Loss


Projected Loss in 20 years Projected Loss in 50 years

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


Timbalier 60,100 22 150,250 56

Penchant 24,900 8 62,250 20

Verret Not Available Not Available

Fields 2,800 11 7,000 29


Total 87,800 14 219,500 36


BASIN PLAN

In the Timbalier Subbasin, protection and restoration of the barrier islands (Isles Dernieres and Timbalier Islands) requires immediate and extensive action, because these landforms provide protection for mainland marshes, and destruction of many of the islands is imminent. Interior marshes will also be protected through a hydrologic restoration zone which will be developed in the vicinity of the independently proposed Terrebonne Parish Comprehensive Hurricane Protection system. In this zone, fresh water and sediment will be used along with marsh protection and passive hydrologic restoration structures to enhance and restore overland and sinuous channel flow. A related action in the Timbalier Subbasin is a proposed barrier to saltwater intrusion in the Houma Navigation Canal.

In the Penchant Subbasin, Atchafalaya River fresh water, sediment, and nutrients will be better utilized through hydrologic restoration to protect marshes and reduce loss rates. To the extent possible, actions will restore historic flow

Figure TE-2: Terrebonne Basin, Strategy Map

patterns and conveyance channels and improve the distribution of sediment-laden water. These actions in Timbalier and Penchant are considered critical for short-term implementation.

In the Penchant Subbasin, at least one major diversion would be built from the Atchafalaya River to bring fresh water and sediment into the subbasin. This is contingent upon adequate addressing of flood problems in the subbasin.

Because these actions will not cover all areas of concern, a supporting short-term strategy is to consider site-specific, small-scale projects in all subbasins where there is a critical need for wetlands protection or restoration, or a significant opportunity for wetlands creation. In the short term, demonstration and pilot projects must also be conducted to develop or test methods and approaches needed for implementing long-term strategies.

In the Timbalier Subbasin, long-term restoration depends on cost-effective importation of sediment by diversions or dedicated dredging, which makes demonstration of sediment extraction, transport, and placement technologies a priority. In addition, the possibility of diverting Mississippi River water and sediment into Bayou Lafourche as a conduit to the Timbalier Subbasin (as well as to the Barataria Basin) must be evaluated, and will be part of a larger study. The establishment of a Mississippi River sediment budget and distribution options, to be initiated by the Task Force immediately, will greatly aid in this effort.

In the Verret Subbasin, pumping to lower water levels is required to protect the swamp forests. This is a long-term strategy, because significant planning activities must precede its implementation. In addition, this action cannot occur until provisions are made for managing outfalls in ways which will not exacerbate flooding in the Penchant Subbasin.

In summary, the Terrebonne Basin Plan includes both a short-term and a long-term phase. The short-term phase focuses on immediate actions needed to protect vulnerable marshes from the proximal causes of loss in the Terrebonne Basin (saltwater intrusion, erosion, and other consequences of significant hydrologic modifications) using a combination of restoration techniques (especially hydrologic restoration and small-scale marsh creation) in the most critical areas or key locations, and barrier island protection. Successful implementation of short-term strategies will reduce rates of wetlands loss, and will provide the foundation for longer-term strategies. The long-term phase focuses on wetlands gains through sediment diversion and import, with the intent of encouraging development of a sustainable wetland ecosystem. Long-term strategies are critical to addressing the primary problem of sediment starvation associated with high subsidence and loss of fluvial inputs, and to achieving no net loss of wetlands in the basin.

Projects included in the Terrebonne Basin Plan are listed in Table TE-2. Table TE-2 indicates the classification (e.g., critical, supportive, demonstration), estimated benefits and costs, and status of these projects. The main elements of the Terrebonne Basin strategy are displayed in Figure TE-2.

A description of the Terrebonne Basin plan formulation process is contained in Appendix E. A complete listing of projects that have been proposed for the Terrebonne Basin can be found in Appendix E, Table 5, including those that were combined with other projects, or were not included in the plan for reasons stated in the appendix. More detailed information on each selected project also is provided in Appendix E.

COSTS AND BENEFITS

An expenditure of approximately $310,000,000 will directly create, protect, or restore more than 32,000 acres of wetlands in the Terrebonne Basin (Table TE-3), with additional wetlands enhancement increasing the benefit to more than 100,000 acres (see Table TE-2). In the Timbalier Subbasin, implementation of critical and supporting projects which compose the short-term phase of the selected plan will offset almost one third (31 percent) of the predicted marsh loss by direct protection, restoration, or marsh creation. Additional efforts will be needed to achieve a sustainable wetlands environment in the Timbalier Subbasin, making the long-term phase of the plan--sediment import projects--and associated demonstrations necessary.

Table TE-3

Estimated Benefits and Costs of the Selected Plan 1/2/


Acres Created, Percent

Protected, or Loss

Restored Prevented Cost ($)

Critical Short-Term

Timbalier Subbasin 16,349 27 225,733,000

Penchant Subbasin 11,406 46 57,272,000

Fields Subbasin na na na

Subtotal 27,755 32 283,005,000

Supporting Short-Term

Timbalier Subbasin 2,269 4 16,971,000

Penchant Subbasin 2,218 9 9,018,000

Fields Subbasin 61 2 815,000

Subtotal 4,548 5 26,804,000

Total 32,303 37 309,809,000


1/ Only projects with estimates of both benefited acres and cost were included in the summary.

2/ Neither costs nor benefits are now known for the key strategies in the Verret Subbasin.

na--not applicable (no critical projects in the Fields Subbasin).

In the Penchant Subbasin, implementation of the short-term phase of the selected plan, including both critical and supporting projects, will avert or offset approximately 55 percent of the predicted loss. After hydrologic restoration is in place and flood control problems are addressed, the long-term strategy of diverting substantial amounts of Atchafalaya River water and sediment into the subbasin can be implemented, conceivably leading to no net loss of wetlands.

Although the costs and benefits for the key strategies in the Verret Subbasin are not currently known, the scale of the strategy in Verret is appropriate to the scale of stress on the cypress swamps and addresses the major portion of the problem. Only site-specific, small-scale projects are currently planned for the Fields Subbasin.

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Dynamics of the Basin

The Terrebonne Basin covers approximately 1,712,500 acres of southern Louisiana, including about 728,700 acres of wetlands (figure 22). About 96% of the wetlands in the Terrebonne Basin are privately owned. The USFWS recently established the 4,618-care Mandalay National Wildlife Refuge located in the Lake Hatch area of central Terrebonne Basin. State-owned land is represented by wildlife management areas (WMA's) and refuges covering about 28,244 acres in the southeastern basin. The state leases additional lands, which it manages as WMA's.

The USACE has constructed and maintains navigation channels in the Terrebonne Basin, which cross sensitive wetland areas. Vessel traffic in the channels is a major source of erosion in wetland areas. These channels also provide an avenue for saltwater intrusion into fragile wetland areas, thereby changing the salinity and nature of these wetlands and leading to deterioration and conversion to open water.

Subsidence occurs at different rates throughout the inactive deltaic plain as unconsolidated sediment dewaters and compacts. Subsidence in the Terrebonne Basin is among the highest in Louisiana at 0.42 inches/year (Penland et al. 1989). As subsidence occurs, flooding in wetlands increases, contributing to marsh loss. Subsidence also impacts the Terrebonne Basin's barrier island chains (Isles Dernieres and Timbalier Islands) that potentially provide protection to fragile inland wetlands. These islands absorb the impact of wave action from the Gulf of Mexico and potentially inhibit erosion of inland shorelines. As these islands shrink from subsidence, inland wetlands may become more vulnerable to the erosive forces of the Gulf of Mexico. Hurricane Andrew in 1992 had a severe impact on these islands, and without restoration, Louisiana's barrier islands will disappear. The extent of protection to interior areas by barrier island is currently being modeled through the Louisiana Barrier Shoreline Feasibility Study.

An abundant supply of fresh water and sediment is an important component to the health of wetlands in the Terrebonne Basin. These resources are supplied to the northern and western areas of the basin by the Atchafalaya River. The formation of the deep organic soils of this basin is a result of vegetative deposition, typically below ground with very limited mineral matter (Nyman et al. 1992, 1993a, 1993b, 1993c, 1994). The primary source of fresh water to the Timbalier subbasin (in the basin's southeast region) is precipitation, which averages 65 inches/year in this area. On average, precipitation is greater than evaporation; however, in the summer months evaporation exceeds precipitation. Sediment input into the southeast Terrebonne Basin occurs only when the Atchafalaya River stage is high and river waters flow down the Houma Navigation Canal. These inputs are small relative to the substantial influence of saltwater intrusion and high subsidence rates in the area. Overall, the southern basin has the most limited freshwater resources and sediment influx in the entire inactive deltaic plain. The absence of overflows from the riverine sources accounts for these freshwater and sediment deficits.

The hydrology of the Terrebonne Basin has been severely influenced by construction of canals and levees. As a result saltwater intrusion has occurred and has led to erosion and ultimate conversion of many areas from fresh marsh to salt marsh or open bodies of water. Barrier islands have also been impacted by erosion. As these islands have absorbed the wave energy of the Gulf of Mexico, they have continued to erode away.

Since 1932, the Terrebonne Basin has lost approximately 20% of its wetlands (Dunbar et al. 1992, figure 27). Current loss rates range from approximately 4,500 (Dunbar et al. 1992) to 6,500 (Barras et al. 1994) acres/year. This loss amounts to up to 130,000 acres during the next 20 years. One-third of the Terrebonne Basin's remaining wetlands would be lost to open water by the year 2040. Losses would be concentrated in the lower basin, where Timbalier Bay could become open to the Gulf of Mexico and the existing shoreline could retreat as much as 10 miles north (LCWCRTF 1993).

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Discussion

Project implementation in the Terrebonne Basin to date has focused on rebuilding barrier islands and creating, protecting, and restoring wetlands in localized areas. Since no projects have been completed in the Terrebonne Basin, there are no results of project performance. Once completed, almost 7,000 acres are anticipated to directly benefit from wetland creation, restoration, and protection through CWPPRA. Additional indirect benefits may include mainland marsh protection resulting from the CWPPRA barrier island projects.

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.