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.
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.