Bay Raccourci Marsh Creation and Ridge Restoration (TE-156)

Post project degraded marsh and open water.

Location

The project is located in Region 3, Terrebonne Basin, Terrebonne Parish.

Problems

High saline waters from Lake Mechant have directly contributed to the loss and/or conversion of much of the historically intermediate marshes to low salinity brackish marshes north of Lake Mechant. Subsidence, canal dredging and storm damage have also contributed significantly to the loss of marsh in the area. The zone of intermediate marsh (transition zone between fresh and brackish marshes) is located just north of Lake Merchant. High salinity water entering Bay Raccourci via Bayou Raccourci/Lake Mechant flows unimpeded into low salinity marshes surrounding Bayou Raccourci, effectively short circuiting the TE-44 Project. The 1984 to 2016 USGS loss rate is -0.32%/yr for the extended boundary area.

Restoration Strategy

The primary goals of this project are; 1) restore marsh habitat in the open water and degraded marsh areas via marsh creation and 2) restore forested ridge habitat along Bayou Decade.

The project would consist of the creation of 341 acres if marsh and 103 acres of marsh nourishment with sediments dredged from Lake Mechant and confined with earthen dikes. The proposed design is to place the dredged material to a fill height of +1.17 ft NAVD88. Dewatering and compaction of dredged sediment should produce elevations conducive to the establishment of emergent marsh and within the intertidal range. Containment dikes will be gapped at the end of construction.

The northern containment dike would be built to a height of +4.0 feet NAVD88 with a 10 feet wide crown and would also serve as a ridge. Material for the ridge would be bucket dredged from both Bayou de Cade and from within the marsh creation cell. The entire 16,350 linear feet (17 acres) of ridge would be planted with saplings and bare root seedlings on the crown and smooth cordgrass along the bayou side slopes.

The project would result in approximately 342 net acres over the 20-year project life.

The project was approved for Phase I Engineering and Design in January 2020.

The project is on Priority Project List (PPL) 29.

The Federal Sponsor is US Fish & Wildlife Service.

The Local Sponsor is CPRA.

Land Loss and Human Impacts

Louisiana’s shrinking coastal zone is due to both natural causes such as rising sea levels and wave erosion, but human activity intensifies these evolutionary processes. Some of the most impactful land loss processes further increased by human activity include salt water intrusion, proliferation of invasive species, and subsidence.

Hurricanes and other storm events push salt water inland, increasing the salinity of wetlands to levels that damage local flora adapted to lower salinities, causing those plants to die, which in turn decreases their potential to reduce storm surge around human settlements. Dredged canals for oil and gas exploration provide easy pathways for salt water to move inland since these canals are often straight. [1] Healthy marshes decrease the distance that storm surges can infiltrate, so any man-made development that diminishes intermediate or salt marshes indirectly affects freshwater wetlands as well.

Invasive species are plants, animals, or other biota that are from other regions of the world that cause harm to our local native environment. One such invasive species with extensive ramifications for our coastal wetlands is the Coypu, or “nutria rat.” This large rodent devastates stands of native graminoids such as cordgrasses (Spartina spp) and bulrushes (Schoenoplectus spp). Coypu specifically target the base of stems and roots, digging for them in soft sediment platforms. [2] Lower root concentration in soils and active disturbance make for weakened substrates that are more susceptible to being washed away. Other invasive species have similar outcomes, but not necessarily by the same method. We have several invasive animals and plants in Louisiana, each introduced by humans either on purpose or accidentally, and each one has a destructive presence along our coast. CWPPRA actively works to counter the destruction of invasive species through research, engineering and reward-based mitigation, such as the Coastwide Nutria Control Program. [3]

Louisiana was built by the Mississippi River over the past several thousand years, depositing layer after layer of soft, uncompacted sediment. Naturally, that sediment will compact, causing the surface to sink. Developing human settlements might speed up this process due to increased weight. Some cities are sinking as fast as 12 millimeters per year. Combined with rising sea levels, these areas are getting 15 millimeters (.6 inches) closer to sea level each year. [4] Combining the natural subsidence rates with unnatural marsh degradation, flooding will continue to worsen in our towns and cities. The Coastwide Reference Monitoring System (CRMS) tracks subsidence as well as several other ecological conditions and CWPPRA project performance over 391 sites along the Louisiana Gulf Coast. [5] Human activity is an integral component of Louisiana’s coastal zone, and CWPPRA works with biologists, engineers, local governments, volunteers, and residents to study those adverse impacts and devise innovative methods to address and deter them.

 

Sources:

[1] http://web.mit.edu/12.000/www/m2010/finalwebsite/background/wetlands/wetlands-degradation.html

[2] http://www.columbia.edu/itc/cerc/danoff-burg/invasion_bio/inv_spp_summ/Myocastor_coypus.htm

[3] https://www.nutria.com/site9.php

[4] https://www.nola.com/news/environment/article_fc2fc043-f0a3-55a5-b1a5-ce96dc712c3e.html

[5] https://www.lacoast.gov/crms/Home.aspx#

 

Featured Image from JennyCuervo [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)%5D