Conservation

Deep Lake Terrace Project

Completed terracing project in the Rainey Sanctuary by building 14 terraces to 5-6 foot elevation to address long term sea-level rise projection.
Photo: Karen A. Westphal
Photo: Karen A. Westphal
Conservation

Deep Lake Terrace Project

Completed terracing project in the Rainey Sanctuary by building 14 terraces to 5-6 foot elevation to address long term sea-level rise projection.

Terraces are typically built no taller than 3 feet NAVD 88, which will soon be overtopped by rising seas and subsiding substrate and will last about 20 years. The terraces built in the Deep Lake Management Unit of the Rainey Sanctuary were built 5 to 6 feet, and for the first time, will address long-term (>50 years) sea-level rise projection.

This project is expected to improve the functionality and resiliency of 4,000 acres of wetlands in Louisiana, providing diversified habitat for fish and wildlife as the climate changes, tidal influences increase and sea level rises.

Designing the structure to either support trees or maintained as bird islands diversifies the future habitats and provides greater opportunities for a variety of species as the climate changes. The material to build the terraces was excavated from submerged areas adjacent to the terrace and resulted in deep water habitat in an otherwise shallow system that provides a refuge for aquatic creatures during periods of low water. The construction in open water with deep areas on either side discourages access by predators such as bobcat and coyote that are becoming increasingly destructive to marsh-breeding and migrating birds.

In July 2019, just three months after terrace construction, Hurricane Barry made landfall at the east end of Rainey. Upon reviewing the terraces for storm damage, the marsh behind the terraces were undamaged, although landscape not protected by terraces was affected. In addition, at least two Least Terns survived on each colony.

In September 2019, volunteers planted smooth cordgrass and Paspalum. In January 2020, Audubon staff planted 313 trees on nine terrraces.

Continued monitoring for this project will include:

  • aerial monitoring by drone
  • elevation and bathymetry monitoring
  • terrace habitat
  • surrounding land changes
  • SAVs
  • birds
  • non-avian wildlife

Additional Resources:

Karen Westphal hosted a webinar sharing the results of the deep lake terrace project.

 
The wetlands at Audubon’s Paul J. Sanctuary in coastal Louisiana sustained substantial damage following hurricanes Rita in 2005 and Ike in 2008. About 1,000 acres of the 26,000 acre sanctuary was converted to open water, putting the remaining marsh at increased risk to future storms.
In response, we have been conducting a series of restoration projects at the 4,000-acre Deep Lake Water Management Unit, one of the more heavily impacted areas of the sanctuary. In addition to some experimental use of small dredges, and working with Ducks Unlimited on hydrologic restoration, we designed a tall terrace restoration project in which 12 tall terraces and 2 marsh terraces would slow wave and wind driven erosion of the remaining marsh.
Normally, terraces are designed to be no more than +3.0 feet NAVD 88, but we had hoped to build taller terrace ridges that would be more resilient in the face of rising seas and increased storm intensity. We submitted our design and permit application to the LA Department of Natural Resources and Army Corps of Engineers, and received our permit after about 8 months of negotiations and waiting.
Funded by the Wildlife Conservation Society, Lamar Family Foundation, and Walton Family Foundation, we began outlining the locations of the terraces with tall poles.
We created a path of least impact to move the excavators into position to begin the project.
We contracted FrogCo, Inc., to build the tall terraces, which began in January 2019 and took about 3 months to complete.
As material was being piled, birds like Forster’s Terns were actively foraging in the slurry, picking fish and invertebrates that were displaced by the activity.
Excavators scooped up multiple layers of soft sediment. We regularly resurveyed each terrace to make sure they exceeded the goal of 5-6 feet in elevation.
Having two excavators working simultaneously increased the efficiency of building terrace of this size.
A completed terrace in March 2019 looks rough with a new layer of clay drying and settling.
We measured new terrace elevations with a Trimble RTK unit to check progress or document final height.
On the left is an image made by combining multiple photos taken by the drone into one image (mosaic) that was brought into the GIS with the terrace plan overlaid to see if the contractor followed our guide poles appropriately. On the right is a plot of the elevations we collected with our RTK unit as soon as the material was consolidated enough to walk on. These were sent to the contractor to either confirm our acceptance and designate it complete, or ask them to try again.
During low water, you could see the tracks where the excavators moved between terraces. Just a few more stacks and these will be complete.
Each terrace was constructed to 5+ feet NAVD 88 and measured 500 feet long. This is how the newly completed terraces looked in the landscape in April 2019. The view is to the north-northeast. For scale, there are boats at the bottom of the image.
Looking here to the northeast, the greener marsh was a recently completed 15-acre small dredge demonstration project. We anticipate that these terraces will bring synergistic benefits to that project, by slowing the movement of water in and out of the wetland management unit.
This July 2019 post-construction drone imagery overlaid on the base imagery took six drone missions to map the area at 400-foot altitude. The wide terrace on the far right (#3) and the left (#14) were built to 3 feet NAVD 88 for marsh as an offset to Essential Fish Habitat that was lost by building the 12 taller terraces. Two of these tall terrace ridges made it to 6 feet NAVD 88, and the remaining 10 each exceeded 5 feet NAVD 88.
Soon after construction was complete, we began seeing wildlife using the terraces. Just about every terrace had an alligator, which also used the deep borrow holes adjacent to the terraces.
About 10 pairs of Least Terns started nesting on three of the tall terraces in 2019, and this colony has expanded in 2020.
At the peak of the nesting season, Hurricane Barry made landfall on July 13, 2019, just 3 months after terrace construction. It passed just east of Rainey making landfall at Marsh Island and the east end of Rainey first before making the second landfall east of Intracoastal City. One pulse of storm surge near midnight brought the water level to 4 feet above normal.
Most of the terraces did not overtop with storm surge, and several of the Least Tern chicks survived, whereas most of the tern colonies along beaches in the region saw widespread devastation.
In September of 2019, 14 volunteers helped us plant the terraces with 11,500 plugs of smooth cordgrass and 2,500 plugs of Paspalum. We also planted 330 bare root seedlings of live oak and sugarberry on 9 of the tallest terraces in February 2020, as future upland habitat for migratory songbirds.
By the following spring of 2020, the plants were thriving, the surrounding water was clearing, and a vibrant wetland community was taking shape.

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