Getting Geotechnical: Rockefeller Wildlife Refuge: A Shoreline Stabilization Success Story

More than 20 years ago, the Louisiana Department of Wildlife and Fisheries discovered the Rockefeller Wildlife Refuge, which borders the Gulf of Mexico for 26.5 miles, was losing about 50 feet of land per year. This loss was estimated to increase to 300 feet annually by 2016. For many, it seemed that soon there would be no land left.

However, a solution was in the works. The Rockefeller Refuge Gulf Shoreline Stabilization Project sought to construct breakwaters to combat the quickening erosion. But this project was not without its challenges—including soil conditions that precluded several breakwater construction techniques. In fact, there were more than 50 proposals for the project, many of which were unfeasible given the project’s parameters.

After narrowing down the options and constructing demonstration tests, the project team found a breakwater with a core formed from lightweight aggregate performed best.

Project Specifics

Through the course of a year, the design team monitored the land loss around a gravel beach model, a reef-style breakwater model and the breakwater with a lightweight aggregate core (LWAC) model. The gravel beach lost 84 feet of land, and the reef-style breakwater failed to break the wave energy as intended and lost 18 feet of land. The LWAC breakwater lost only 3 feet of land and met all the design team’s criteria.

After this demonstration, the team upscaled the 500-foot model LWAC breakwater into a series of 1,500-foot-long segments constructed approximately 150 feet offshore. These segments were separated by narrow gaps to allow the natural flow of water and marine wildlife into and out of the coastal area.

The LWAC consists of stacks of bags made from a woven geotextile and filled with approximately 5 cubic yards of expanded shale, clay and slate (ESCS) aggregates. After being filled offsite and transported to the construction area by barge, the bags were wrapped in another layer of geotextile material for protection and anchored with armor stones weighing between 1,000 and 5,000 pounds.

 

Why ESCS Lightweight Aggregate?

In very soft subsurface conditions such as Rockefeller Refuge, traditional breakwater methods just didn’t cut it. Ordinary stone sinks into shell hash and mud. For some projects, stone would sink so fast that a project team could never break the water line. In these situations, heavy stone represents wasted time and money, which never bodes well for future budget approvals.

Due to its network of internal voids, ESCS is approximately half the weight of common geotechnical fills. It’s also chemically inert and water insoluble, which made it particularly well-suited for the LWAC breakwater. The geotextile bags help keep the material in place and add extra protection against tides and storms.

It’s also important to note the material, transportation and labor costs associated with LWAC breakwaters. For this project, the availability of material and the simplicity of its transportation helped the winning proposal cost less than expected. This meant the initial three-mile length was able to be extended by an additional mile.

 

 

The Right Stuff

The combination of ESCS lightweight aggregate and geotextile bags has been a saving grace for the Rockefeller Wildlife Refuge. These materials helped project teams solve challenges in the preservation of a vital wildlife sanctuary. As extreme weather continues to threaten coastlines along the Gulf of Mexico and elsewhere around the United States, it’s important to have several solutions on hand to mitigate the damage.

For the Rockefeller Wildlife Refuge, ESCS lightweight aggregate and geotextile bags were the right products to protect this precious resource. Their use, in conjunction with the engineering knowledge of the design team and producer, prove that humankind can help preserve nature.