Caillou Lake Headlands (Whiskey Island) Coastal Restoration
Reinforcing a critical barrier island with the largest volumetric coastal restoration project in Louisiana history.
Louisiana is losing coastline at a rate of 16 square miles a year—or almost a football field of land every hour. Like many critical barrier islands, Whiskey Island was slowly disappearing into the Gulf of Mexico. GeoEngineers worked with teaming partners and the state’s Coastal Protection and Restoration Authority (CPRA) to design Louisiana’s largest volumetric restoration project ever and restore the island.
Barrier islands in the Gulf are on the front line of coastline erosion. They help protect communities, oil and gas infrastructure and the sensitive wetland and near-shore habitats on which the fishing industry depends. Whiskey Island in the Caillou Lake Headland is part of the Isle Derniere barrier island chain, remnants of a single, expansive barrier island (Isle Derniere means “Last Island” in French) which once protected the coast. Over the past 150 years erosion, subsidence, sea level rise and other factors have reduced the chain to a handful of relatively small, isolated barrier islands that provide just a fraction of the coastal protection offered in the past.
Historically, sediment transport from the Mississippi River would offset coastal erosion by naturally accumulating over time. Unfortunately, changes to sediment pathways and other impacts have blocked natural deposition and island building in the region. Without human intervention, experts at the US Geological Services (USGS) National Wetlands Research Center predict the gradual disappearance of coastal land in the region and increased impacts from storms and tides.
Standing at the extreme southern edge of the Caillou Lake Headland, and a 40-minute boat trip away from the nearest landing, is Whiskey Island. The island had been slowly disappearing for years, and CPRA believed restoring and reinforcing this frontline barrier island would help protect habitat and people in Terrebonne Parish. The coast harbors abundant marsh and swamp habitats and serves as an important base for the Louisiana fishing and oilfield services industries. CPRA contracted with Coastal Engineering Consultants (CEC) and GeoEngineers to design a dredge-and-fill plan and restore the island.
Restoring Whiskey Island meant coordinating a complex interplay of coastal, geotechnical and habitat-design elements for this thin barrier island. The team’s first challenge was the geotechnical investigation and the travel required to collect samples. Just getting to and from the island for the first of two phases of sampling meant a 40-minute boat trip from the nearest boat launch. During the initial stage, sampling was partially subject to U.S. Army Corps of Engineers (USACE) criteria, so GeoEngineers completed half the soil borings with a 5-inch diameter fixed piston sampler to satisfy regulations from the local district. This required extensive coordination and specialized equipment to collect the samples and reduce the risk of sample disturbance during the choppy boat trip back to the launch.
Reduced Costs with CPT Sampling
A review of the first phase of sampling data revealed a gap in the team’s knowledge of the area, so GeoEngineers suggested supplementing the existing soil borings with cone penetrometer testing (CPTs). CPTs provide in-situ soil data instantly on location without any need to transport samples to the lab for testing, which allows for faster data collection and removes the risk of sample disturbance during transport. Including CPTs in the design process reduced the number of days on site and testing costs while still providing high quality data. CPRA recognized the effectiveness of this sampling approach and has since included the use of CPTs in their Marsh Creation Design Guidelines.
Geotechnical Recommendations and Permitting
To restore Whiskey Island, the project team needed to find and transport a record-breaking amount of fill material to reinforce almost six miles of barrier habitat. Fortunately, they found plenty of high-quality sand at Ship Shoal, a submerged barrier island left over from an ancient era of lower sea levels. Unfortunately, the shoal was 10 miles away. Collecting fill material from so far away and pumping it back to Whiskey Island was not easy. Fortunately, CEC designed a highly effective conveyance system to pump the fill sediment from the offshore shoal borrow area to Whiskey Island.
Before the fill could be transported, however, CEC had to secure access to the shoal through the Bureau of Ocean Energy Management’s complex sand-leasing program. Although it was time-consuming, going through the program reduced the potential for multiple-use conflicts and environmental impacts. Following the bureau’s recommendations also helped preserve the remaining sand resources of the shoal so other parties can use it for future coastal protection projects.
In total, the team dredged, transported and carefully placed more than 10 million cubic yards of dredged sand on Whiskey Island—enough to fill the New Orleans Super Dome twice. All that sand built 600 acres of new beach, dune and marsh habitat to reinforce the island.
The team knew that dynamic tidal and storm forces meant rapidly changing conditions—barrier islands are always on the move. In addition to normal erosion, Tropical Storm Cindy and Hurricanes Harvey and Nate came barreling into the Gulf of Mexico in 2017 while restoration work was ongoing, shifting the landscape of Whiskey Island significantly. In the time between design completion and the beginning of construction, the eastern side of the island regressed northward 200 to 400 feet from sediment transport and erosion. The team quickly adapted the project design to the changing conditions by shifting the fill area northward while the contractor mobilized equipment to begin the construction phase. Despite efforts to keep the project moving, weather-related delays are often a cost of working on the coast. In all, the project team logged more than 100 days of lost time due to severe weather.
Protecting and Expanding Natural Habitat
Whiskey Island’s mangroves and marsh land have historically been a productive habitat for migratory and wintering birds, and the project’s construction timeline conflicted with nesting and breeding season. The team worked with the Louisiana Department of Wildlife and Fisheries (LDWF) and United States Fish and Wildlife Service (USFWS) to identify restricted nesting areas and permitted work areas. To avoid disrupting the fragile mangrove ecosystem during construction, the engineering team designed an alternate restoration strategy. Instead of adding fill on top of the existing island footprint—a traditional approach to restore barrier islands that might cover sensitive bird habitats—the team opted to add fill outward from the perimeter of the existing island. Already, this new habitat has attracted threatened native wildlife like the least tern, which is establishing a colony on the island after a long absence.
The project successfully turned an island on the verge of disappearing into the Gulf into a fortified barrier island with more than 600 acres of new coastal habitat—Louisiana’s largest restoration project by volume ever. All that new barrier habitat is providing enhanced storm protection for coastal communities and critical marsh habitat for native wildlife. There is much more coastal restoration work to do in Louisiana, and GeoEngineers’ work has helped CPRA define a standard approach to these projects. For example, CPRA now encourages bird-nesting considerations in future designs, and a bird-mitigation specialist is required for most coastal restoration project proposals. CEC and GeoEngineers’ passion, dedication, and innovative approaches contributed to the state of the art for future large-scale coastal restorations.