3 minute read
Redistribution and Impacts of Nearshore Berm Sediment
Chandeleur Barrier Islands, Louisiana, United States
As part of the emergency response plan to the Deepwater Horizon oil spill, the Louisiana Office of Coastal Protection and Restoration Authority (CPRA) suggested that the State of Louisiana construct sand berms to mitigate the effects of oil. Two berms were built between 2010 and 2011; one to protect the highly sensitive Chandeleur Island chain— located in the Chandeleur Sound along the eastern edge of southern Louisiana—and the other to protect inland wetland ecosystems. Though it was not the primary intended purpose, the constructed berms mimicked nearshore beneficial use of dredged material (BUDM) applications that are utilized for barrier island nourishment and restoration. A study was undertaken to evaluate the evolution of the Chandeleur Island berm and the impacts of berm sediment on the northern Chandeleur Island. It was found that substantial amounts of the sand berm moved westward; most of its sediment was ultimately transported onto existing island features or across the island and into Chandeleur Sound. The project became a living example of the benefits nearshore berm placement offers, especially to adjacent habitat nourishment.
Article cover: Oblique aerial photo depicting the overwashing and redistribution of berm sediment onto and bayside of existing Chandeleur Island features. (Photo provided by Karen Morgan, USGS)
Producing Efficiencies
It is estimated that approximately 86 million cubic feet of dredged sediment was utilized to construct the Chandeleur Island sand berm. Berm sediment lifted the elevation of existing emergent marsh and shoals and created new shoals, resulting in increased colonization and productivity of emergent vegetation and submerged aquatic vegetation. The redistributed sediment also encouraged island feature stability in the maintenance of areal extent and the quantity and quality of vegetation present.
Using Natural Processes
Most of the berm eroded within 18 months of construction. If the berms are evaluated as BUDM applications, then marine and coastal processes (i.e., storm-induced scouring, breaching, overwashing) can be used to describe how berm sediment was redistributed within the island system, thereby nourishing the horizontal and vertical extent of existing and newly created island features. These changes are largely due to the influx of high-quality quartz sediment that was redistributed from the berm.
Broadening Benefits
Vegetation plays an important role in a marsh’s capacity to act as a buffer for high-energy wave activity and extreme weather impacts. The build up and reinforcement of the island also ensures its availability as a habitat for wildlife such as the brown pelican, the least tern, the piping plover, and loggerhead sea turtles.
Promoting Collaboration
The effort to create the berms and evaluate them was a collaboration between multiple federal and state agencies, including the U.S. Army Corps of Engineers, the Louisiana Office of Coastal Activities, the Louisiana Office of CPRA; Louisiana Governor Bobby Jindal; and others. Data for the study were collected and processed by federal and state agencies, universities, and private sector companies.
