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Integrating Mapping and Modeling to Support the Restoration of Bird Nesting Habitat
Breton Island, located at the southern end of the Chandeleur Islands, Louisiana, is part of the Breton National Wildlife Refuge (NWR) established in 1904 by President Theodore Roosevelt. Breton NWR is recognized as a globally important bird habitat, hosting one of Louisiana's largest brown pelican nesting colonies. Recent surveys indicate that this colony has declined dramatically. Between 2008 and 2012, there was a reduction of approximately 50% in breeding pelicans. Loss of island area through relative sea level rise, diminished sediment supply, and storm impact constitutes a major and ongoing threat; in 2005, Hurricane Katrina completely submerged the island. Since the 1920s, the island area has been reduced by up to 90% (Terrano et al., 2016). Without actions to restore sand into the island platform system, Breton Island is expected to completely submerge over the next two decades and evolve into a re-emerging sand bar, rendering the island unusable by nesting seabirds.
As part of the Deepwater Horizon Oil Spill Natural Resource Damage Assessment and Restoration (NRDAR) program, the Department of the Interior U.S. Fish and Wildlife Service (USFWS) was tasked with mitigating environmental damaged caused by the spill. That effort includes restoring beaches, dunes, and back-barrier marsh habitats in Louisiana, in part to increase the populations of birds such as brown pelicans, terns, skimmers, and gulls injured by the spill (learn more). Through this program, the USFWS, in collaboration with other Federal and state agencies, has initiated a plan to restore bird habitat by enhancing the beach, dune, and back-barrier marsh of Breton Island through sediment replenishment.
It is estimated that the proposed island restoration would require over 3 million cubic yards of sand, to be acquired from offshore sources. In 2014, the CMHRP collected geophysical data and sediment samples to characterize the geologic structure of the shelf and nearshore around the island, and provide information necessary to evaluate potential restoration resources.
Offshore sand extracted for restoration will leave a depression (dredge pit) in the seafloor that may affect the characteristics of the waves (e.g., wave height and direction) in the region. Changes to the wave field may alter sediment transport, resulting in new erosional or accretional patterns along the beach. CMHRP researchers developed numerical models to assess the impacts of sand extraction on waves near the shore. Impacts were assessed over a range of wave conditions, and gauged, in part, by changes in significant wave height and wave direction inshore of the sand extraction sites.
The CMHRP also partnered with a private engineering firm to conduct a decadal-scale numerical modeling study to evaluate the response of potential island restoration scenarios to a range of oceanographic conditions. The CMHRP worked closely with the USFWS to develop a modeling approach that addressed the specific needs of the project and could be effectively used to support decisions made while implementing the project. The model simulations assessed the evolution of the restoration scenarios to winter and tropical storms and the cumulative impact of multiple storms. The modeling study helped predict longevity of each design option and provided better understanding of where the nourishment material will be transported over time. In addition, the modeling and decision-support approaches developed at Breton Island are being improved and extended to other locations in the northern Gulf of Mexico as part of USGS-wide efforts to provide science and science-based products in support of projects to restore coastal ecosystems and enhance coastal community resilience.