Integrating Mapping and Modeling to Support the Restoration of Bird Nesting Habitat at Breton Island National Wildlife Refuge

Sources/Usage: Public Domain. View Media Details

This research is part of the Geologic and Morphologic Evolution of Coastal Margins project.

Studying Habitat Change at Breton Island

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 Theodore Roosevelt. Breton NWR is recognized as a globally important bird habitat because of the resources it provides, and hosts one of Louisiana's largest historical brown pelican nesting colonies. However, recent surveys indicate that this colony has declined dramatically, including a reduction of approximately 50 percent of breeding pelicans between 2008 and 2012. Loss of island area through relative sea-level rise, diminished sediment supply, and storm impact constitutes a major and ongoing threat (Lavoie, 2009; Martinez and others, 2009; Kindinger and others, 2013); in 2005, Hurricane Katrina completely submerged the island. Since the 1920s the island area has been reduced by over 90 pecent (Martinez and others, 2009). 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 (Lavoie, 2009), rendering the island unusable by nesting seabirds.

In order to restore Breton Island to pre-Katrina conditions, the U.S. Fish and Wildlife Service proposes rebuilding the shoreface, dune and back-barrier marsh. It is estimated that the proposed restoration would require over 3 million cubic yards of sand, to be acquired from offshore sources. Studies have shown that sediment deposits within Breton NWR suitable for shoreline nourishment are rare (Twichell and others, 2009), and are constrained to buried distributary channels, terminal spits and tidal deposits (Flocks and others, 2009). The USGS will use high-resolution geophysical investigations to characterize the geologic framework of the shelf and nearshore around the island, and provide information necessary to evaluate potential restoration resources.

Offshore sand resources extracted for restoration will leave a depression (borrow area) in the seafloor that may affect the wave climate in the region (e.g., Bender and Dean, 2003; Benedet and List, 2008). These perturbations to the shallow-water bathymetry can impact the wave field in a variety of ways, and may result in alterations in sediment transport resulting in new erosional or accretional patterns along the beach. Initially, a scenario-based numerical modeling strategy will be used to assess the impacts of the borrow area scenarios on the nearshore wave field. Impacts will be assessed over a range of wave conditions, and gauged, in part, by changes in significant wave height and wave direction inshore of the borrow sites.

The USGS will also perform a numerical modeling study to evaluate the response of potential island restorations to a range of oceanographic conditions. The model simulations will assess the evolution of the restoration scenarios to winter and tropical storms and the cumulative impact of multiple storms. The modeling study will help determine the predicted longevity of each design option and provide better understanding of where the nourishment material will be transported over time.

Objectives

• Understand the geologic evolution of the inner shelf

• Define the evolutionary relationships between the near-surface geology and seafloor morphology

• Identify resources for shoreline restoration

• Measure and monitor topographic and bathymetric changes through comparison of historical data to present

• Evaluate how restoration scenarios respond to various oceanographic conditions

• Determine if extraction of sand resources offshore will impact the wave climate at Breton Island

• Model natural processes, such as wave action and sediment transport, to evaluate how the island will respond to restoration

Methodology

Geophysical investigations of the island platform were conducted in 2007 as part of the Louisiana Barrier Island Monitoring Project (Kindinger and others, 2013). Data collection included single-beam bathymetry, chirp subbottom profiling, lidar, and aerial photography. This information will be greatly enhanced by additional high-resolution geophysical investigations and topobathymetric lidar, both on the island platform and offshore. Instrumentation will include interferometric swath bathymetry, sidescan sonar, and chirp subbottom profiling. Platforms used to collect the data range from a large research vessel providing 24-hour acquisition support, to a jet-ski equipped with a single-beam bathymetric transducer for shallow water and surf zone acquisition, to recently developed aircraft-mounted Riegl topo-bathymetric lidar. Sediment cores will be collected to ground truth the geophysical data.

A scenario-based numerical wave modeling study will be used to evaluate the impacts of proposed borrow area designs on the wave climate around Breton Island. The oceanographic scenarios are derived from a wave climatology developed for the region (Long and others, 2014). This methodology will be used to investigate the spatially variable wave climate under a range of wave conditions including both the low energy conditions that typically prevail in the northern Gulf of Mexico as well as during storm events (hurricanes and winter storms). The wave climate will be compared under the current conditions (no borrow area) and with adjusted bathymetry including proposed borrow areas to determine what changes in nearshore wave height and direction may result from the utilization of offshore sand resources in this area.

The new bathymetric data and a set of potential island restoration scenarios will be used to initialize a morphological model (Roelvink and others, 2009) that will simulate island evolution under a variety of meteorological and oceanographic conditions. The evolution of each restoration scenario will be simulated for a range of wave and water level conditions chosen from the same wave climatology used for the borrow area wave impact assessment. These simulations will quantify how each of the proposed island configurations will respond to individual and repeated storm events. The combination of model scenarios and geophysical observations will help quantify sediment transport processes and erosion/accretion of the seafloor and shoreline.

Data Synthesis

• Characterize the near surface stratigraphy, island and seafloor morphology using high resolution geophysical and sedimentologic investigations

• Evaluate the influence of the geologic framework on the past and present morphology and sediment processes

• Provide geologic and morphologic information for:

  • effective restoration design and implementation

  • incorporation into morphologic models of coastal change to determine island response to construction, storms, and long-term processes

  • identification of dominant sediment transport processes, pathways, and budgets, and understand the role played by geologic processes and constraints.

This research is part of the Geologic and Morphologic Evolution of Coastal Margins project.