Alabama Barrier Island Restoration Study

Science Center Objects

Scientists are collecting geologic data and developing a numerical model framework to understand the evolution of Dauphin Island over the last 15-20 years and assess the future evolution of the island over the next 15-50 years, including the impacts of potential restoration scenarios.

Dauphin Island is a barrier island along the northern Gulf of Mexico with a population of over 1,200 people concentrated on its eastern end. The island protects mainland Alabama’s coastal communities and resources from storms, while also providing recreational opportunities (fishing, beach tourism, etc.) and other economic benefits to the local community and state. Dauphin Island (including its undeveloped western end) provides expansive coastal habitat, with over 200 acres of beach, dunes, overwash fans, intertidal wetlands, maritime forest and freshwater ponds in addition to shallow-water nearshore areas. These habitats support many living coastal and marine resources, including threatened and endangered species such as the piping plover. The island also protects the eastern Mississippi Sound ecosystem by reducing the wave energy and preserving the salinity structure for submerged aquatic vegetation (SAV), oysters, shrimp, crabs, and other species. Extreme events have severely impacted Dauphin Island over the past decades, including most recently Hurricanes Ivan, Katrina, and Isaac and the Deepwater Horizon Oil Spill. Focused efforts to maintain and restore the island have occurred, but a comprehensive plan for restoration of Dauphin Island focusing on its importance to the State’s natural coastal resources has not been developed.

Recently the National Fish and Wildlife Foundation (NFWF) provided funding for an inter-agency effort (State of Alabama, U.S. Army Corps of Engineers [USACE], and the U.S. Geological Survey [USGS]) to create an island-wide comprehensive restoration plan. This effort will investigate viable options for the restoration of Dauphin Island as a sustainable barrier island to protect and restore island resources, including habitat and living coastal and marine resources. Three USGS science centers are engaged in this project including the St. Petersburg Coastal and Marine Science Center (SPCMSC), the Wetland and Aquatic Research Center (WARC) and the Alabama Cooperative Fish and Wildlife Research Unit. The SPCMSC is working on research efforts focused on:

  • Characterizing the geology and bathymetry surrounding Dauphin Island;
  • Sediment characterization and distribution;
  • Conducting a historical shoreline change analysis of the gulf and estuarine shorelines;
  • Developing hydrodynamic and morphological models that can predict the decadal-scale evolution of Dauphin Island.

These activities will be integrated with other aspects of the Dauphin Island project to allow stakeholders and decision-makers to evaluate potential restoration alternatives in a comprehensive and data-driven way. For example, the morphodynamic modeling of the island receive and provide feedback to a USACE structural response model being used to evaluate the likely evolution of a rock-wall structure currently closing Katrina Cut (an extensive breach in the island). The modelled physical evolution of the island will also be used by USACE numerical modelers to evaluate the potential impacts of restoration alternatives on water quality in Mississippi Sound. Each of these frameworks will inform a habitat model that will be used to evaluate how restoration alternatives will influence the Dauphin Island ecosystem. Ultimately, the results of all of the numerical modeling efforts will be incorporated into a formal decision-support framework that directly uses the results of the study to provide quantitative evaluation of the benefits and uncertainty of different restoration alternatives.

Bathymetric and Geophysical Characterization

Trackline map showing the final 2015 singlebeam and multibeam coverages around Dauphin Island.

Trackline map showing the final 2015 singlebeam and multibeam coverages around Dauphin Island.(Public domain.)

In an effort to provide an updated and detailed physical characterization of the Alabama Barrier Island system, acoustic surveys were conducted to profile bathymetry and stratigraphy in areas that contribute to the evolution of the island, including the Mobile ebb-tidal delta, the inner shelf and shoreface, and within Mississippi Sound. Using these data, a comprehensive, high-resolution bathymetric Digital Elevation Model (DEM) around the island will be generated and then integrated with elevation data of the island to provide a complete up-to-date bathymetric/topographic DEM. These data were collected in collaboration with the U.S. Army Corps of Engineering (USACE) and support many overall project objectives including:

  • Monitor recent topographic and bathymetric changes to the island and the surrounding seafloor and determine amounts and directions of sand redistribution through comparison of present elevations to historical data.
  • Provide up-to-date elevations for wave modeling, restoration design, and coastal management.
  • Characterize the morphology and geologic framework of the innershelf, Mobile Pass ebb-tide delta, and Mississippi Sound. Geophysical investigations (including swath bathymetry, swath backscatter, sidescan sonar, seismic sonar) will provide information necessary to constrain the physical drivers influencing island evolution, and identify subsurface features that contribute to local perturbations in shoreline integrity.
  • Provide information necessary for locating and evaluating sediment resources suitable for island restoration efforts. 
  • Characterize changes in seafloor texture related to variability in depositional processes and habitat.
Merged topographic and bathymetric digital elevation model of Dauphin Island.

Merged topographic and bathymetric digital elevation model of Dauphin Island. (Public domain.)

Sediment Characterization and Distribution

Interpolated spatial distribution of median grain size

Interpolated spatial distribution of median grain size (in micrometer or microns) for the 300 plus samples collected on and around Dauphin Island, Alabama.(Public domain.)

In order to aid in defining sedimentological gradients around the Alabama barrier island platform and surrounding water bodies, sediment samples were collected along shore-normal transects that incorporate the basal shoreface (seaward to depths that encompass the active transport zone during both fair-weather and storm conditions), middle shoreface, upper shoreface (mean low water; MLW), beach berm, dune, interior flats, washover platform, back-barrier marsh, back-barrier flats and other sub-tidal back-barrier environments along the entire extent of Dauphin Island as well as Sand/Pelican Island and Little Dauphin Island. These data are processed for water content, dry bulk density, percent organic matter, and textural characterization. This extensive dataset provides a foundation for restoration monitoring and planning, modeling coastal processes, and developing a better understanding of sediment-dispersal patterns around Dauphin Island. Specifically, these data can be used to:

  • characterize longshore and cross-shore variability in sediment characteristics;
  • help evaluate habitat distribution and vegetation composition on the island and associated flats;
  • identify geotechnical properties that influence restoration approaches.

Shoreline Change Analysis

lidar-derived shorelines for Dauphin Island 1998-2013

Lidar-derived shorelines (red lines) for the period 1998-2013 for Dauphin Island, Alabama. (Public domain.)

An updated analysis of historical (decadal resolution since 1940s) and recent (annual to semi-annual scale since 1998) gulf and estuarine facing shoreline positions is being conducted to provide the basis for assessing short-term and long-term shoreline change along the coast of Dauphin Island, Alabama. The analysis includes the use of shorelines derived from both airborne lidar and historical aerial photography and will identify trends in shoreline erosion/accretion of both sides of the island as well as island width. These data and analysis are intended to aid in establishing island-scale sediment budgets and validate numerical models of morphologic change.

In addition to these observations of horizontal shoreline position, back-barrier marsh cores are collected to understand vertical accretion/erosion of marsh environments. Vegetation (e.g., marsh grasses) help trap sediment transported across the barrier island from the ocean side to the estuarine side during storm overwash and inundation. The combination of these data will help identity how the island is migrating through time, how the two sides of the island interact with each other, and whether the marsh areas are eroding or accreting as the island migrates.

Hydrodynamic and Morphological Modeling

Domains for the individual models

Domains for the individual models coupled in the hydrodynamic/morphodynamic simulation framework. Black points represent the outer northern Gulf of Mexico wave model domain, red points are the inner wave, circulation, and sediment transport model domain, and the depth colormap indicates the nested beach and dune storm response model. (Public domain.)

The development of a numerical modeling framework that can simulate the evolution of Dauphin Island is needed to support evaluation of restoration alternatives. Prior to the start of this project, there was no established or scientifically-validated model framework available that was suitable and robust for predicting the evolution of a barrier island on the time-scale of decades, including the impacts of storms and longer-term processes, and considering both restoration and no-action scenarios. The model framework will include a series of components to capture the dominant processes that dictate how the island evolves, including:

  • Delft3D to capture longshore sediment transport and the impacts to island morphology during quiescent periods and extratropical storms;
  • XBeach to predict the response of the island to tropical storm events, including dune erosion and overwash;
  • EDGR, a new Empirical Dune GRowth model developed for this project, to capture dune building during non-storm conditions.

The model framework will be calibrated/validated by hindcasting the island evolution between 1998 and 2013 and comparing to the 15 lidar surveys collected during that time period. Model simulations will provide a quantitative understanding of the processes governing the past and present Dauphin Island barrier system. Restoration alternatives will then be incorporated into the present day island configuration and simulations using the same model framework will be performed to estimate the island response in the future over various time periods.

Hydrodynamic and morphologic output from this model framework will be passed to other contributors of the project to inform the modeling of water quality, structural response, and habitat parameters, all of which change as the island morphology evolves.