Science Support for the Mississippi Coastal Improvement Project Completed

Sources/Usage: Public Domain. Visit Media to see details.

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

Coastal Mississippi is protected by a series of barrier islands (Petit Bois, Horn, East Ship, West Ship, and Cat) that comprise the Gulf Islands National Seashore (GUIS). The islands support an ecologically diverse shoreline that provides habitat for wildlife including migratory birds and endangered animals. The majority of GUIS is submerged, and aquatic environments include dynamic tidal inlets, ebb-tide deltas, sandy shorelines, and seagrass beds. The islands are in a state of decline, with land areas severely reduced during the past century by storms, sea-level rise, and anthropogenic alteration. Since the mid-1800s up to 64 percent of GUIS surface area has been lost. Heavy damage is inflicted on the islands by frequent hurricane events, including Hurricane Katrina in 2005, which passed by as a Category 3 storm and battered the islands with winds of more than 160 kilometers per hour and a storm surge up to 9 meters.

Since 2007, the USGS, in collaboration with the National Park Service, has been mapping the seafloor and substrate around the islands as part of the USGS Northern Gulf of Mexico Ecosystem Change and Hazard Susceptibility project (NGOM). The purpose of these investigations is to characterize the near-surface stratigraphy and identify the influence it has on island evolution. Since 2009, this effort provided the basis for a collaborative effort with the U.S. Army Corps of Engineers (USACE) to expand the investigation outside of GUIS boundaries as part of the Mississippi Coastal Improvement Project (MsCIP). The MsCIP program consists of structural, nonstructural, and environmental project elements to restore portions of coastal Mississippi and GUIS affected by storm impacts. In order to mitigate shoreline erosion and breaching, the project includes the placement of sand along the islands, both on the existing beaches and within the littoral zone. This action requires the location and assessment of offshore sediment deposits that can provide suitable material for shoreline renourishment. The geophysical and sample information collected by the USGS through its geologic investigations provides this information. As part of the MsCIP program, the USGS mapped over 500 square kilometers (km²) of seafloor around GUIS between March 2010 and June 2013. Interferometric swath bathymetry, sidescan sonar, and Chirp sub-bottom profiling were used to characterize seafloor elevations, texture, and the underlying stratigraphy. Seafloor and subbottom features identified in the studies include shoals, sandsheets, lowstand valley fill, tidal delta deposits, and abandoned barrier deposits. Understanding the relationships between these features and the physical processes responsible for their formation is important in characterizing the history and evolution of the barrier islands and inner shelf.

Sources/Usage: Public Domain.

Sources/Usage: Public Domain.

Objectives

• Understand the geologic evolution of the inner shelf
• Define the evolutionary relationships between the nearsurface geology and seafloor morphology
• Identify resources for shoreline restoration

Methodology

Sources/Usage: Public Domain.

As part of the USGS NGOM project and the USACE MsCIP Project, extensive geological investigations were conducted along the Mississippi Barrier Islands and offshore between 2008 and 2013. The seafloor within the study area was mapped using single-beam and interferometric swath bathymetric systems and sidescan sonar. Vessel motion was recorded continuously using motion referencing units (for heave, pitch and roll), and differential Global Positioning Systems (navigation). To account for variability in sound velocity due to oceanographic conditions, water density was monitored continuously using a sound mini sound velocity sensor mounted at the swath head, and in profile through the water column through regular casts.

In the subsurface, two-dimensional profiles of the stratigraphy were acquired using seismic reflection (chirp) systems. For complete details on the data acquisition and processing, see DeWitt and others (2012); Twichell and others (2011); Pendleton and others (2010); Forde and others (2011), and Pfeiffer and others (2011).

The offshore geophysical data were collected onboard the Research Vessel (R/V) Tommy Munro, shallow-water surveys were conducted onboard the R/V Gilbert and smaller-draft research vessels. Survey lines for most of the study area were spaced 100 to 150 m apart in waters less than 10 m, and 300 m in deeper waters, parallel to the island shoreline. Tie-lines (shore perpendicular) were acquired 1 km apart. The study area covers 330 km².

Sediment profiles were collected offshore by the USACE and inshore by the USGS using vibracore systems. The cores were 3 to 6 m in length. Each core was described macroscopically and photographed. Subsamples were collected and analyzed for grain size. The sediment cores provided correlation between the geophysical data and the stratigraphy to ground truth the remote sensing data. Details on the collection and processing of the cores can be found in Kelso and others, (2015). 

Data Synthesis

• Bathymetric data were used as the foundation for the seismic, side-scan sonar, and sediment data. After the bathymetry data was processed all data were referenced to the seafloor elevation.
• Seismic data were used to create a 3-dimensional structure of the geology beneath the seafloor. Depositional units were identified and mapped in order to interpret the geologic framework.
• Stratigraphic profiles from the vibracores were used to ground truth the seismic data and to help recreate the depositional environments of each unit identified. Physical descriptions and grain-size analysis were used to characterize the physical properties of the units.
• Using seismic data, areas and volumes were calculated for the four sand units identified in the study, including calculating depth from modern seafloor to the tops of the units.

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