USGS – USACE partnership: Mapping offshore of the Mississippi barrier islands in support of MSCIP

Mississippi Coastal Improvements Program

The loss of land mass on the barrier islands has been documented and the continued loss will result in a change in the ecology of the Mississippi Sound. The options have been based on three basic concepts for island restoration:

• Adding additional land mass to the existing islands by using sand dredged and transported from an off-shore location. This option would be used to protect historic Fort Massachusetts and fill in Camille Cut. Potential sources of sand are being sought just offshore of Ship Island. USGS is investigating the quantity and quality of the sand using geophysical and coring methods.
• Adding sand into the littoral zone at specific locations between the islands based on additional sediment transport modeling. This option would allow the littoral currents to move the sand onto the islands where the natural process of island building could take place. This would not affect the present-day islands and would help mitigate any effects of dredging the ship channels that pass through the chain of islands where sand may have been lost from the system.
• Planting native vegetation to help provide environmental restoration on existing dunes several feet above the beach. These dunes would be planted with sea oats to help in the re-establishment of the dunes on the beaches.

The MsCIP plan was completed during the fall of 2007. After completion of sand placement and planting vegetation, the project will be subject to an 11- year monitoring and adaptive management program to be finalized during FY 2010.

USGS

The U.S. Geological Survey (USGS) is collecting high resolution geophysical data to investigate the shallow stratigraphy of the inner shelf region offshore of the Mississippi Barrier Islands in collaboration with the U.S. Army Corps of Engineers (USACE), Mobile District, during March and April, 2010. The purposes for this study include establishing the role the underlying framework geology has played in the development of these islands; describing the surficial geology and morphology of the inner shelf to define sediment transport pathways and sediment exchange with the islands; and providing a state-of-the-art baseline dataset (including bathymetry and surface features) for future monitoring and modeling studies. This study compliments and expands an ongoing USGS study, the Northern Gulf Coast Ecosystem change and Hazard Susceptibility Project. One of the goals of this USGS project is to predict future landscape change in the broader northern Gulf coastal region in response to changes in sea level and climate. The Mississippi coastal region is a central part of this study, but presently available data on the morphology and geology of the offshore region are inadequate to achieve project goals.

Results from the work presently underway will provide information necessary for the USACE to develop the shoreline restoration component of the MsCIP. The study area, shown in Figure 1, covers two areas: the inner shelf off of Petite Bois Pass, and the inner shelf offshore of Cat Island, extending to Horn Island. The study area covers approximately 425 km². The data being collected include DGPS navigation, high-resolution chirp seismic-reflection profiles for mapping the shallow stratigraphy, interferometric swath bathymetry to provide a detailed understanding of the seafloor morphology, and sidescan sonar imagery for mapping the seafloor geology.

The shallow stratigraphy of this coastal region is the product of the interplay of changes in sea level, sediment supply, and sediment redistribution by oceanographic processes. Subsurface features formed during the last sea-level lowstand and subsequent sea-level rise include drowned fluvial channels, tidal deposits and offshore shoals. The dense grid of data is necessary to ensure that the seafloor geology and shallow stratigraphy are resolved in adequate detail to understand the geometry of surficial and buried features. As an example, a backscatter image collected from an area immediately shoreward of the proposed study area shows discontinuous high-backscatter areas that are interpreted to be surficial expressions of sandy deposits. Because of the wide line spacing, it is not possible to map their orientation and full extent with confidence. Understanding the thickness, orientation, extent, and origin of these features are critical to this study. Close line spacing is also be needed for the analysis of the seismic data to define the origin of buried channels. Channels, which can be fluvial or tidal in origin, contain sand-rich deposits. Tidal channels (in this area) are 10-100 m in width and less than 1 km in length and are comprised of 50-100% sand. Fluvial channels are 100 m -1 km in width, extend over long distances, and their sand content is 25-75%. Distinguishing these two channel types is essential to improving the understanding of the evolution of this coast, and will provide a high resolution physical parameterization of these deposits.

From this field work USGS will recommend a selection of core sites based on interpretation of the seismic data during and immediately following the cruise. These sites will be selected to provide ground truth information for the geophysical data in areas of geologic importance, and will include areas where potential sand resources for MsCIP planning will be found. Other immediate deliverables include: digital copies of the field data and a draft version of a GIS (ESRI’s ArcMap) including navigation and JPEG images of seismic profiles. Final products including bathymetry and an analysis of shallow seismic stratigraphy and related papers will be completed by the end of 2010 and available sometime in 2011.