Shoreline and Wetland Change Assessment

Shoreline changes document subaerial dynamics of the land-water boundary in response to various coastal processes, particularly those associated with storm events. Applied Coastal personnel specialize in quantification and interpretation of historical trends in shoreline position, bathymetry, and wetland area relative to dominant coastal processes. Innovative strategies have been developed to take advantage of state-of-technology advances in surveying, image analysis and interpretation, and geographic information systems (GIS), combined with a comprehensive and detailed understanding of the processes impacting coastal geomorphic change. Topographic and hydrographic surveys of coastal and nearshore environments provide a direct source of data for quantifying system response. Applied Coastal personnel analyze these data to evaluate coastal change trends at various locations along the U.S. coast in relation to normal incident coastal processes, storm impacts, relative sea-level change, and human modifications of the coast.

Historical shoreline and bathymetry data; dredging records; tropical cyclone records; coastal process data sets; and wave, current, and morphologic change modeling results were analyzed to evaluate the potential impact of channel dredging across the Mobile Pass Outer Bar Channel on erosion processes along Dauphin Island, Alabama, and on the ebb-tidal delta west of the navigation channel. Based on historical shoreline and bathymetry surveys, two distinct periods were evaluated; one representing conditions prior to significant construction and maintenance dredging activities to determine natural changes (1847/48 to 1917/20), and the other representing conditions after significant changes to the outer bar channel had been imposed (1917/20 to 2002) to quantify beach response along Dauphin Island. A sediment budget was developed for the period 1917/20 to 1984/87 to document sediment transport pathways and quantify sediment volume changes and fluxes throughout the study area.

Analysis of shoreline position and bathymetric change between 1847/48 and 2002 revealed a common link associated with geomorphic evolution of Dauphin Island. Major changes in island configuration west of Pelican Island and shoal development on the ebb-tidal delta were always associated with hurricanes or tropical storms. Bathymetric changes indicate consistent deposition for the entire ebb-tidal delta prior to and after channel dredging, except for two periods where the end dates were close to major storms. Wave, hydrodynamic, and sediment transport modeling results illustrated net transport to the west throughout the study area, and patterns of net deposition and erosion are well-documented when comparing wave-current simulations and bathymetric change results. Overall, net sediment transport from east-to-west between 1917/20 and 1984/87 has been supplying sand quantities necessary to produce net deposition on the islands and shoals of the ebb-tidal delta, infill and nourish storm breaches and washover surge channels on Dauphin Island, and promote growth of western end of the island, even though channel dredging has been active.