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Evaluation of Channel Dredging on Shoreline Response at and Adjacent to Mobile Pass, Alabama

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.

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