Channel Dredging and Geomorphic Response at and Adjacent to Mobile Pass, Alabama

The entrance to Mobile Bay, Alabama, between Mobile Point on the western end of the Morgan Peninsula and Pelican Point on the eastern end of Dauphin Island, is an extensive natural inlet that has been improved by channel dredging activities since 1904, primarily through the outer bar at the seaward extent of the ebb-tidal delta. The purpose of this study was to evaluate the potential impact of construction and maintenance dredging activities for the Federal navigation project in Mobile Outer Bar Channel on ebb-shoal changes and shoreline response along Dauphin Island, Alabama. Ebb-shoal changes and shoreline response relative to storm and normal forces, and dredging in the outer bar channel, were evaluated to determine the extent to which beach erosion along Dauphin Island could be attributed to U.S. Army Corps of Engineers (USACE) channel construction and maintenance dredging operations. 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 1986/2002) to quantify changes on the ebb shoal and beach response along Dauphin Island. Overall, net sediment transport from east-to-west for the entire period of record 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. Based on all available information, there appears to be no measurable negative impacts to ebb-tidal shoals or Dauphin Island beaches associated with historical channel dredging across the Mobile Pass Outer Bar.