Net Longshore Sediment Transport and Textural Changes in Beach Sediments along the Southwest Alabama and Mississippi Barrier Islands, U.S.A.

Luigi E. Cipriani, Gregory W. Stone

Abstract


A nearshore sediment transport model is developed and presented for the southwest Alabama and Mississippi barrier island coast along the northern Gulf of Mexico, USA. A cellular-type nearshore transport system, supplied by differential sediment sources, characterizes the present day study area, in contrast with previously formulated hypotheses of a net unidirectional, integrated nearshore transport system supplied by a single sediment source.

Computer simulations of net longshore sediment transport between Dauphin Island, Alabama, and West Ship Island, Mississippi predict six distinct transport cells characterized by net westward longshore sediment transport. Along eastern Dauphin Island, net longshore transport is eastward toward Mobile Pass. Granulometric trends and changes in the composition of foreshore (step), beach (mid-tide level) and foredune sediments support transport predictions and suggest the possibility of onshore sediment transport along the western flank of the study area. Step and mid-tide sediment grading (coarsening downdrift) is evident along Dauphin Island, and shows a strong relationship with predicted breaker wave height. West of Dauphin Island, sediment samples are characterized by higher concentrations of calcium carbonate (shell) by weight, and heavy minerals, coinciding with a decrease in the inner shelf slope. Field observations and historic shoreline trends are in agreement with longshore sediment transport predictions. For example, chronic shoreline retreat along Dauphin Island, coincides with an increase in net longshore sediment transport, and the highest erosion rate is localized at the net longshore sediment transport reversal (nodal point).

Contemporary drift cells appear to experience minimal net sediment exchange because of net longshore transport values approaching zero at most cell termini and ongoing maintenance dredging at the inlets, implying that they function as sediment sinks. Alternative sources of sediment appear to be internal on these barriers.

Keywords


Beach and foredune sediment texture; northeast Gulf Coast; wave refraction; numerical model; WAVENRG; Gulf of Mexico.

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