Net longshore sediment transport and textural changes in beach sediments along the southwest Alabama and Mississippi barrier islands, USA

A nearshore sediment transport model is developed and presented for the sou thwest Alabama and Mississippi barrier island coast along the northern Gulf of Mexico, USA. A cellular-type nearshore transport system, supplied by di fferential sediment sources, characterizes the present day study area, in c ontrast with previously formulated hypotheses of a net unidirectional, inte grated nearshore transport system supplied by a single sediment source. Computer simulations of net longshore sediment transport between Dauphin Is land, Alabama, and West Ship Island, Mississippi predict six distinct trans port cells characterized by net westward longshore sediment transport. Alon g eastern Dauphin Island, net longshore transport is eastward toward Mobile Pass. Granulometric trends and changes in the composition of foreshore (st ep), beach (mid-tide level) and foredune sediments support transport predic tions and suggest the possibility of onshore sediment transport along the w estern flank of the study area. Step and mid-tide sediment grading (coarsen ing downdrift) is evident along Dauphin Island, and shows a strong relation ship with predicted breaker wave height. West of Dauphin Island, sediment s amples are characterized by higher concentrations of calcium carbonate (she ll) by weight, and heavy minerals, coinciding with a decrease in the inner shelf slope. Field observations and historic shoreline trends are in agreem ent with longshore sediment transport predictions. For example, chronic sho reline retreat along Dauphin Island, coincides with an increase in net long shore 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 te rmini and ongoing maintenance dredging at the inlets, implying that they fu nction as sediment sinks. Alternative sources of sediment appear to be inte rnal on these barriers.