Changes in Estuarine Sediment Dynamics with a Storm Surge Barrier

Gated storm surge barriers are increasingly being considered as part of coastal protection plans for urbanized areas located on estuaries. Barriers remain open to tidal exchange during non-storm periods, but blockage of the cross-section causes locally accelerated velocities and associated drag results in decreased tidal amplitude inside the estuary. This modeling study of a storm surge barrier near the mouth of the Hudson River estuary examines potential impacts of the alterations to flow on sediment dynamics in the system. Within about a tidal excursion of the barrier, increased bottom stresses result in erosion and coarsening of the bed. Reduced availability of fine sediment on the bed causes a reduction in suspended sediment concentration (SSC) near the barrier. Inside the estuary, reductions in tidal amplitude result in increased trapping of sediment input from the watershed due to increased stratification and a landward shift in the salinity intrusion. Despite the increased trapping, SSC in the estuary decreases due to the reduction in tidal bed stresses. Sharp velocity and stress asymmetries at the barrier openings create a divergence in sediment transport capacity that reduces export of sediment from the estuary during high discharge periods and reduces the return of sediment from offshore during low discharge. In the Hudson, the reductions in SSC (~ 10 to 20%) are modest and not expected to have major impacts on water clarity or marsh sustainability. Surge barriers may have more substantial impact on sediment dynamics in estuaries with lower background SSC or limited watershed inputs.