Storm-induced geomorphological changes in a shallow, back-barrier estuary

Zafer Defne, US Geological Survey, Woods Hole, MA, United States and Neil K Ganju, Department of the Interior Washington DC, Washington, DC, United States
Sediment redistribution in estuaries due to storms is critical for evaluating impacts to navigation, wetland sediment supply and fate of contaminants. Storm-induced sediment transport in Barnegat Bay, New Jersey during Hurricane Sandy was modeled using the Coupled Ocean-Wave-Sediment Transport (COAWST) system. Simulations with and without the storm were compared to evaluate the direct effect of the storm. The storm caused high waves and strong near-bottom currents offshore of the barrier island that were attenuated in the interior of the bay. The amplified hydrodynamic forcing during the storm caused redistribution of estuarine bed material especially across preexisting sharp gradients in sediment fraction. Storm-induced bottom stress resuspended fine sediment resulting in net erosion from the deeper areas with ensuing net deposition over fringing wetlands (up to 0.01 m accretion) and shoals throughout the bay. Waves, although significantly smaller in the bay than offshore, were the main driver of erosion along the intertidal shoals on the mainland. Erosion along the main channel, however, was intensified (up to 0.5 m bed scour) primarily by the excess flow associated with storm surge. Before the storm made landfall, the storm surge offshore created a large pressure gradient across the inlets causing strong landward currents. These currents transported the coarser sediment mobilized offshore into the bay and prevented the suspended sediment in the bay from exiting. During the subsequent surge outflow, most of the sediment captured in the first half of the storm was exported out of the bay yielding net erosion that was ~30 times less than at the peak of storm. The model results support the idea that barrier islands help retain sediment in estuaries by providing shelter from the intensified offshore currents and wave stresses caused by large storms.