Suspended Sediment Dynamics, Marsh Accretion, and Cold Fronts in Coastal Louisiana

In a regime of rapid sea level rise, marshes must accrete vertically in order to maintain their position in the tidal frame. It is well established that storms have the potential for increasing mineral sedimentation on the marsh platform. The high water level associated with storms, however, creates two competing effects; high water levels increase the hydroperiod on the marsh platform, but decrease the wave bed shear stress and thus the ability to resuspend sediment. As such, it is not clear which storm conditions are the most favorable for marsh sedimentation. This is of particular importance in coastal Louisiana, where marshes are struggling to keep pace with sea level rise. The purpose of this study was to explore the role of cold fronts on sediment availability and deposition in coastal Louisiana bays and marshes. Sensors were deployed in Barataria Bay and Terrebonne Bay (LA) to measure turbidity and pressure. The highest suspended sediment concentrations in the bays (or open water) occurred during northerly winds (i.e. during the initial phase of cold fronts), but the low water level limited the transport of this sediment on to the marsh surface. The high sediment concentration during this low water period is explained by a combination of enhanced bed shear stress and increased marsh edge erosion. Conversely, in the post-frontal period, water levels increased and the suspended sediment concentration decreased. The high water levels allowed sediment to deposit on the marsh surface, but this did not occur at peak suspended sediment concentrations. The mismatch in timing between high suspended sediment concentration and high water levels result in less-than-optimal sediment accretion on the marsh surface during cold front passage. Based on these data, we developed a model to determine optimal wave heights and water levels that will result in maximum sediment deposition on the marsh surface.