The Effects of Waves and Tidal Inundation on Sediment Deposition and Flux across a Bay-Marsh Boundary

Sediment deposition processes acting on salt marshes adjacent to larger bodies of water, such as bays, differ from the processes acting in tidal creek marshes, but have not been well characterized by previous research. This difference in interface alters the primary controls on suspended sediment concentration (SSC), an indication of sediment availability, as well as sediment flux to and deposition on the marsh. This study was conducted along the boundary between a shallow, microtidal coastal lagoon, Hog Island Bay, and a microtidal salt marsh on the edge of Chimney Pole, a marsh island on the Eastern Shore of Virginia. This research sought to characterize the hydrologic regime at the bay-marsh boundary, determine changes in SSC due to changing wave-generated bottom shear stress, establish the effect of tides and currents on sediment transport to the marsh, and estimate changes in bottom shear stress due to relative sea level rise (RSLR) and storm surge.

Two primary differences in bay and tidal creek environments are the presence of waves and unconstrained flow. The results from this study indicate a strong correlation between wind direction and wave formation, whereby the largest waves formed when westerly winds (i.e. the direction of longest fetch) blew across Hog Island Bay at relatively high speeds (> 8 m s-1). Maximum wave-generated bed shear stress on the tidal flat during times of strong westerly winds occurred at water elevations near the marsh platform height (~0.5 m above MSL), above which shear stress declined. Our findings showed that waves primarily force SSC, but the impact of wave events on sediment deposition at Chimney Pole is limited by the fact that these events typically occur when the marsh is not flooded. When the marsh is flooded, wave height is rapidly diminished as waves propagate across the bay-marsh boundary due to vegetation and shallow water depths. Modeling changes in wave-induced bed shear stress with depth revealed that shear stress is likely approaching a maximum given present-day sea level, wind and tidal conditions. If the marsh keeps pace with RSLR, sediment deposition will be maximized by present-day conditions or slow RSLR. However, if the marsh elevation remains constant, potential deposition may increase with increasing RSLR and storm surge.

Winds also forced currents, whereby weaker, southerly winds were associated with an alternating northward flood, southward ebb current pattern, whereas strong, northerly winds pushed currents southward regardless of tidal phase. Overall, the unconstrained flow present in Hog Island Bay limited sediment deposition on Chimney Pole, because currents carried sediment towards, but also away from, the marsh during flooding conditions. High bed shear stress likely prevented sediment transported onto the marsh from depositing near the marsh edge; therefore it was deposited further into the marsh interior. Calculated (0.025 g cm-2) and measured (0.028 g cm-2) deposition for the March 2014 deployment showed that the marsh is accreting further away from the edge. The results indicated that bed shear stress was sufficient to remobilize sediment after it deposited, however grain size analysis revealed that this process is likely limited by bed cohesiveness at the site.