Hydrodynamics and Sediment Transport in the Tidally-Influenced James River

The tidally influenced James River is an important economic, ecologic, and cultural resource for VA residents. Tidal rivers have been historically understudied, however they are critical transition zones, the dynamics of which have implications for freshwater supply and sediment trapping. Globally, estimates suggest that >30% of fluvial sediment is trapped in the tidal zone, the location and dynamics of which are actively changing due to sea level rise and saltwater encroachment. In addition, analysis of historical water levels on the James River has shown a decrease in the tidal range since 1940. The present study combines >1-year's worth of hydrographic measurements collected using sensors deployed in 2018-2019 with more-recent ship-based observations, to evaluate the dynamics of saltwater intrusion and sediment trapping. Analysis of these data provides a better understanding of the mechanisms behind the location of the ETM and extent of saltwater in the James River. The cross-channel measurements of velocity collected across a complete tidal cycle are used to describe the internal water structure and mixing processes observed during the transition from ebb to flood tide and vice versa. Visualizations of these data depict a lag in the tidal reversal in the main thalweg during the transition from ebb to flood tide. From the time-series data, we observe a turbidity peak in sync with the highest water level of the tidal cycle, while the salinity maximum typically lags by ~3 hours. This offset, combined with the uneven cross-sectional tidal reversal, demonstrate some of the system's complexity, and will have important implications for predicting sediment deposition. Subsequent analysis will combine the time-series observations with the ship-based measurements to extend our understanding of the tidal dynamics in both space and time.