A modeling study of a tidal intrusion front and its impact on larval dispersion in the James River estuary, Virginia

A tidally-induced frontal system regularly develops in a small area off Newport News Point in the lower James River, one of the tributaries of the Chesapeake Bay. In conjunction with the front, a strong counter-clockwise eddy develops on the shoals flanking the northern side of the channel as the result of tidal interaction with the local bathymetry and estuarine stratification. A three-dimensional hydrodynamic model was applied to simulate the eddy evolution and front development, and to investigate time-varying circulation and material transport over a spring-neap tidal cycle. The model results show that variation of tidal range, together with periodic stratification-destratification of the estuary, has a significant impact on the residual circulation of the lower James River. The net surface water circulation, which takes the form of a counterclockwise eddy on the Hampton Flats, is stronger during neap tide than during spring tide. Strong stratification and weak flood current during neap tide results in a dominant ebb flow at the surface, which delays flooding within the channel and advances the phase lead of flood tide on shoals adjacent to the channel, thus increasing both period and intensity of the eddy. Front development in the area off Newport News Point provides a linkage between shoal surface water and channel bottom water, producing a strong net upriver bottom transport. The existence of the vertical transport mechanism was independently demonstrated through tracer experiments. The impact of the dynamics on larval dispersion was investigated through a series of model simulations of the movement of shellfish larvae over multiple tidal cycles following their release at selected bottom sites. These results show that eddy-induced horizontal circulation and vertical transport associated with the frontal system are important mechanisms for the retention of larval organisms in the James River.