Modeling study of the influences of tide and stratification on age of water in the tidal James River

Abstract

The concept of the age of water parcels is applied to the tidal James River estuary, a western branch of the Chesapeake Bay, to compute the timescale quantifying transport processes of a dissolved substance in the estuary. The age is defined in this study as the time elapsed since a water parcel entered the headwater of the estuary. A theoretical deduction shows that the vertical age profile can be explained by the extended Hansen and Rattray's Central Region theory, which is mainly controlled by gravitational circulation in the estuary. The age distributions in the James River under different hydrological and hydrodynamic conditions are then calculated through a series of model simulations using a real-time three-dimensional hydrodynamic model. The influences of tide, freshwater inflow, and density-induced circulation on age distribution have been investigated. The model results show that the vertical age distribution resembles the stratification pattern of the salinity in the regions where stratification persists. The age difference between the surface and bottom is larger during neap tide than during spring tide. The vertical difference is about 5 days under mean river flow conditions. The model results indicate that river flow is one of the dominant factors that influence mean age distribution. It takes 95 and 35 days, respectively, for a water parcel discharged into the estuary to be transported out of the estuary under mean and high flow conditions. The tidal influence is more significant in the downstream portions of the estuary. The density-induced circulation contributes to the long-term transport substantially in the downstream estuary where the stratification persists. The influence of density-induced circulation on age distribution can be more than 45 days under the mean flow condition. The results provide useful information for understanding the characteristics of the transport processes and the influences of gravitational circulation on long-term transport processes in the estuary.