Intellectual Merit: This study is the first proposed field effort to quantify the role of secondary circulation in driving the estuarine exchange flow. Although recent numerical studies in a limited estuarine parameter space suggest that the lateral advection is a leading-order term in the subtidal along-channel momentum balance, it has not been confirmed by observations. Observational test of this model result is critical to the field of estuarine oceanography because it raises a fundamental question regarding the validity of the classic theory of estuarine circulation. Numerical and scaling results have suggested that the effects of lateral advection are nearly always balanced by internal stresses so that a simple theory based on the momentum balance between longitudinal pressure gradient and bottom stress provides an accurate prediction for the estuarine residual velocity. The central hypothesis is that the near cancellation between secondary flows and interfacial stress is a manifestation for a possible interaction or feedback between the advective accelerations due to secondary circulation and diffusive momentum transfer due to small-scale turbulent flows. This project will investigate the co-variability between the lateral advection and interfacial stress over the spring-neap tidal cycle and assess their roles in the estuarine exchange flow. We plan to conduct a series of field experiments in the James River estuary, complemented by ROMS (Regional Ocean Modeling System) and LES (Large Eddy Simulations) modeling simulations. Field efforts include moored and shipboard observations as well as dye-release experiments and microstructure profiling. ROMS modeling will focus on the effects of secondary circulation on the estuarine exchange flows in this wide estuary, while LES modeling will examine the interactions and possible coupling between secondary flows and small scale turbulent flows. Results obtained from this project will provide a definitive answer on the role of secondary circulation in estuarine circulation.
Broader Impacts: Estuarine dispersion is largely driven by the exchange flow and the ability to predict dispersion is critical in many applied problems such as determining the Maximum Total Daily Load (TMDL) permissible to an estuarine system. More generally, this project will yield much-needed information regarding the circulation and mixing processes in estuaries and help develop state-of-the-art numerical models for simulating estuarine flows, which are required for predicting water quality, contaminant and fish larvae transport. For the field work in the James River, we will involve participation of high school teachers and students from the areas near the estuary. Through the participation of one of the investigator at the University of Florida, this project enhances the involvement of minority groups in science. This project will also provide training to three graduate students and undergraduate interns.
Dynamics and Societal relevance of Estuarine Circulation Robert J. Chant Rutgers University Estuarine systems are among the most productive ecosystems on earth. Over 75 percent of commercially viable fish and shellfish use estuarine habitats and provide habitats for many migratory birds. Estuarine systems are also the most anthropogenically impacted marine systems in the word. They are used as waste receptacles, modified by dredging and land use, dammed, used to cool nuclear power plants, used for commerce, recreation and are highly valued for their ascetic value. Wetlands that fringe their boundaries, whose existence rely on the delivery of sediment by estuarine circulation, act as filters between terrestrial and marine ecosystems and provide protection from floods. Despite their small size, estuaries are disproportionally important ecologically, economically and recreationally. Moreover, with 75% of the world's population living within estuarine watersheds they are increasingly impacted by humans both by our activities within the watershed and by global climate change that is raising sea-level and modifying precipitation patterns. Many aspects of estuaries, from their geomorphology, their ability to disperse pollutants and the extent that salt water extends landward is strongly influenced by the estuarine circulation. This circulation is characterized by a mean landward flow of saline water at depth in the estuary. The physics that drive this flow has been a central topic in physical oceanography for over a half a century and these studies have produced a standard model that predicts the strength of this circulation. However, results of our study of circulation in the James River estuary (Figure 1) challenge several aspects of this classic model. In particular we have found that the estuarine exchange flow exhibits weak, if any, variation over the tidal spring/neap cycle and this is in contrast to what the classic model predicts which would predict a 5-10 times stronger circulation during neap tides. Our results suggest that "non-linear" dynamics associated with tidal period cross-channel flows transfers energy from the oscillatory tidal flows to the steady exchange flows. While previous modeling results have demonstrated this our results are the first that use both field studies to confirm these dynamics. These results suggest that to accurately manage the many economic services that estuaries provide, such as drinking water and dispersal of municipal discharges, requires accurately capturing the exchange flow and thus the use of models with the sophistication to include these non-linear dynamics.
