Curves, Coriolis, and Cross-Channel Circulation in the Hudson River Estuary

Cross-channel (secondary) circulation in estuaries can have an important influence on the along-channel momentum balance, dispersion, and transport, despite its relatively small magnitude. We compare secondary circulation at two sites in the Hudson River estuary with different degrees of curvature, contrasting the patterns and magnitudes of secondary circulation during varying conditions in tidal amplitude, river discharge, and resulting stratification. Spring tides and low discharge reduce stratification, whereas neap tides and high discharge enhance stratification. We find well-defined flows during flood tides at both sites, characterized by mainly two-layer structures during less stratified spring tides and three- or more-layer structures during more stratified neaps and transitions between tidal stages. Ebb tides have generally weaker and less definite flows, except at the site with stronger curvature, where curvature and Coriolis reinforce each other to produce strong flows during spring tide ebbs. Secondary flows have opposite directionality at the two sites, demonstrating the importance of curvature in driving cross-channel flows that differ from those expected due to Coriolis at the site with stronger curvature. A maximum in cross-channel flow tracks the peak in along-channel flow during flood tide at both sites. Using a simplified momentum balance including acceleration, Coriolis, curvature, barotropic pressure gradient, and vertical eddy stress divergence terms, we are able to explain observed flows only for time periods with relatively simple vertical profiles in shear. Using a more detailed data set from a previous study at one of our sites, we illustrate the importance of the baroclinic pressure gradient and non-linear advective terms in completing the momentum balance, especially during ebb tides and transitions between tidal stages. An improved understanding of the dynamics and drivers of cross-channel currents informs estuary modeling and management.