The Influence of Centrifugal and Coriolis Forces on the Circulation in a Curving Estuary

A three-dimensional hydrodynamic model (ECOMSED—of the POM family of models) was used to investigate the physics of estuarine circulation and salt intrusion around a bending region of an estuary. The simulated estuarine channel modeled was long and had a flat bottom. The focus was the effect of transverse circulation created by channel curvature on the dynamics of salt transport, with and without Coriolis forcing. Model results show how, in the absence of Coriolis force, the centrifugal acceleration, directed toward the bend's outer bank during both flood and ebb, tends to give rise to a balancing cross-estuary, barotropic sea level gradient. The vertical shear stress then sets up a vertical plane transverse circulation cell directed toward the outer bank at the channel's surface and toward the inner bank at the channel's bottom. Shear in the along-channel velocity accompanies secondary circulation. In the presence of stratification, the secondary circulation tends to initially produce upwelling of salt at the inner part of the bend, giving rise to a baroclinic pressure gradient directed opposite to the barotropic one at the surface. During slack, when the centrifugal acceleration diminishes, the baroclinic pressure gradient slowly forces the salt at the inner bank towards the bend's outer bank. Thus, secondary circulation creates upwelling of salt, at both banks of the bend, at different times, decreasing the along-shore vertical steady shear dispersion of salt. It may also lead to overturning and intense vertical mixing. Both processes, in turn, lead to a decrease in salt intrusion. Results also show that the Coriolis force can significantly influence secondary circulation around a bend. The Coriolis force, by itself, can be a significant cause of secondary circulation and lateral mixing of salt, even in the absence of a bend. The creation of secondary circulation by Coriolis force alone can also reduce salt intrusion. If the channel bend is present with a curvature that creates centrifugal acceleration in the direction of the Coriolis acceleration, secondary circulation tends to increase, leading, in turn, to even less salt intrusion within the bend. The opposite is also true.