Channel curvature effects on estuarine circulation in a highly stratified tropical estuary: The Sao Francisco river estuary (Brazil)

摘要

The effects of the channel curvature on estuarine circulation were assessed in the Sao Francisco river estuary. This estuary is a deltaic front system, with a highly stratified salinity distribution. The assessment was based on field experiments where currents and salinity and temperature were monitored during a semi-diurnal tidal cycle in two cross-sections with distinct curvatures (radii of 2 and 8 km). The least-squares method was used to determine the mean velocity (residual velocity), and the tidal velocity amplitude and phase for the M-2 harmonic. The results for the M2 harmonic were compared to an analytical solution that considers a simplified momentum balance between barotropic pressure gradient and bottom friction. The model skill for these two results was 0.44 and 0.63 for sections #A and #B, respectively. The differences between these results indicate that there are other dynamically influential forces for the longitudinal momentum balance besides those included in the analytical solution. The baroclinic pressure gradient presented values two orders of magnitude greater than the bottom friction, demonstrating the dynamic importance of this force. Although centrifugal acceleration is a component of the lateral momentum balance, it also plays a role in the distribution of longitudinal currents in the cross-section, displacing the maximum currents towards the exit of the curve. In addition to the channel curvature effect, the influence of the bathymetry was also observed on the tidal amplitude in the cross-sections and on the depth-averaged residual longitudinal velocities along the sections, with the highest average speed in the shallowest areas of each section.