Residual flow over a bump in Quellón Bay

Abstract

Measurements of velocity and density profiles were used to describe the tidal and residual flow structure in Quellon Bay, southern Chile (43 o 10'S), an eastward opened bay with ca. 6 km width (north-south) at its mouth and a low river discharge at its head, associated to a productive marsh- like branch. Current profiles were obtained in winter and spring with a 307.2 kHz Acoustic Doppler Current Profiler (ADCP) during two semidiurnal cycles along a repeated triangular circuit. The eastern side of the triangle consisted in an across- bay transect oriented in the north-south direction, and two along-bay (aprox. east-west oriented) transects formed the other sides identified as northern and southern transects. The triangle circumscribed a small bump of ca.10 depth, surrounded by bathymetry of the order of 20-30 m. Density profiles were obtained at the westernmost corner of the triangle. The longitudinal mean flow in the eastern transect at the mouth showed a lateral exchange structure that consisted in inflow (westward) over the northern side and outflow (southeastward) over the southern side. Lateral residual flow showed increases of the southward component that seems to be related to the presence of the bump. The longitudinal residual flow was mostly westward in the southern transect and eastward in the southern transect, suggesting that flow enters the bay through the northern side and exits through the southern side. Over the southern transect there were evidences of flow divergence over the bump which promotes upward velocities over this feature. Upward velocities over the bump and downward velocities over the eastern flank were estimated using the continuity equation. There were evidences that divergence is induced by flow separation yielded by a pair of counter-rotating vortex over this feature. Nonlinearities seem to dominate the tidal dynamic balance. The intra tidal variability of flow and density at the repeated station showed no significant differences between the low and high rainy seasons, suggesting that dynamics of the system is less affected by river discharge. Small increases of salinity were associated to the effect of tidal flood phase. There were no significant differences between samplings on July and November, nor in residual flow patterns or in density field.