Impact of a rocky outcrop on hydrodynamics and geomorphology in a mesotidal channel

Abstract

The purpose of the present work is to study the effect of an isolated depression and a related rocky outcrop on the flow field in a mesotidal channel of the Bahía Blanca Estuary, Argentina. To this end, measurements of current velocity (ADCP), swath bathymetry (multibeam), and seismostratigraphic surveys (3.5 kHz) were made in the study area. In general, the channel bed studied (Cabeza de Buey Channel) is flat and mainly composed of clayey silts. In the inner sector, there is a depression 6 m-deep and 50–60 m in diameter, and on either side (in a streamwise sense) there are two isolated rocky outcrops that extend up to 2 m in height relative to the prevailing bed level. The two features differ in that the up-estuary outcrop is longer and extends over the whole width of the hole, while the down-estuary outcrop is shorter and only extends over part of the diameter of the hole. Seismostratigraphic analysis, in correlation with bathymetric and lithological data, indicates that these high points correspond to the outcropping of relict deltaic materials that formed part of a regional Late Pleistocene drainage system. Fine-grained estuarine sediments deposited above the old deltaic materials show evidence of the present-day sedimentological conditions related to the hydrodynamics of the channel. The flow patterns within the depression observed over a complete tidal cycle demonstrate that the flow behaves differently depending on the stage of the tide. The depth-averaged flow shows higher velocities during the ebb tide, generating a net flow directed towards the sea. Results reveal the generation of return flows (turbulent eddies) at the bottom of the hole, which expand immediately towards the water surface. During the ebb tide, intense vertical currents develop inside the hole. The eddy formation is attributed to the unequal size of the outcrops. Hence, the larger upstream channel outcrop plays an important role in eddy generation, just as downwelling-upwelling interaction produces erosion and transport of sediments, which exerts a strong influence on the deepening of the hole.