CONICET | Buscador de Institutos y Recursos Humanos

The purpose of the present work is to study the effect of an isolated depression and a related rocky outcrop on theflow field in a mesotidal channel of the Bahía Blanca Estuary, Argentina. To this end, measurements of currentvelocity (ADCP), swath bathymetry (multibeam), and seismostratigraphic surveys (3.5 kHz) were made in thestudy area. In general, the channel bed studied (Cabeza de Buey Channel) is flat and mainly composed of clayeysilts. In the inner sector, there is a depression 6 m-deep and 50?60m in diameter, and on either side (in astreamwise sense) there are two isolated rocky outcrops that extend up to 2m in height relative to the prevailingbed level. The two features differ in that the up-estuary outcrop is longer and extends over the whole width ofthe 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 highpoints correspond to the outcropping of relict deltaic materials that formed part of a regional Late Pleistocenedrainage system. Fine-grained estuarine sediments deposited above the old deltaic materials show evidence ofthe present-day sedimentological conditions related to the hydrodynamics of the channel. The flow patternswithin the depression observed over a complete tidal cycle demonstrate that the flow behaves differently dependingon the stage of the tide. The depth-averaged flow shows higher velocities during the ebb tide, generatinga net flow directed towards the sea. Results reveal the generation of return flows (turbulent eddies) at the bottomof the hole, which expand immediately towards the water surface. During the ebb tide, intense vertical currentsdevelop inside the hole. The eddy formation is attributed to the unequal size of the outcrops. Hence, the largerupstream channel outcrop plays an important role in eddy generation, just as downwelling-upwelling interactionproduces erosion and transport of sediments, which exerts a strong influence on the deepening of the hole.