Physical forcing mechanisms over a subaqueous dune field in Guanabara Bay - RJ

Abstract

Guanabara Bay (RJ) is a depositional coastal system where large subaqueous sand dunes occur in response to the hydrodynamic conditions prevailing on its central channel. Current velocity measurements, taken 1 meter above the bottom, allowed the computation of shear stresses during the period comprised between October 1994 and June 1995. Shear stress is expressed in terms of its associated shear velocity (u*). Assuming that the surface sediment is mainly composed of medium sands, the critical shear velocity for sedimentary particles with mean diameter of 0.25mm (lower limit of medium sands), was calculated and yielded a u*cr equal to 0.91 cm.s-1. It was estimated that the shear velocity exceeded the critical value during 64% of the duration of the measurements. When frontal systems reach the study area, the combined effect of strong winds and long period waves potentially increase sediment mobilization and transport. Thus, tidal currents can be considered the most important forcing mechanisms for this portion of Guanabara Bay sediment dynamics.