A 3-DIMENSIONAL MODEL OF TIDAL CURRENTS IN THE MOUTH OF THE TAGUS ESTUARY

Three-dimensional simulations of barotropic tides are used to investig ate the complex circulation at the mouth of the Tagus estuary. Compari son with held data shows that elevations are well represented in the m ain area of interest, but velocities are slightly over-predicted due t o a simplified treatment of intertidal flats. Model results show the e xistence of strong velocity phase lags (up to 1.75 hours for the semi- diurnal constituents) between the shallow areas and the deep channel. These phase lags are partially responsible for the generation by advec tive accelerations of a strong residual velocity field (velocities rea ch 0.5 m s(-1)) with well defined eddies. The interaction between the residual and tidal velocity fields in turn generates strong chaotic st irring. Localized sigma coordinates (LSC), a recently proposed method which allows the number of nodes per vertical to vary horizontally, ar e used for the first time in a three-dimensional application. A previo usly proposed criterion for the horizontal distribution of the total n umber of vertical nodes is shown to be inadequate when advective accel erations are important. However, with an alternative criterion, the us e of LSC reduces maximum errors by a factor of two relative to traditi onal sigma coordinates. (C) 1998 Elsevier Science Ltd. All rights rese rved.