THE FORMATION OF ESTUARINE TURBIDITY MAXIMA DUE TO DENSITY EFFECTS INTHE SALT WEDGE - A HYDRODYNAMIC PROCESS STUDY

By means. of a numerical model of an idealized flat-bottom estuary, th e paper studies the hydrodynamic control of the turbidity zone by the combined effect of the salt wedge and tidal movements. The model is of two-dimensional (x, z) finite-difference type with high resolution in time and space. It computes momentum, surface elevation, salinity, su spended particulate matter (SPM), turbulent kinetic energy, and dissip ation rate as prognostic state variables. At the seaward boundary a ti dal forcing is applied. At the landward boundary a weir is situated wh ere a constant freshwater discharge is prescribed. The initial SPM con centration is horizontally homogeneous. After simulating a few tidal p eriods the model results exhibit the evolution of a stable SPM peak (t he estuarine turbidity maximum or ETM) at the tip of the salt wedge. A n inspection of the tidal mean velocity profiles around the ETM shows that this trapping of SPM is due to a residual near-bottom upstream cu rrent in the region of the salt wedge. Three physical causes for this residual countercurrent are investigated in greater detail by numerica l experiments, namely, (i) the residual gravitational circulation, (ii ) the tidal velocity asymmetry, and (iii) the tidal mixing asymmetry. The first mechanism is related to the baroclinic part of the longitudi nal pressure gradient. The second and third mechanism are based on the differences between the vertical profiles of velocity and SPM, respec tively, at flood and ebb tide. For the macrotidal estuary considered h ere, the consideration of both (i) and (ii) could be Shown to be neces sary for the establishment of an ETM in the considered idealized estua ry. It could further be shown that (iii) affects the ETM formation onl y quantitatively but not qualitatively and appears to be not necessary for the existence of an ETM.