The Formation of Estuarine Turbidity Maxima Due to Density Effects in the Salt Wedge. A Hydrodynamic Process Study

Abstract

By means of a numerical model of an idealized flat-bottom estuary, the 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, suspended particulate matter (SPM), turbulent kinetic energy, and dissipation rate as prognostic state variables. At the seaward boundary a tidal forcing is applied. At the landward boundary a weir is situated where a constant freshwater discharge is prescribed. The initial SPM concentration is horizontally homogeneous. After simulating a few tidal periods the model results exhibit the evolution of a stable SPM peak (the estuarine turbidity maximum or ETM) at the tip of the salt wedge. An inspection of the tidal mean velocity profiles around the ETM shows that this trapping of SPM is due to a residual near-bottom upstream current in the region of the salt wedge. Three physical causes for this residual countercurrent are investigated in greater detail by numerical 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 longitudinal pressure gradient. The second and third mechanism are based on the differences between the vertical profiles of velocity and SPM, respectively, at flood and ebb tide. For the macrotidal estuary considered here, the consideration of both (i) and (ii) could be shown to be necessary for the establishment of an ETM in the considered idealized estuary. It could further be shown that (iii) affects the ETM formation only quantitatively but not qualitatively and appears to be not necessary for the existence of an ETM.