Kinematics of The Suspended Particulate Matter in the Quequén Grande River Estuary (Argentina)

In salt wedge estuaries, the instantaneous position of the intrusion front is determined by the balance between the baroclinic pressure gradient due to longitudinal density differences and the restoring force induced by the river discharge. This position is also controlled by the oscillatory free surface gradient. Under these conditions, an interface that separates an upper layer of brackish water from the saline intrusion is developed. Its thickness is determined by the competition between shear stress and stratification. Variable amounts of freshwater and saltwater affect the transport and settling of sediments, estuarine morphology, and water quality. Hence, the characterization of the suspended particulate matter (SPM) behaviour is a key point to determine the relative importance of the vertical mixing and the stratification in the estuarine turbidity maximum area where macro- and micro-scale physical mechanisms coexist. These mechanisms include re-suspension, delay effects in settling/scour, stratification, turbulence reduction, river discharge drag force, baroclinic effects, asymmetric tidal mixing, etc. In this context, the aim of this study was to describe the dynamic behaviour of the turbidity and saline fronts by analysing the underlying physical mechanisms associated with the mixing of waters and their effects on the SPM transport in the Quequén Grande River estuary. Measurements of velocity, backscatter, SPM concentration and salinity were performed during several tidal cycles with a 1200 kHz ADCP and a multi-parametric probe in two fixed locations: one around the tidal reach ant the other in the harbour zone. Organic and inorganic particles transported by the river interact with salt water, generating aggregations and/or flocs that are transported seaward in the brackish water layer, where they interact with the locally added organic matter to form larger particles and settle especially in the deeper harbour zone. They are trapped there, unable to recirculate upstream and re-incorporate to the turbidity front. This SPM behaviour is caused by bathymetry of the estuary lower part and the time-space variability of the SPM properties (e.g. size and concentration) and tidal flows.