Saltwater intrusion and ETM dynamics in a tidally-energetic stratified estuary

Abstract

We explore the dynamics of the salt wedge and estuarine turbidity maxima (ETMs) in the Rotterdam Waterway using three-dimensional model simulations. These are compared to 13-h time series of profiles of velocity, salinity and suspended particulate matter (SPM) at a number of boat stations along the estuary, and long-term water level and salinity records. Evaluation of the numerical results shows that while good agreement is found between predicted and measured water levels and tidal discharges, the model under-predicts saltwater intrusion and stratification, and it over-predicts the height of the pycnocline above the bed. This leads to deficiencies in predictions of (1) the magnitude and vertical distribution of the baroclinic pressure gradient and subsequently of local shear and (2) vertical SPM gradients and concentrations near the bed because salinity stratification determines this distribution. However, the stability of the salt wedge during tidal excursions, the dominant role played by currents caused by the baroclinic pressure gradient and the damping of turbulence at the pycnocline with subsequent trapping of fluvial SPM at the head of the salt wedge are all well reproduced. A single stable ETM is formed when the salt wedge remains in the Rotterdam Waterway at low water slack. When saltwater intrudes farther up-estuary, multiple stable along-channel ETMs are maintained by localized trapping of fluvial SPM at the respective heads of saltwater by salinity gradients. Our results demonstrate that (1) the saltwater intrusion length is one of the main parameters controlling the SPM trapping probability and (2) the ETM is an advective phenomenon determining the timing of the availability of SPM for exchange with harbours. The model results indicate that all sediments deposited in the harbours along the Rotterdam Waterway and New Meuse are of fluvial origin.