Modelling the transverse distribution of velocity and suspended sediment in tidal estuaries

An estuary is a semi-enclosed coastal body of water which has a free connection with the open sea and within which sea water is measurably diluted with fresh water derived from land drainage. Examples are the Western Scheldt River Estuary and the Chesapeake Bay. Within these environments complex patterns of velocity and suspended sediments are observed in the transversal plane (across-estuary and vertical), and sediments are trapped laterally (across-estuary). The transverse structure of velocity is relevant to the transport of salt, sediment, contaminants, oxygen and other material. High sediment concentrations affect water quality, ecology and wildlife, and may cause siltation of navigation channels and harbors. This work aims at a fundamental understanding of the transverse distributions of estuarine velocity and suspended sediment. The thesis provides two-dimensional (cross-sectional) analytical models to identify the effect of individual forcing mechanisms on the transverse distribution of velocity and suspended sediment in tidally-dominated estuaries. The models are based on the shallow water equations and sediment mass balance. Considered are the residual and the semi-diurnal tidal components of the along-estuary, across-estuary and vertical velocity and of the suspended sediment concentration. The models apply to partially to well-mixed tidal estuaries, relatively uniform along-channel conditions and weakly to moderately nonlinear flow. Horizontal density gradients are prescribed based on numerical or observational data. The analytical flows are decomposed into components induced by individual mechanisms. Considered are tides, horizontal residual density gradients, river discharge, stokes return flow, wind, the earth’s rotation, tidal variations in the across-channel density gradient and channel curvature. In addition, two tidally rectified along-channel residual flow mechanisms are considered, which result from net advection of along-channel tidal momentum by the Coriolis-induced transverse tidal flow and by the density-induced transverse tidal flow, respectively. The models were validated against observations in the James River and Chesapeake Bay, and against a three-dimensional numerical model for various estuarine conditions. An important finding is that the residual across-channel density gradient is crucial for the lateral distribution and trapping of sediment in many estuarine cross-sections. The gradient tends to trap sediments in fresher areas of the cross-section. Tidal variations in the across-channel density gradient were found to cause a double circulation pattern in the transverse tidal flow during slack tides. The gradient also affects along-channel residual velocity via density-induced tidal rectification. This rectification component features landward currents in the channel and seaward currents over the slopes, and is particularly effective in deeper water. Coriolis-induced tidal rectification was found to induce residual flows that are up-estuary to the right and down-estuary to the left of an estuarine channel (looking up-estuary in the northern hemisphere). The process fundamentally changes the transverse structure of along-channel residual flow for stronger tides or steeper channels, as the flow becomes internally asymmetric. For weaker tides, along-channel residual flows are typically dominated by a gravitational circulation, i.e., landward flow in the channel and seaward flow over the shoals, or river flow. Stokes return flow, which resembles river flow, is particularly important for strong tides in shallow wate