Observational study on mixing in a stratified scour hole due to the wind-driven lateral circulation in a semi-closed estuary

Salt intrusion occurring in estuarine environments can be aggravated by the presence of scour holes in the bed. Saltwater has a higher density than freshwater. Therefore saltwater accumulates in scour holes which may exacerbate the salinization of freshwater once it is entrained by wind or other forcings. Wind can contribute to estuarine circulation and stratification through three main mechanisms: direct wind mixing, wind straining and wind-driven lateral circulation. Previous studies (Csanady, 1982; Winant, 2004), suggest that horizontal wind circulation mostly comes into play in estuaries with laterally varying bathymetry, which is the case for our study site. The Haringvliet estuary in the Rhine-Meuse delta is a former tidal basin in the western part of the Netherlands; it varies in bathymetry and has been closed off by floodgates. The gates are opened during ebb tide to discharge river into the sea, and also for a short period of time during flood tide for ecological purposes. The complex geomorphology of the estuary is composed of shoals and deep scour holes. An extensive field campaign was carried out for over six months in the Haringvliet, at the locations of the scour holes, in which we measured flow velocity, salinity, discharge, and wind speed and direction. Results indicate that, under an axial wind over the estuary, a horizontal circulation forms by downwind flow over shoals and upwind flow in the deep channels. Based on the collected dataset, a change from a down-estuary to an up-estuary wind direction occurred while the floodgates were closed. As a result of the wind influence, the flow direction in the stratified deep channel changed quickly, which provided sufficient shear and turbulence in the whole water column for vertical mixing. The sharp drop in the salinity concentration corresponding to the mixing and flushing in the scour hole occurred due to the wind-induced lateral circulation without having high river discharge. This research shows that, in a semi-closed estuary like the Haringvliet, lateral currents and the momentum transfer corresponded to that can exert a predominant control on estuarine circulation and stratification.