Fateme Ebrahimi Eramia*, Vasileios Kitsikoudisa, Bart Vermeulenb, Suzanne J. M. H. Hulschera
a Water Engineering and Management, Faculty of Engineering Technology, University of Twente; b Hydrology and Quantitative Water Management, Wageningen University and Research
The presence of scour holes (bathymetric depressions) in estuaries adds complexity to the salt intrusion phenomenon . Denser saltwater accumulates in the deeper parts of the scour holes in each flood tide and it may not be flushed back to the sea by the subsequent ebb tide. Understanding the flushing mechanism of saltwater from scour holes is of particular interest in semi-closed estuaries, such as the Haringvliet in the Netherlands, where the trapped saltwater in scour holes may pose a threat to the freshwater supply. Forcings such as high river discharge and strong wind can initiate the mixing process by providing enough shear and turbulence at the pycnocline of the highly stratified scour holes. In this study, we investigate the flow velocity and salinity dynamics in a scour hole in the Haringvliet, at times when the floodgates are either closed or partially open.
The measurement instruments were deployed in a scour hole in the Haringvliet for 73 days from November 2022 to January 2023 to capture saltwater trapping and flushing events. The flow velocity data were collected using bed-mounted acoustic Doppler current profilers (ADCPs) with a vertical resolution of 30 cm and a sampling frequency of 4 Hz. In order to be able to track the pycnocline, we used high-resolution echosounder data along the center-beam of the ADCP with a resolution of 5 cm. In addition, salinity data at three different heights through the water column were recorded by CTD instruments.
The flow patterns in the water column over a stratified scour hole will be analyzed by relating mean velocities and turbulence intensities to salinity concentrations and echosounder data. Such an example is shown in Figure 1.
Figure 1: Flow velocity in east-west direction and echosounder amplitudes in a stratified scour hole in the Haringvliet Estuary over one day. The dashed horizontal line indicates the upper part of the scour hole.