The Haringvliet Estuary in the western part of the Netherlands was closed with a series of floodgates (Figure 1) more than half a century ago to ensure coastal protection as part of the Deltaworks. In the years following the construction of the floodgates, the estuary was turned to a freshwater body to support freshwater needs of the surrounding area. While this Engineering intervention improved substantially the coastal safety and the freshwater availability, the ecological consequences for the estuary have been detrimental due to the disconnection of the estuary with the open sea. Until recently, the operating protocol of the floodgates dictated that the floodgates should remain closed during flood tide, to prevent saltwater reaching the freshwater intakes, and opened during ebb tide to discharge the river flow. A few years ago, Rijkswaterstaat changed the operating protocol to enhance the ecological functions of the estuary. Specifically, the intention now is to keep the floodgates open for a limited amount of time during flood tide to support fish migration from the open sea to the rivers. However, this would inevitably introduce saltwater into the system, which can be undesirable. Ideally, the saltwater that is introduced during the flood tide should be flushed out of the system with the following ebb tide and the river discharge. However, the incoming saltwater can be trapped into deep bathymetric depressions in the estuary (Figure 1) because saltwater is heavier than freshwater. In such cases, it is not clear under what conditions the trapped saltwater can be subsequently flushed out of the bathymetric depressions. Understanding this is crucial for the management of the estuary, since it has been observed that the saltwater can be flushed out by wind-driven recirculation of the flow in the estuary when the floodgates are closed. Such a flushing event may spread the saltwater landwards and reach the freshwater intakes, which would be particularly problematic.
The research aim of this project is to understand under what flow conditions the saltwater that is trapped in the bathymetric depressions can be flushed out of them, either by the river discharge or by wind-driven recirculation in the estuary. The objective of this project is to analyze flow velocity and salinity concentration measurements that Rijkswaterstaat has carried out. The flow velocity measurements were conducted with ADCPs that measured the flow velocities across the depth at the bathymetric depressions shown in Figure 1. Preliminary analysis of the available data has shown that several flushing instances have been captured. This study will build on the study of Kranenburg et al. (2023), which reported the flushing conditions in bathymetric depressions D and F (Figure 1). The available dataset provides additional flushing data in bathymetric depressions D and F, which are the most interesting ones since the salt tends to settle there more, and provides additional data from other bathymetric depressions, such that the flow dynamics across the whole estuary can be assessed.
Figure 1. The Haringvliet Estuary with details about (a) the floodgates, (b), (c) the location of the study area, and (d) the bathymetry of the estuary. The letters A – H denote the bathymetric depressions with available flow velocity measurements across the depth. The figure was obtained from Kranenburg et al. (2023).
References
- Kranenburg, W. M., Tiessen, M. C. H., Blaas, M., Van Veen, N. P. (2023). Circulation, stratification and salt dispersion in a former estuary after reintroducing seawater inflow. Estuarine, Coastal and Shelf Science, 282, 108221.
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