Circulation, mixing and salt transport in a former estuary after reintroduction of seawater inflow: a 3D modelling study

Around the world, estuaries have been partially or completely closed-off from the sea and their number may increase with rising sea levels. Concurrently, there is a trend to reintroduce seawater inflow into enclosed former estuaries for ecosystem improvement. This is also the case in the Haringvliet, a former estuary in the Rhine-Meuse Delta, closed-off in 1970 with floodgates blocking seawater inflow and regulating outflow. As the reintroduced salt water inflow may threaten fresh water intake from the basin, the dispersion of salt through the system needs to be well understood and carefully managed.

To study the circulation, mixing and salt transport in the Haringvliet, we developed a high-resolution 3D numerical model using the unstructured hydrostatic modelling software DFlow-FM. The model has a horizontal grid with typical cell side lengths of 60 m and a combination of z- and σ-layers in the vertical with a typical thickness of 0.125 m. In agreement with observational data, the model results show that the incoming salt water reaches the deeper parts of the system, induces a strong stratification and is only flushed out of the system after multiple events of large outward floodgate discharges. When the floodgates are closed during the low river discharge season and salt is still present in the system, wind becomes the dominant forcing of mixing and transport. For axial winds, the model results show a considerable horizontal circulation, with downwind currents over the shallow parts and significant upwind currents over the deep parts of the system. These upwind currents are an important mechanism for inland transport of salt after upward mixing, and increased salinity values are found at landward locations for seaward wind. Using the model to explore the mixing mechanisms, we found that the current-related shear is generally not strong enough to induce interfacial mixing directly above the deep parts. Mixing mostly occurs when salt water reaches less deep areas after tilting of the pycnocline. We will explore how to relate this competition of mixing processes to non-dimensional parameters like the Wedderburn number.

With scenario analyses, we study the dynamics for a range of wind conditions and determine which condition provides most risk for fresh water intake. We also investigate for what rates of seawater inflow and outward floodgate discharges dynamic equilibria can be reached in which the incoming salt mass equals the mass flushed out during the subsequent ebb. The insights in circulation, mixing and salt transport due to forcing by floodgate discharges and wind are relevant for other semi-enclosed former estuaries.