Circulation, stratification and salt dispersion in a former estuary after reintroducing seawater inflow

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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 can threaten fresh water intake, inflow, flushing and dispersion need to be well understood and carefully managed. Here we investigate stratification, flow circulation and salt transport in the Haringvliet by analyzing ADCP data collected in two former tidal channels, together with salinity time series and profiles. The profiles show that the incoming water reaches the deeper parts and that the system tends to be strongly stratified. Over time, the interface levels deepen in steps, mainly coinciding with floodgate discharge events, which are strongly correlated with the primary current velocities in the channels. However, even floodgate discharges for above average Rhine discharge conditions were insufficient to quickly flush or mix the salt out of the channels. This is consistent with calculated gradient Richardson numbers, which barely get in the range of critical values. For closed floodgates with no outward discharge, we found considerable depth-averaged upwind currents in the channels for axial winds. This reveals a dominant horizontal circulation, with downwind currents over the shallow parts and upwind currents over the deep parts of the system, explained by a local imbalance between the wind stress and pressure gradient force at both shoals and channels. This horizontal circulation is an important driver for inland salt transport, as increased salinity values were found at landward locations for seaward wind. This implies this is a condition with increased risks for fresh water availability. Analytical calculations confirmed the upwind currents in the channels can become sufficiently strong to transport salt mixed up at one side of the system to the other within the duration of a wind event. However, the current-related shear is likely not strong enough to induce interfacial mixing directly above the deep parts, and we hypothesize mixing mostly occurs when salt water reaches less deep areas after tilting of the pycnocline. The insights from this study are relevant for other formerly enclosed estuaries for which reintroduction of seawater inflow is considered, as well as presently open systems for which (partial) closure is discussed.