Circulation, stratification and salt dispersion in a former estuary after reintroduction of seawater inflow to improve fish migration

The Haringvliet is a former estuary in the Rhine-Meuse Delta. Since 1970, seaward outflow is regulated with floodgates, while seawater is kept out. To improve fish migration and the ecological quality of the Rhine-Meuse system, limited seawater inflow during flood has been reintroduced again in 2018. The incoming salt water progresses through the former tidal channels, arrives in deep pits and is partially flushed out again by the outflow, especially during high river discharge. However, the remaining salt water can gradually spread through the system due to wind-induced mixing and circulations, especially when the gates are closed also for outflow during low river discharge. As this can threaten fresh water intakes, inflow, flushing and dispersion of salt need to be well understood and carefully managed.

In this study we analyse velocity measurements from ADCPs at two former tidal channels in the Haringvliet, together with salinity time series and profiles at multiple locations. The salinity profiles show that the system tends to be strongly stratified. Using the ADCP backscatter, we estimated the time development of the interface level to relate this to the local velocity, floodgate discharge and wind. For peak discharges and low wind speed, the velocities show a clear relation with the discharge and the interface can lower abruptly. However, for lower discharges and higher wind speed, the relations are less clear, and the profiles are highly affected by the wind. In case of wind but closed sluices, flow against the wind was found for wind in the systems longitudinal direction. We explain this from the large area of (former) shoals, leading to flow with the wind in shallow parts and against the wind in deep parts due to a local imbalance between stress divergence and pressure gradient. This turns out to be an important driver of landward salt transport, as increased salt concentrations were found at landward locations for seaward wind. Next to that, indications were found of exchange between former tidal channels and transport over sills due to wind driven tilting of the salinity interface.

Enhanced understanding of the salt transport dynamics in this former estuary after reintroduction of limited seawater inflow is an essential element to manage this system, protect the fresh water availability and keep the confidence of critical stakeholders, which is essential for the success of this ecosystem improvement program.