Dynamic control of salt intrusion in the Mark-Vliet river system, the Netherlands

The Volkerak-Zoom Lake is an enclosed part of the estuarine delta in the southwest of the Netherlands and exists as such since 1987. The current freshwater lake experienced a deterioration in water and ecological quality. Especially cyanobacteria are a serious problem. To solve this problem it is proposed to reintroduce salt water and tidal dynamics in the Volkerak-Zoom Lake. However, this will affect the water quality of the Mark-Vliet River system that drains into the lake. Each of the two branches of the Mark-Vliet River system is separated from the Volkerak-Zoom Lake by a lock and drainage sluice. Salt intrusion via the locks may hamper the intake of freshwater by the surrounding polders. Salt intrusion can be reduced by increasing the discharge in the river system. In this study we used the hydrodynamic SOBEK model to run different strategies with the aim to minimize the additional discharge needed to reduce chloride concentrations. Dynamic control of the sluices downstream and a water inlet upstream based on real-time chloride concentrations is able to generate the desired discharges required to maintain the chloride concentrations at the polder intake locations below the threshold level and to reduce the amount of water required by more than 50% compared to a situation with a constant discharge. Other effective measures consist of relocating the most downstream polder intakes more upstream, reducing the downstream cross section of the Vliet to increase flow velocities and measures that reduce the inflow of salt water via the locks. This study shows that dynamic control is a promising technique in regulated streams to alleviate water quality problems by controlled flushing of the syste