Buffering fresh water at the Volkerak-Zoommeer to mitigate drought

Abstract

The Volkerak-Zoommeer is a former estuary located the Rhine Meuse Delta in the southwestern part of the Netherlands. After the completion of the Volkerakdam, Philipsdam and Oesterdam as part of the Delta Works, this estuary was cut of from the Oosterschelde and a fresh water lake was formed in a sea water environment. This new lake became the Volkerak-Zoommeer: a lake that acts as a fresh water supply for the regional water systems of the water boards Hollandse Delta, Scheldestromen and Brabantse Delta as well as providing a tidal free shipping connection between Antwerp and the Rhine. The operational water manager of the lake is Rijkswaterstaat. They manage the water level and water quality by letting fresh water in from the Hollandsch Diep at the Volkeraksluizen and release it at Bath into the Westerschelde. This mechanism enables Rijkswaterstaat to flush the system and adhere to the target levels, as well as managing a maximum chloride concentration of 450 mg Cl/L during the growing season. In 2018, the Netherlands experienced one of the most severe droughts since the beginning of weather measurements started. Low discharges at the Rhine resulted in a decreasing water supply and more salinization in the Rhine Meuse Delta. When the discharge of the Rhine at Lobith reach below 800 m3/s, it is no longer allowed to let in fresh water through the Volkeraksluizen. This could lead to problems in the fresh water supply to the regional system of the water users of the Volkerak-Zoommeer. The question came up whether creating a fresh water buffer by temporarily heightening of the water level could be used to overcome a period with no fresh water supply from the Hollandsch Diep. In autumn 2020, the dynamics of the system were researched during a practical trial. The water level was heightened up to +0.15 m NAP. Next, the inlet was closed and it was measured how long it would take until the water level reached -0.10m NAP with normal flushing operations at Bath. During a second trial, the outlet at Bath was closed too. Based on the insights of these trials, a prototype for a Decision Support System was created. This system is able to give insights in the development of the water level at the Volkerak-Zoommeer according to operational water management decisions. This Decision Support System was used to evaluate the impact of delta and climate scenarios in 2050 and 2085 on the water level of the Volkerak-Zoommeer. The maximum possible duration for a period without a fresh water supply was researched per scenario within the target levels of -0.10 m NAP and + 0.15m NAP. Different flushing regimes at Bath were evaluated. For normal flushing operations, this duration is 5 till 7 days. For flushing every second low tide, this period could be lengthened to 8 to 13 days. When the outlet at Bath is closed, the maximum duration is 20 till 60 days. From the trials, it was found that the chloride concentration will gradually increase in this period. Unfortunately, sufficient insights in the dynamics of the chloride concentration lack at the moment to sufficiently incorporate the chloride concentration in the prototype. From historical data, the maximum duration of a period with discharges lower than 800 m3/s measured at Lobith was 16 days during the growing season and 82 days for all year data. Within the current target levels, this duration cannot be reached. It was therefore researched what initial water level theoretically could be achieved based on historical data. This turned out to be +0.50 m NAP. Here, the Volkerak-Zoommeer was considered as a closed system, neglecting the open connections with the Dintel and the Vliet. With an initial water level of +0.50 m NAP, the duration without fresh water supply could be significantly lengthened. With normal flushing operations, the period can take between 12 and 19 days. For flushing operations every second low tide, this period takes 20 till 34 days. When the outlet at Bath is closed, a period up to 120 days could be bridged.