Delta Decision for the IJsselmeer Area
The IJsselmeer area fulfils a number of crucial functions for Dutch flood risk management and water management. The Delta Decision for the IJsselmeer Area provides direction for the measures that preserve and strengthen those functions.
The Delta Decision for the IJsselmeer Area addresses water level management in the IJsselmeer in relation to freshwater supplies and water discharge to the Wadden Sea.
Water discharge in a context of sea level rise
Starting in 2025, water will be discharged from the IJsselmeer to the Wadden Sea with a combination of gravity discharge and pumping at the Afsluitdijk barrier dam. New pumps are being installed in the Afsluitdijk barrier dam at Den Oever. Completion will be in 2025 at the latest. Research shows that, with the current lock complexes and the new pumps, flood risk management is assured. The lock complexes are due for replacement in 2050.
Water level management
A new level decree has been in effect since 2018. It determines the average winter level in the IJsselmeer and the Markermeer lakes for the years to come. After 2050, the average winter level in the IJsselmeer and Markermeer may rise by a maximum of 30 centimetres in line with the sea level in order to make it possible to respond to new developments if necessary. Any rise will take place gradually. In addition, this new level decree sets a flexible summer level, which significantly enhances the buffer capacity. The underlying idea is that this will make it possible to cope with droughts with a probability of occurrence of once every twenty years. However, research in the Freshwater Delta Programme from 2020 shows that this buffer capacity may no longer be adequate. On the basis of the Steam2050 climate scenario, the buffer capacity will have to be increased further in the years to come. This means that a large rise in the lake level should already be taken into account now for dike upgrades and future pumping capacity in the Afsluitdijk barrier dam.
Fresh water
Rijkswaterstaat is using the opportunities afforded by flexible level management in the IJsselmeer and Markermeer to safeguard the freshwater buffer. The option remains open of increasing the bandwidth of the flexible water level in the IJsselmeer and Markermeer to a maximum of 50 centimetres – from NAP -0.40 m to NAP +0.10 m – after 2050.
In the event of a freshwater shortage, Rijkswaterstaat, the water authorities and the provincial authorities will make joint decisions about how the available water will be allocated to the IJsselmeer area.