Expansion of De Onlanden water storage facility to start in September
The Noorderzijlvest Water Authority can start increasing water storage capacity in De Onlanden in September 2025. On August 14, the Provincial Executive of Drenthe approved the project decision for the realization of additional water storage in De Onlanden. The additional water storage will increase the storage capacity of De Onlanden within the existing quays from 7.5 to 12.7 million cubic meters. This expansion will make it possible to temporarily store more water during extreme weather conditions before it is discharged into the sea. The additional water storage capacity is expected to be partially operational by the end of 2025/beginning of 2026 and fully completed in 2027.
Work activities
The Hooiweg variant has been chosen for the implementation of the work. With the project decision, phase one of this variant, the expansion of the water storage capacity, can be carried out. Two new weirs will be added at the Hooiweg. When the extra water storage capacity is used, these weirs will be raised to retain water. The water board will also modify the Doolhof weir in the Peizerdiep and raise the quays in the water storage area. In addition, the water board will increase the pumping capacity of the Zanddijk pumping station, raise the access roads of TenneT and Gasunie, and take protective measures for homes on Roderwolderweg and Moleneind. Noorderzijlvest will also modify the capacity of the Sandebuur pumping station. Although this pumping station is located outside the project area, it does discharge into De Onlanden. That is why the water board is including these adjustments.
Additional capacity to collect water
The additional water storage capacity provides space for another 5.2 million cubic meters of water. During heavy rainfall, the water level can rise to a maximum of +18 cm NAP (previously -0.20 NAP). This means that in extreme situations, more water can be temporarily stored (for a number of days) before it is discharged into the sea.
Nature
The expansion will take the natural environment in De Onlanden into account as much as possible. Refuge mounds will be created for animals during high water levels, the water board will fill in ditches to retain seepage water for longer, soil will be stripped along ditches, a slenk will be constructed, and the water level will be raised in parts of the area. These measures, which have been developed in collaboration with nature organizations and site managers, will strengthen the marshland nature and make the ecosystem more robust. The area between the Hooiweg and the Leekstermeer will remain outside the additional water storage area, as it is an important breeding and living area for birds. Some of the nature measures will be implemented in phase one, at the same time as the expansion of the water storage area. The remaining measures will follow in phase two of the project.
From design to project decision
The draft project decision for the additional water storage facility was available for inspection from December 19, 2024, to January 30, 2025. A total of eighty opinions were received. These opinions were carefully considered by the province when adopting the project decision. Based on the opinions and discussions with local residents, additional measures and customized solutions were included in the final draft. The project decision will take effect four weeks after publication. Appeals against the project decision can be lodged with the Administrative Law Division of the Council of State for a period of six weeks (from August 21 to October 1, 2025).
Limiting flooding
Accelerated climate change is increasing the risk of flooding in the northern Netherlands. As part of the Dry Feet 2050 project, the northern provinces and water boards have investigated what measures are needed to limit flooding until 2050. These include additional water storage, improved control of pumping stations, raising quays, and reinforcing dikes. The optimization of De Onlanden, which has been designated as a water storage area since 2012, is an important part of this project.