Expansion of De Onlanden balancing lake to start in September
Noorderzijlvest Water Authority can begin expanding balancing lake De Onlanden in September 2025. On 14 August, the Provincial Executive of the province of Drenthe approved the project decision for the construction of the additional balancing lake in De Onlanden. The expansion of balancing lake De Onlanden’s increases the storage capacity within the existing flood embankments from 7.5 to 12.7 million cubic metres. This expansion will make it possible to temporarily store more water during extreme weather conditions before it is discharged into the sea. The additional balancing lake is expected to be partially operational by 2025/early 2026 and fully completed by 2027.
Work
The Hooiweg option has been chosen for the implementation of the work. With the project decision, phase one of this option, the expansion of the balancing lake, can be carried out. There will be two new weirs at the Hooiweg. When the extra balancing lake is used, these weirs go up to retain water. The water authority will also modify the Doolhof weir in the Peizerdiep and raise the flood embankments in the balancing-lake area. In addition, the water authority 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. Therefore, the water authority includes these modifications.
Additional capacity to collect water
The additional balancing lake can accommodate another 5.2 million cubic metres 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 stored temporarily (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, the water authority will fill in ditches to retain seepage water for longer, soil will be turfed 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 organisations and site managers, will strengthen the marshland nature and make the ecosystem more robust. The area between the Hooiweg and lake Leekstermeer will remain outside the expansion of the balancing lake, 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 balancing lake. The remaining measures will follow in phase two of the project.
From design to project decision
The draft project decision for the expansion of the balancing lake was available for inspection from 19 December 2024 to 30 January 2025. A total of eighty opinions were received. These opinions were carefully considered by the provincial executive when it adopted the project decision. Based on the opinions and discussions with local residents, additional measures and customised 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 21 August to 1 October 2025).
Limiting flooding
Accelerated climate change is increasing the risk of flooding in the northern Netherlands. As part of the Droge Voeten (Dry Feet) 2050 project, the northern provinces and water authorities have studied which measures are needed to limit flooding until 2050. For example, expanding balancing lakes, improved control of pumping stations, raising embankments, and reinforcing dykes. The optimisation of De Onlanden, which has been designated as a balancing lake since 2012, is an important part of this project.