De Onlanden gets new weirs to prevent flooding
Work began this week on the expansion of the De Onlanden balancing lake. On the instructions of Noorderzijlvest Water Authority, contractor Oosterhuis is building two new weirs at Hooiweg. The existing Doolhof weir at Peizerdiep will also be modified. The work will continue until the spring of 2026. Additional measures and nature conservation measures will follow. The entire expansion will be completed in 2027.
Len van Rosmalen, Director of Water Systems and Water Safety at Noorderzijlvest Water Authority:
“Together with our regional partners, we have gone through an intensive process to reach this point. By working together with other government agencies, consultants, and nature organisations, we are now able to take this step. De Onlanden is a unique area where nature and water management go hand in hand. It is of great importance that both functions remain in balance. By increasing the capacity of the balancing lake from 7.5 to 12.7 million cubic metres of water, we are building a future in which we keep our feet dry and give nature room to thrive.”
Smart water management
Two new weirs will be built on Hooiweg/Matsloot: the Vischlust weir (north) and the Rowolder weir (south). Under normal circumstances, these weirs are down, allowing water to flow freely. During heavy rainfall, they are raised to retain water temporarily. The Doolhof weir will be fitted with a new valve and will be raised, together with the flood embankments on either side.
Adrie Masteling, operations manager at Oosterhuis: "We have started with the northern weir. We will then continue with the southern weir and the Doolhof weir. The work on the weirs will be completed in the spring of 2026. We are delighted with this contract for Noorderzijlvest. We worked on De Onlanden before. We feel a close connection, because in 2012, when the balancing lake was nearly ready, we had a lot of flooding here. We worked through the night to lower a flood embankment so that the balancing lake could fill. We know why we are doing this."
Follow-up measures and attention to nature
At the beginning of 2026, the embankments in the De Onlanden balancing lake will be raised. At the same time, we will take additional measures such as protecting homes. Nature conservation measures will also be implemented. For example, refuge mounds will be created for animals during high water levels and the area will be enriched with marshland, which will benefit species such as the bittern.
Martijn Bakker, team leader at Staatsbosbeheer (Dutch Forestry Commission), explains: “These interventions make nature more resilient and better able to withstand extreme weather conditions. The partners are now making the most of this for water and nature. That’s important because we’re working together on a larger, integrated challenge: water availability, water quality, and water safety. In De Onlanden, water and nature must form a single robust system.”
Noorderzijlvest Water Authority and nature organisations have agreed that nature will be taken into account as much as possible during construction. Earthworks will take place outside the breeding season. In addition, fences will be installed to prevent otters and other animals from crossing the Hooiweg during construction.
Larger balancing lake because of climate change
Due to climate change, we are experiencing more extreme precipitation and need more space for balancing lakes. That is why Noorderzijlvest is increasing the capacity of balancing lake De Onlanden from 7.5 to 12.7 million cubic metres. This will be done within the existing flood embankments. Part of the expansion will be completed by early 2026. The entire project will be completed in 2027. The extra space is expected to be needed in full once every 25 years and in part once every 10 years. The collected water will then be discharged in a controlled manner via lake Lauwersmeer to the sea.
Part of Droge Voeten (Dry Feet) 2050
The expansion of De Onlanden is part of the Droge Voeten (Dry Feet) 2050 programme, in which the northern provinces and water authorities work together on solutions to flooding. As part of this programme, measures include modifying pumping stations, creating new balancing lakes, and raising and reinforcing flood embankments and dykes.
Stay informed of the work
During the project preparation phase, Noorderzijlvest held extensive discussions with residents in the area and local partners. The environmental manager will also be available to answer questions during the construction phase. Anyone interested can stay up to date on the progress of the work via the website and the balancing lake De Onlanden newsletter.
Activities | Noorderzijlvest Water Authority
Expansion of De Onlanden balancing lake | Noorderzijlvest Water Authority