Additional Water Storage
De Onlanden is both a water storage facility and a nature reserve. Its storage capacity is being expanded from 7.5 to 12.7 million cubic meters of water.
More water storage in De Onlanden
De Onlanden is already a water storage area. However, climate change has led to more frequent and heavier rainfall. This means that more space is needed to store water in this nature reserve. By increasing the water storage capacity in De Onlanden, we will be better prepared for these extreme weather conditions.
Capturing more water within the existing quays
De Onlanden can currently collect 7.5 million cubic meters of water. This will be expanded to 12.7 million cubic meters. This will be done within the existing quays. During heavy rainfall, the water level can be temporarily raised. This allows more water to be collected in De Onlanden.
To make this possible:
- Are we raising the quays surrounding the area?
- Are we building two new weirs?
- Are we modifying an existing dam?
- Are we taking nature-based measures so that plants and animals can also cope better with the amount of water?
- Are we taking additional measures such as modifying pumping stations and protecting homes in the area?
When is the extra water storage used?
During periods of extreme rainfall, we retain the water in the storage facility for a few days. It is then released into the sea in a controlled manner. This prevents unwanted flooding in Groningen and the surrounding area. Weexpect that the entire extra capacity will be needed once every 25 years. Once every 10 years, only part of the capacity will be needed.
Protecting nature and collecting extra water
De Onlanden is a water storage area and nature reserve. It is an important link in the water system from peat to sea. The combination of functions creates a robust area that enables us to cope with climate change.
It isimportant that both functions continue to reinforce each other in the future.Forest rangers Kees van Son of Staatsbosbeheer and Bart Zwiers of Natuurmonumenten talk about the balance between water and nature. Marieke van Leeuwen of the Noorderzijlvest water board shows how an additional 5.2 million cubic meters of water can be stored.
Controlled water storage
During heavy rainfall, large amounts of water flow into streams and ditches. This water flows naturally from the higher-lying province of Drenthe to the lower-lying Onlanden, which temporarily collects this water. The current water storage area therefore fills up automatically.
The additional water storage area will soon be a controlled water storage area. This means an area where water is deliberately and controlled collected during heavy rainfall or high water levels. The difference with the current water storage area is that an extra layer of water can be retained. The extra water storage facility will not fill up automatically. Extra water will be added by actively 'controlling' it, for example by opening or closing weirs. This will allow us to collect the water in a more targeted manner and discharge it gradually.
Creation of water storage area
In 2012 and 2022, De Onlanden proved its usefulness as a water storage facility.
The De Onlanden area (see mapGeneral overview map of the planning area.pdf), on the border between Groningen and Drenthe, was developed between 2007 and 2012 for water storage in combination with nature. The construction of De Onlanden has created a robust climate buffer that helps to keep the surrounding region dry. In February 2022, a large amount of waterwascollected in De Onlanden after heavy rainfall. This once again demonstrated the usefulness of water storage (see Map of De Onlanden with photos of the high water situation in February 2022.jpg).
Climate scenarios increasingly predict heavy and prolonged rainfall. This means that more space is needed to store water. De Onlanden can currently store 7.5 million cubic meters of water. The plans for additional water storage will provide space for another 5.2 million cubic meters of water. This will allow more water to be stored temporarily (for a number of days) in extreme situations before it is discharged into the sea. The additional water storage will be partially operational in early 2026. All work on the additional water storage will be completed in 2027.
Parties involved and decision-making
Initiator Noorderzijlvest Water Authority and competent authority Province of Drenthe are working together with the Province of Groningen and the municipalities of Tynaarlo and Noordenveld to realize additional water storage in De Onlanden. There is also cooperation with site managers Staatsbosbeheer, Natuurmonumenten, and Stichting Het Drentse Landschap.
In 2021, the boards of these parties decided to further develop the creation of more water storage within the existing boundaries of De Onlanden, with attention to the vulnerable nature. This decision was made after it became clear that alternatives outside the boundaries of De Onlanden were not feasible in the short term.
The parties are also working together to strengthen and create a more natural water system outside the boundaries of De Onlanden. Read more about this under Kop van Drenthe.
How did the plan for De Onlanden come about?
In 2022, the procedure for the project decision started with an exploratory phase. Various alternatives for expanding the water storage capacity were investigated to determine which solutions would work well for expanding the water storage capacity in De Onlanden. Nature and environmental organizations actively contributed ideas for promising solutions.
This exploratory study resulted in three possible variants. These were assessed in an environmental impact assessment (EIA). Based on the assessment, the Hooiweg variant was chosen. From December 2023, this variant will be further elaborated in part 2 of the EIA.
What has been decided?
In November 2024, the Provincial Executive of Drenthe adopted the draft Project Decision and the accompanying EIA. On August 14, 2025, the final project decision was made. This decision confirmed that the expansion of the water storage facility in De Onlanden will go ahead.
View the De Onlanden Project Decision on the Environment Portal
Other documents can be viewed on the page De Onlanden - Province of Drenthe
Expansion of De Onlanden water storage facility to start in September (news item)
The project decision was published on September 17, 2025 (four weeks after publication). Appeals against the project decision may be lodged with the Administrative Law Division of the Council of State within six weeks (from August 21 to October 1, 2025).
Planning for additional water storage
The work have already commenced. The additional water storage capacity will be partially operational by early 2026. All work on the additional water storage capacity will be completed in 2027.