Additional balancing lake

De Onlanden is both a balancing lake and a nature reserve. Its storage capacity will be expanded from 7.5 to 12.7 million cubic meters of water.

More balancing lake De Onlanden

De Onlanden is already a water storage area. However, climate change has resulted in more frequent and heavier rainfall. This means that more space is needed for water in this nature reserve. By creating more balancing lake De Onlanden, we will be better prepared for these extreme weather conditions.  
 
Capturing more water within the existing embankments 
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 weir an existing weir ?
  • 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 protecting homes in the area? 

When is the extra balancing lake ?
During periods of extreme rainfall, we retain the water in the balancing lake 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 balancing lake nature reserve. It is an important link in the water system from peat bog 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 water authority shows how 5.2 million cubic meters of extra water can be stored.

Controlled balancing lake

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 balancing lake therefore balancing lake up automatically.
The additional balancing lake a controlled balancing lake. 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 balancing lake also balancing lake fill up automatically. Extra water is added by actively 'controlling' it, for example by weirs or closing weirs . This allows 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 balancing lake.

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 balancing lake demonstrated the usefulness of the balancing lake (see Map Onlanden met foto's hoogwatersituatie februari 2022.jpg).

Climate scenarios increasingly predict heavy and prolonged rainfall. This means that more space is needed for water. De Onlanden can currently hold 7.5 million cubic meters of water. The plans for the additional balancing lake space for another 5.2 million cubic meters of water. This will allow more water to be temporarily stored (for a number of days) in extreme situations before it is discharged into the sea. The additional balancing lake will be balancing lake operational in early 2026. All work on the additional balancing lake 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 on the realization of balancing lake additional balancing lake De Onlanden. There is also cooperation with land managers Staatsbosbeheer, Natuurmonumenten, and Stichting Het Drentse Landschap. 
 
In 2021, the boards of these parties decided to further develop the creation of more balancing lake 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 the expansion of the balancing lake investigated to determine which solutions would work well for the expansion of the balancing lake 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 balancing lake De Onlanden will go ahead. 

View the De Onlanden Project Decision on the Environment and Planning Portal 
Other documents can be viewed on the page De Onlanden - Province of Drenthe 
Enlargement of balancing lake Onlanden balancing lake 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 an additional balancing lake 
The work have already commenced. The additional balancing lake will be partially operational balancing lake 2026. All work on the additional balancing lake completed in 2027.