Expanding the 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 metres of water.

Expanding balancing lake De Onlanden

De Onlanden is already a balancing lake. 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 expanding the 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 metres of water. This will be expanded to 12.7 million cubic metres. This will be done within the existing embankments. During heavy rainfall, the water level can be raised temporarily. This allows more water to be collected in De Onlanden. 

To make this possible: 

  • We raise the embankments surrounding the area
  • We will build two new weirs
  • We adjust an existing weir
  • We take nature-based measures so that plants and animals can also cope better with the amount of water
  • We take additional measures such as modifying pumping stations and protecting homes in the area 

When is the extra balancing lake used?
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. We expect 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 balancing lake  and a 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 is important that both functions continue to reinforce each other in the future. Forest rangers Kees van Son of Staatsbosbeheer (Dutch Forestry Commission) and Bart Zwiers of Natuurmonumenten (Dutch Society for Nature Conservation) talk about the balance between water and nature. Marieke van Leeuwen of the Noorderzijlvest Water Authority shows how 5.2 million cubic metres 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 collects this water temporarily. The existing balancing lake fills up automatically.
The additional balancing lake will be a controlled balancing lake. This means an area where water is collected deliberately and in a controlled manner during heavy rainfall or high water levels. The difference with the existing balance lake is that it can retain an extra layer of water. The additional balancing lake cannot fill up automatically. Extra water is added by actively 'controlling' it, for example by opening or closing weirs. This allows us to collect the water in a more targeted manner and discharge it gradually. 

Creation of a balancing lake

In 2012 and 2022, De Onlanden proved its use as a balancing lake.

The De Onlanden area (see overview map of the planning area.pdf map), on the border between Groningen and Drenthe, was developed between 2007 and 2012 as a balancing lake 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 water was collected in De Onlanden after heavy rainfall. This demonstrated the usefulness of the balancing lake (see Onlanden with photos of high-water situation February 2022.jpg  map).

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 metres of water. The plans for the expansion of the balancing lake provide space for another 5.2 million cubic metres 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 balancing lake will be operational in early 2026. All work on the expansion of the balancing lake will be completed in 2027. 

Parties involved and decision-making

Initiator Noorderzijlvest Water Authority and the competent authority the Drenthe Provincial Executive are working together with the Province of Groningen and the local councils of Tynaarlo and Noordenveld to expand De Onlanden balancing lake. There is also cooperation with site managers, Staatsbosbeheer (Dutch Forestry Commission), Natuurmonumenten (Dutch Society for Nature Conservation), and Stichting Het Drentse Landschap. 
 
In 2021, the boards of these parties decided to further develop the creation of more balancing lake space within the existing boundaries of De Onlanden, with attention to the vulnerable landscape. 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 were studied to determine which solutions would work well for the expansion of the balancing lake De Onlanden. Nature and environmental organisations actively contributed ideas for promising solutions. 
This exploratory study resulted in three possible options. They were assessed in an environmental impact assessment (EIA). Based on the assessment, the Hooiweg option was chosen. From December 2023, this option will be worked out in detail in part 2 of the EIA. 

What has been decided? 
In November 2024, the Drenthe Provincial Executive adopted the draft Project Decision and the accompanying EIA. On 14 August 2025, the final project decision was taken. 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 (Dutch only) 
Other documents can be viewed on the page De Onlanden - Province of Drenthe (Dutch only) 
Expansion of De Onlanden balancing lake to start in September (Dutch only) (news item)

The project decision became effective on 17 September 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 21 August to 1 October 2025). 

Schule for expansion of balancing lake 
The work has already started. The expanded balancing lake will be partially operational by early 2026. All work on the expansion of the balancing lake will be completed in 2027.