Smart mowing for good water management

For the Water Framework Directive (WFD), we are working to improve ecological water quality. An important measure for the WFD is to leave part of the vegetation in place during ditch maintenance, if there is room in the profile. By leaving some of the aquatic plants in place, there is still room for aquatic animals and fish to shelter and reproduce. The presence of aquatic plants also provides oxygen in the water, which is good for water quality.

By mowing smartly, we ensure good water supply and drainage and contribute to ecological water quality and biodiversity.

How do you know where vegetation can remain?

HHNK is working on updating the Legger Wateren, the digital map of the water system with the rules for maintenance. For each polder area, we identify bottlenecks and calculate which part of the ditch profile is necessary for proper inflow and outflow and where there is room for aquatic plants. For the first areas, the degree of vegetation can be found here: Per maximum degree of vegetation (available for inspection).

The analyses show that there is room in the profile of many ditches to allow aquatic plants to remain. 

Guidelines for three types of locks

Based on the new analysis, we distinguish three types of locks with different maintenance guidelines:

  1. Open ditches play an important regional role in water supply and drainage. In these ditches, a maximum of 25% of the profile may be covered with aquatic plants. During maintenance for inspection, a maximum of 10% of vegetation may remain in the entire watercourse (5% on each side).
  2. Moderately overgrown ditches provide more space for aquatic plants and may be overgrown by up to 50%. During maintenance, a maximum of 25% of the vegetation may remain in the profile of these ditches.
  3. Plant-rich ditches are mainly of local importance for water supply and drainage and have plenty of space in the profile for aquatic plants. A maximum of 90% vegetation is allowed here, and maintenance must reduce this to a maximum of 50%.

If more vegetation is allowed to remain, it is also possible to carry out maintenance in phases. For example, one side of the ditch one year and the other side the next year. For all types of ditches, however, it is important that the structures (such as culverts, bridges, and weirs) are well maintained. This also applies to phased mowing.

HHNK will work according to the new guidelines, carrying out maintenance itself. The new maintenance guidelines also apply to farmers and private individuals who are responsible for maintaining ditches, as well as other owners such as Natuurmonumenten, Landschap Noord-Holland, and municipalities.

Implementation by area

Carefully updating the register takes time. That is why the new guidelines for ditch maintenance are being introduced on a region-by-region basis. You can find the current regions in the Water Register.

By the end of 2026, the new maintenance guidelines for the entire management area (56 areas) will be available. The goal is to implement Smart Mowing throughout our entire management area by 2027. In this way, we are working together to improve ecological water quality and biodiversity.

Frequently asked questions about smart mowing

1. What is the purpose of smart mowing for good water management?

By working according to the updated register, you ensure proper water supply and drainage and contribute to ecological water quality by leaving vegetation in place wherever possible. In this way, we work together to comply with the Water Framework Directive.

2. What is the basis of Smart Mowing for good water management? 

The basis for smart mowing for good water management is the HHNK register. To obtain an up-to-date, reliable, and complete picture, we perform analyses and calculations for each area. In the past, watercourses were only assessed in terms of supply and drainage capabilities. Nowadays, it is also important to take ecological water quality into account in maintenance (see question 6). When updating the register, it is made clear for each watercourse whether, and if so, how much space there is for vegetation. 

In practice, it appears that in many watercourses—particularly secondary and tertiary ones—there is space left over where vegetation can remain. In addition, there are watercourses that require more intensive maintenance.

3. How were the calculations for the register performed? 

First, detailed hydrological models are used to calculate the volume of water inflow and outflow per watercourse. Next, a required profile is calculated for the applicable standards. The practical knowledge of the area managers is used to test the analyses.

4a. Does this have any consequences for water supply and drainage? 

The basic principle is and remains that the water supply and drainage must comply with the applicable standards. That is also the criterion for the inspection.

4b. And what about extreme precipitation? During a cluster storm, the ditch must be completely clean, because more water has to be drained away, right?

Water drainage in a polder is limited by the capacity of the pumping station or an intermediate weir. If the degree of vegetation in the ditch complies with the register, the vegetation does not limit the drainage capacity during heavy rainfall. In other words, in such exceptional circumstances, the water cannot be drained more quickly if there is less vegetation.

5. Does vegetation limit the storage capacity of watercourses? After all, where there is a plant, there cannot be water.

Submerged and floating plants limit inflow and outflow, but not storage capacity. This only applies to plants that protrude above the water (such as reeds). A study once showed that storage capacity is reduced by around 2% in a densely overgrown reed bed. With an average storage capacity of 30 mm of precipitation in the water system, you lose 0.6 mm. So it's very limited.

6. How does this contribute to water quality?

For the Water Framework Directive (WFD), water quality is assessed on two aspects: ecological water quality and chemical water quality. Smart mowing is important for ecological water quality. Ecological water quality is assessed based on the presence of aquatic plants, small aquatic animals, fish, and algae. By leaving more vegetation in place where possible, existing plants can recover more effectively and there is less disruption to the habitat of fish and other aquatic animals. 

7. Are there any other advantages?

As the party responsible for maintenance, you have more flexibility in choosing when to maintain a watercourse, as long as the vegetation remains below the limit.

It is no longer necessary to apply for an exemption from inspection in order to manage a ditch ecologically, as long as it fits within the image of the register.

Less intensive maintenance and more vegetation also contribute to stronger banks, resulting in less erosion.

8. Is it mandatory to leave vegetation in place where possible according to the register?

There is no obligation for those responsible for maintenance to leave vegetation in place. 

9. How is this used in the chimney sweep?

In areas where the cadastral register has been updated, the new approach forms the basis for the inspection. The inspectors have received instructions on this. Over the coming years, the number of areas operating in this way will be gradually expanded.

10. What constitutes a healthy ditch environment?

A healthy ditch needs oxygen. Not only for the fish, plants, and other living organisms, but also for processing dead aquatic plants and tree leaves on the bottom; the decomposition process. The remains of the decomposition process remain on the bottom and form the sludge layer, or dredging material.

When there is little water in the ditch, the oxygen is quickly depleted. Fish, plants, and other living organisms can no longer survive there. The quality of the water then deteriorates rapidly. For a healthy ditch environment, it is therefore important that there is sufficient water in the ditch.

11. In what other ways can water quality be taken into account during maintenance?

The following practical guidelines are good for ecological water quality:

  • Phased mowing, for example, one side one year and the other side the next year
  • Remove grass clippings from the edge and dispose of them
  • Preference for cutting or mowing over scraping and chopping.

Nature conservation legislation

When carrying out ditch maintenance, you must take into account nature conservation legislation (Omgevingswet). You must not cause any damage to protected animals and plants present and you must take breeding birds into account. If part of the ditch cannot be maintained due to breeding birds or protected animals or plants, please report this to the inspection team.

For more information on this subject, please refer to the Nature and Biodiversity page on the website of the Ministry of Economic Affairs.

12. What should I do if I disagree with the specified vegetation coverage for the ditch I am responsible for maintaining?

If you disagree with the classification of the locks and bottlenecks, please contact us. The location will then be reassessed. If necessary, the classification will be adjusted. Adjustment of the register remains a continuous process.