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. Leaving some of the aquatic plants in place leaves room for aquatic animals and fish to shelter and reproduce. Also, the presence of aquatic plants provides oxygen to the water. This 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 an update of the Legger Wateren 2024, the digital Map of the water system with the rules for maintenance. For each polder area we will Map bottlenecks and calculate which part of the ditch profile is necessary for good supply and drainage and where there is room for aquatic plants. For the first areas the degree of overgrowth can now also be found here.

The analyses show that in many ditches there is room in the profile to leave aquatic plants. This is recorded on the Map .

Guidelines for three types of locks

Based on the new analysis, we distinguish three types of ditches, which are subject to different maintenance guidelines:

  1. Open ditches have an important regional function for water supply and drainage. During maintenance for the inspection, a maximum of 10% of aquatic plants may remain here (5% per side).
  2. In moderately vegetated ditches, there is more room for aquatic plants and up to 20% of aquatic plants may remain (10% on either side).
  3. Plant-rich ditches are especially important locally for water supply and drainage and have plenty of space in the profile for aquatic plants. Here a maximum of 50% of the aquatic plants may remain during maintenance.

If more vegetation is allowed to remain, it is also possible to carry out maintenance in a more phased manner. For example, one year one ditch side and the next year the other side.

HHNK itself is going to work according to the new guidelines where we do the maintenance ourselves. 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.

Introduction by area

The careful updating of the legger takes time. Therefore, the new guidelines for ditch maintenance this from 2024 will be implemented per area. For the following areas, the analyses are ready and incorporated into the legger:

  • Uitgeester and Heemskerkerbroek
  • Assendelft
  • Westzaan
  • Westerkogge
  • Hoorn
  • Drieban.

These six areas are subject to the updated maintenance and inspection guidelines beginning in 2024.

In the coming years we will work on updating the other areas. By the end of 2026, the new maintenance guidelines for the entire management area (56 areas) will be available. The goal is that Smart Mowing will be applied throughout our management area in 2027. In this way we work 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 network you ensure a good supply and drainage of water and you contribute to the ecological water quality by leaving vegetation where possible. This is how we work together on 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 legger of HHNK. 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 for their inflow and outflow possibilities. Nowadays it is also important to consider ecological water quality during maintenance (see question 6). When updating the legger, we provide insight into each watercourse to determine whether, and if so, what room there is for vegetation. 

It turns out that in practice in many - especially secondary and tertiary - watercourses there is space left over where vegetation can remain. In addition, there are watercourses that actually require more intensive maintenance.

3. How were the calculations performed for the legger? 

First, detailed hydrological models are used to calculate water inflows and outflows for each watercourse. Then a required profile is calculated for the applicable standards. The practical knowledge of the area managers is used to test the analyses.

4a. Will this affect water supply and drainage? 

The basic principle is and remains that water supply and drainage must meet the applicable standards. This is also the criterion for the inspection.

4b. What about during extreme precipitation events? In a cluster downpour, the ditch has to be completely clean, right, because more water has to leave?

Water discharge 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 legger, the vegetation is not limiting the discharge capacity during intensive precipitation. In other words, the water cannot be drained faster in such an exceptional situation if there is less vegetation.

5. Does vegetation limit the storage capacity of watercourses? After all, where there is a plant, there can be no water.

Submerged and floating plants limit inflow and outflow, but not salvage capacity. These are only the plants that rise above the water (such as reeds). A study once showed that it is about 2% storage loss in a densely grown reed bed. For an average of 30 mm of precipitation in the water system, you lose 0.6 mm. Very limited.

6. How does this contribute to water quality?

For the Water Framework Directive (WFD), water quality is assessed on two components: 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 where possible, existing plants can recover better and there is less disruption of habitat for fish and other aquatic animals. 

7. Are there any other benefits?

As the maintainer, you are more flexible in choosing the maintenance time for a watercourse, as long as the vegetation remains below the boundary.

It is not (or no longer) necessary to apply for a waiver from the inspection to manage a ditch ecologically, as long as it fits within the image of the legger.

Less intensive maintenance and more vegetation also contributes to firmer banks, reducing erosion.

8. Is it mandatory to leave vegetation - where the legger allows it - in place?

It is not an obligation for maintainers to leave vegetation in place. 

9. How will this be worked with in the chimney?

In the areas for which the legger has been updated, the new approach is the starting point for the survey. The surveyors have received instructions about this. In the coming years, the number of areas working in this way will be gradually expanded.

10. What is a healthy ditch environment?

A healthy ditch needs oxygen. For the fish, plants and other living organisms, but also to process dead aquatic plants and tree leaves on the bottom; the decay process. The remains of the decay process remain on the bottom and form the silt layer, or dredge.

When there is little water in the ditch, oxygen is quickly depleted. Fish, plants and other living organisms can no longer live there. The quality of the water then deteriorates rapidly. So for a healthy ditch environment, it is important to have enough water in the ditch.

11. In what other ways can water quality be considered in maintenance?

The following practical guidelines are good for ecological water quality:

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

Nature protection legislation

When carrying out ditch maintenance, you must take into account nature protection legislationOmgevingswet). You must not harm any protected animals or plants 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 survey team.

For more information, please see the Nature and Biodiversity page on the Ministry of Economic Affairs website.

12. What do I do if I disagree with the specified vegetation level for the ditch I am supposed to maintain?

If you disagree with the classification of ditches and foresee bottlenecks, please contact us. The site will then be reassessed. If necessary, the classification will be adjusted. Adjustment of the legger remains an ongoing process.