What if ecological mowing restricts water flow? |
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Hollandse Delta Water Board faces challenges with new ditch maintenance rules
Hollandse Delta Water Board has changed the way it mows a large proportion of its ditches since this month. The reason is the new Code of Conduct for Species Protection in Sustainable Management and Maintenance. The code imposes stricter requirements on activities in and alongside waterways in order to protect protected plants and animals. In practice, this means less flail mowing and more vegetation being left in place. Especially during periods of drought, this raises questions for the water board about water flow.
| Image for illustration, generated with AI |
The new code of conduct has applied nationwide since 1 April 2025 and provides water boards with rules for carrying out management and maintenance under the Environment Act. Organisations working according to the code do not need to apply for a separate environmental permit for every regular management and maintenance activity involving flora and fauna. Hollandse Delta has carried out its mowing activities according to the new approach since August 2026.
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Vegetation must remain in almost all waterways wider than one metre
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From flail mowing restrictions to leaving strips in place
For managers and contractors, the consequences are quite concrete. In locations where special plant or animal species occur, flail mowing can no longer simply be used. Hollandse Delta has created an ecological map for this purpose. According to the water board, around 40 percent of waterways may no longer be flail mowed. On the remaining 60 percent, this is still possible under certain conditions.
Management below the waterline is also changing. In almost all waterways wider than one metre, part of the vegetation must remain in place. In ditches between one and four metres wide with protected species, this concerns 25 percent of the vegetation. If no protected species are present, at least 25 centimetres must remain on one side. For waterways wider than four metres, this becomes 50 centimetres when no protected species are present. In special situations, even 50 percent of the vegetation may need to remain.
Differences between waterways
Not every ditch will therefore receive the same type of management. Waterways that are essential for water supply and drainage, referred to by Hollandse Delta as critical ditches, may be fully mowed. The same applies to ditches that are less than one metre wide at the waterline.
Protected species play an important role in determining the approach. The water board mentions species including the weatherfish, natterjack toad, moor frog, crested newt, green emerald dragonfly and various protected aquatic plants. Ecological maps will be expanded further in the coming years, including through eDNA research.
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Not every ditch will receive the same mowing regime in the future
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Cut material remains for several days
The processing of cut material is also changing. Mowed vegetation remains in place for three to five days, allowing animals present in the material to escape. In areas where flail mowing is not permitted, for example, tall reeds are left unchopped on the bank.
For contractors, the code of conduct therefore means not only a different mowing frequency, but sometimes also different machinery and working methods. Hollandse Delta itself warns that maintenance will take more time as a result.
Tension with water management
This ecological approach can conflict with another important function of waterways: allowing water to be supplied and drained quickly. Especially in smaller ditches, remaining vegetation can restrict water flow. 'Then you increasingly get a situation where water remains stagnant and there is insufficient flow,' says board member Piet van der Eijk to Rijnmond.
This is particularly relevant this summer due to drought. Since mid-July, the Netherlands has been experiencing an actual water shortage. Water boards are therefore trying to retain water while also supplying fresh water to areas where needed.
Supervisor Jan Vos of Hollandse Delta sees a practical tension here. 'I may still have an old-fashioned view: a ditch is there for supplying and draining water.'
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We take nature into account, but supplying and draining water remains the priority
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Intervention remains possible
The code of conduct does not mean that water managers must leave an overgrown waterway untouched under all circumstances. Hollandse Delta emphasises that additional maintenance remains possible when problems occur in main waterways. In the event of imminent emergencies, such as extreme drought or flooding, immediate action can be taken. If there is more time, an ecologist can advise on how to minimise damage to protected species.
This creates a management challenge for water managers and contractors that differs from one waterway to another. One ditch may be fully mowed, while only a few metres away a strip of vegetation must remain or different machinery is required. According to Van der Eijk, the functioning of the water system remains leading: 'We want to comply with this as much as possible, but our main task remains water management.'
This article was previously published on 26 August 2026 on the website of Stad + Groen.
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