Dirk van der Plas, area coordinator at the Rijnland Water Authority, explains the role this ‘invisible infrastructure’ plays in water management and preventing nuisance during extreme weather conditions.

Balance in the Water Level Area

As area coordinator at the Rijnland Water Authority, Van der Plas deals with area developments in the Haarlemmermeer polder and advises on infrastructural interventions that affect the water system.

‘In this case, we came into the picture when Rijkswaterstaat approached us about replacing the culvert under the A4,’ says Van der Plas. ‘We manage the water system in the area, Rijkswaterstaat owns the culvert. This requires good coordination.’

The culvert is one of the three underground connections that drain water from east to west in this area, a polder of about 2,200 ha. ‘If one of those connections does not work properly, the balance in the water level area is disrupted. This can lead to water nuisance and deterioration of water quality. Both situations are unfavorable for the use of the area and disrupt the ecological balance.’

Replacing the Culvert

To replace the culvert, we dig a construction pit 12 m wide. The pit is kept dry with sheet piles and pumps. Then, 108 prefab concrete culvert parts are placed, some weighing more than 11,000 kg.

Structure for Water Quantity and Quality

Even under busy infrastructure like a highway, the water system must continue to function. A culvert makes that possible. ‘The beauty is that with such a structure, you keep an eye on water quantity and quality, even in places where space is scarce,’ says Van der Plas. ‘It may seem like a simple pipe, but it is an essential part of the water system.’

Even in drought or heavy rainfall, the culvert plays a key role. ‘We must be able to supply or drain water when necessary,’ explains Van der Plas. ‘Especially in light of the changing climate, it is important that such connections have sufficient capacity and flow possibilities.’

The culvert under the A4 is approximately 130 m long and 3 by 2 m in diameter, and provides the connection between different parts of the polder. ‘On the west side, there are wells. These are places where salty groundwater comes up. If we cannot flush that with fresh water, the area will slowly become saline. That has major consequences for the ecology, as many plants and animals are sensitive to brackish water. But also for agriculture: if the water is not sufficiently refreshed, farmers cannot use it to irrigate their crops.’

Swift Collaboration

The collaboration between Rijkswaterstaat and Rijnland went very smoothly according to Van der Plas. ‘We only really started talking concretely in mid-2024. That we could go from design to implementation within a year is remarkable. That our wishes and concerns were taken seriously says a lot about the collaboration.’

In practice, this means frequent consultations, clear agreements, and short lines. ‘It is nice that the work takes place in the summer,’ says Van der Plas. ‘A short execution in a dry period has minimal impact on the water system and water quality. We also think along about (emergency) measures such as mobile pumps and alternative drainage routes. It is truly a teamwork effort.’

Invisible and Indispensable

Van der Plas emphasizes how significant the impact of a seemingly simple pipe can be on water management, nature, and the living environment. ‘Flow means fresh water, and fresh water means less salinization and a healthier ecology. For the surrounding area, the new culvert will be hardly visible, but if it were not there, the consequences would quickly become noticeable.’

What he finds most remarkable about this project? ‘How quickly it was arranged, and how many people effectively collaborated in a short time. It shows how much happens behind the scenes to keep water, roads, and landscape in balance.’