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Rijkswaterstaat currently reinforces the Houtrib Dike in Lake IJssel, located in the center of the Netherlands. Before the start of the reinforcement, a test section was first laid along the dike. The reason to do this was simple: we knew quite little about sand reinforcements in a freshwater lake. Thanks to the pilot that has recently been completed, this has now changed.

The Houtrib Dike will have wide, sandy banks on the western half. These so-called foreshores reduce the wave action against the dike, making it much less heavily attacked by waves. Because sandy banks have not been used before in a freshwater lake for flood risk management, Rijkswaterstaat has commissioned a pilot for which a test section has been constructed along the dike.

This test section has been monitored for four years during which, among other things, the water levels and wave attacks have been continuously measured. Cameras took a picture every hour and the body of sand was measured several times a year. The trial area has now merged into the wide, sandy banks. The pilot has been formally completed this week with the publication of a research report.

Surprising wave action

Henk Steetzel was the project leader of this pilot on behalf of EcoShape. EcoShape is a knowledge consortium that develops and combines knowledge about Building with Nature, i.e. taking nature as a starting point for the design of infrastructure, for example by using natural materials from the environment of the project. Steetzel started his career as a hydraulic engineer at the old Waterloopkundig Laboratorium in The Netherlands and later went on to work at Alkyon and finally at Arcadis, one of the market parties that share their knowledge in EcoShape. Steetzel: 'We already know very well how sandy reinforcements in the coastal area respond to the waves. And what this means for the desired profile of, for example, sand reinforcements along the coast. But in a lake without a tide, such as the IJsselmeer or Markermeer, it works very differently. The water level is constant and wave movements are less predictable, partly because of the influence of the wind. Thanks to the test section along the dike we found out that you have to reinforce the dike especially just below the waterline. The action happens between the water line and a meter below it. ' Rijkswaterstaat has incorporated this result in the design of the Houtribdijk reinforcement.

Skepticism of the Houtribdijk

The sandy, vegetated banks along the Houtribdijk improve water quality and ecological diversity. A nice gain compared to traditional quarry stones. Yet there is still skepticism within organizations, Steetzel noted. But: 'Thanks to the trial, people got a feeling for this solution in time. My motto is: collect not only knowledge but also acquaintances. In other words: go talk to people and keep explaining and asking questions. Especially with the people who have to manage the dike when we are gone’. The knowledge about sandy banks that has been gained in the pilot is published in guidelines about the desired dike design, but also, for example, about the licensing process, management and maintenance. As a follow-up to the pilot, Rijkswaterstaat is now examining how the sandy banks on a large scale are developing in practice and what this means for water safety, ecology, sand drift and vegetation development.

Application in other reinforcement projects

The knowledge gained in the Pilot Houtrib Dike has also been used in other nature and dyke improvement projects, such as the Marker Wadden and the reinforcement of the Markermeer dikes. In addition to the IJsselmeer region, Steetzel sees more applications for sandy banks in the future: 'Think of the tidal environment of dykes along the Wadden Sea coast in the North of The Netherlands, for example in the provinces of Friesland and Groningen. There are also opportunities in the water of the province of Zeeland waters (southwest of The Netherlands) and perhaps along major rivers. Technology is often not the problem; the point is that you discuss these innovations in time and that you take the time to develop together with everyone involved.'

Source: www.rijkswaterstaat.nl (translated from Dutch)