Dikes protected by vegetated foreshores require less raising and reinforcing than is often planned presently, according to civil engineer Vincent Vuik in his PhD thesis. He received his doctorate from Delft University of Technology on Wednesday, 27 March.
For three years, Vuik carried out continuous measurements on wave height across mudflats (vegetated outer dike land accretions) in the Western Scheldt estuary and salt marshes in the Wadden Sea. He established that the wave load of waves crashing against dikes with 300-metre wide foreshores during heavy storms is over 50% lower.
Effect of vegetation limited
It is primarily the higher elevation of sediment of these foreshores that makes the difference. Any vegetation only has an indirect effect. 'Planting vegetation on these foreshores is often seen as a promising form of dike reinforcement,' suggests Vuik. 'The idea is that the water first has to wrestle its way through dense vegetation before reaching the dike with much less force.'
But in practice, it turns out that most of the vegetation has little effect on heavy waves. It does, however, ensure that the foreshores better retain sediment, causing them to be elevated over time, which contributes to dike protection. This protective effect is already measurable on foreshores that are just a few dozen metres wide.
Reviewing dike raising proposals
According to Vuik, his findings could lead to reviews of planned dike raising and reinforcing projects. 'If there are foreshores already, it would be a shame not to include them in calculations for dike raising projects,' says the researcher.
If there are no foreshores, it may well be worth creating them. That's already happening at the Houtrib dike (pictured above) between Enkhuizen and Lelystad.
Building with nature
The district water boards and Rijkswaterstaat are very interested in the results of Vuik’s research, which fits in well with the wider trend of building with nature. Vuik: 'We don't need to regard nature as our enemy. As civil engineers, we should be making more positive use of the forces and properties of nature.'
But Vuik is not yet willing to conclude that the creation of a wider foreshore along the Markermeer dike would provide a solution. There has already been a lot of discussion about this dike reinforcing project (read in Dutch): Ophef over Markermeerdijken).
Photo: Rijkswaterstaat
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