On Tuesday, 18 December, the ‘Green Shield’ project was launched in the Biesbosch nature reserve. The project will further the understanding of how forebanks with vegetation – such as willow trees – can break waves. It is known that vegetation can mitigate wave impact.
There are models that have been validated in the Delft wave flume, but they have not yet been assessed in the field. The current work in the Biesbosch will remedy that. All the data will be entered in a model that can be used to calculate precisely how forebanks can break waves.
Under supervision from Deltares, students from the Delta Academy of Hogeschool Zeeland and the Delft University of Technology are collecting data. The vegetation near the Kop van het Land dike near Dordrecht is being mapped out in different ways: from good old manual approaches with tape measures and slide rules to innovative techniques. A handheld GPS is also being used to measure the height of the area to within a centimetre. In January, an unmanned plane from Fugro will use an infrared camera to determine the current height profile of the river and the forebank. Finally, sensors will be installed to measure the wave impact and the river flow. The sensors can be read remotely.
Wouter van der Star is the Deltares project leader working on the ‘Green Shield’. ‘Knotted willows produce robust branches that can absorb a considerable proportion of the wave impact. A green shield of this kind can make a major contribution to flood protection and dike preservation,’ he explains. ‘We know willows can be effective breakwaters and help to prevent the internal erosion of the dikes (piping), but we now want to make precise calculations for dike managers such as Rijkswaterstaat and the water authorities. Even small woods can easily cut back a metre-high wave by half. But we will be obtaining more precise information. And that is good news for the dike managers because it means that dike upgrades or can be postponed, or even cancelled altogether, in some places. Traditional dike upgrades cost between three and five million euros per kilometre. So this project could lead to enormous savings.’
The trial with the Green Shield near Dordrecht is a joint venture with the research project Building with Nature and Flood Control 2015. Arcadis and Fugro are also involved with this study.
For the follow-up, an application has already been submitted for an EFRO subsidy through Ecoshape. It will also look at how green forebanks can be used to maximise the effect on river flows, dike protection, ecology and recreation.
Press Release, December 27, 2012