In the spring of 2024, Researchers from the Department of Civil Engineering of the University of Twente investigated how salt marsh vegetation affects wave impacts on a Wadden Sea dike in the Delta Flume of Deltares. The tests showed that the wave height decreased by 5 per cent during the peak of a storm. These results will result in considerable cost savings on future reinforcements of sea dikes with salt marshes.
The wave attenuation effect of the salt marsh vegetation was measured by simulating a superstorm in the Delta Flume. Blocks of soil with salt marsh vegetation were removed from the salt marsh outside the dike near the Frisian village of Peazens-Moddergat, an area owned by It Fryske Gea. The blocks were transported to Delft and placed in the Delta Flume. The Delta flume is a 300-metre-long research facility in which the world’s largest waves can be generated. This allows the tests to be conducted at full scale.
The results
The tests showed that salt marshes could withstand even the most extreme storms, while the toe of the dike remained stable. During a superstorm, the wave impact was 1.90 meters on a salt marsh without vegetation and 1.79 meters on a vegetated salt marsh. The salt marsh vegetation reduced the wave height on the dike by more than 5 per cent. Besides wave attenuation, salt marsh vegetation also retains silt at high tide, enabling the salt marsh to grow along with sea level rise.
The results benefit future dike improvements. In some cases, for example, the stone revetments at the toe of the dike do not have to be replaced thanks to the natural stability of the salt marsh. In addition, less clay is needed for dike reinforcements when a salt marsh is present while upholding the strict safety requirements.
Cooperation
This research was carried out by the Living Dikes Research programme, led by the University of Twente, in collaboration with knowledge institute Deltares, TU Delft, NIOZ and Wetterkip Fryslân. The experiments were funded by the Flood Protection Programme (HWBP). The parties provided guidelines for application to Wetterskip Fryslân and HWBP, which can support reinforcement projects along the Zeeland coast and the Wadden coast.
Flood protection programme
This innovation research is funded by the Flood Protection Programme (HWBP). Rijkswaterstaat and the water boards are facing the largest dike reinforcement operation ever. More than 2000 kilometres of dikes and 400 sluices and pumping stations must be upgraded by 2050. This can only be achieved by a new view on the Dutch dike and dike reinforcements. They should be faster, better and more efficient. This is done in the Flood Protection Programme (HWBP), a reinforcement programme for which the Netherlands is allocating 13.7 billion euros.
More information about the study of Civil Engineering.
