The University of Twente, in partnership with Wetterskip Fryslân and the knowledge institute Deltares, is researching the effect of saltmarsh grass and foreland on the energy of the waves that reach the dyke. This may make it possible to reduce the space required by dykes. Last month, the water authority has been removing quantities of substrate from the saltmarsh beyond the dyke to the east of the village of Peazens-Moddergat. The substrate is being taken to the Delta Flume in Delft for research. This innovation research is being funded by the National Flood Protection Programme (known as the HWBP).
The area covered by the Wetterskip Fryslân water authority includes many dykes with foreland in the form of saltmarshes. A saltmarsh is a strip of land beyond the dyke which borders the sea directly. Saltmarshes that are overgrown with sea couch, a type of blue-grey grass, make an important contribution to the ability of the dyke to hold back the water. The Delta Flume simulates conditions in a superstorm, recreating extreme waves for instance, and this enables the wave-neutralizing effect of the saltmarsh vegetation to be measured. UT Researcher Bas Borsje predicts that in addition to the height of the saltmarsh, the vegetation also contributes to the reduction of wave stress on the dyke. As well as this dampening effect, saltmarsh grass also retains silt during high tides, so that it keeps pace with rising sea levels. The grass is expected to reduce the force of the waves that reach the dykes.
If the research shows that the saltmarsh grass does indeed have a dampening effect on the waves, then dykes could be redesigned to make them take up less space and work more effectively in the long term. Wetterskip Fryslân and other water boards will incorporate the research findings into their designs and dyke reinforcement work in saltmarsh areas.
Substrate extraction is currently in full swing in the saltmarshes at Peazens-Moddergat, with a possible extension into October. A total of approximately 270 square metres of substrate has been removed. That amounts to 68 bins each measuring 2 by 2 metres and 40 to 70 centimetres deep. The bins are being transported to the Delta Flume of Deltares in Delft by truck on working days (1 to 2 shipments per day). The wave tests will be carried out in the Delta Flume at the end of this year.
Rijkswaterstaat and the water boards are undertaking the largest dyke reinforcement operation ever. More than 1,500 kilometres of dykes and 500 locks and pumping stations need to be tackled by 2050. This can only be achieved with a fresh approach to dykes and dyke reinforcement. The work needs to be carried out better, more quickly and more efficiently. It is part of the National Flood Protection Programme (HWBP), a dyke reinforcement programme to which the Netherlands is allocating €13.7 billion.
Photo caption: The wave tests are taking place in the Deltares Delta Flume in Delft.