How long can our clay dykes withstand severe storms with high waves? To make dyke reinforcement more effective, sustainable and efficient, the Brabantse Delta water authority, together with Deltares, Fugro and Arcadis, is investigating how quickly grass-covered clay dykes erode.
This series of tests is part of a long-term study into the erodibility of dykes, initiated by the Dutch Flood Protection Programme (HWBP) in collaboration with regional water authorities. The research was prompted by earlier experiments in the Deltares Delta Flume , which showed that the existing method for determining clay strength is not a reliable predictor of erosion caused by wave action.
This made it necessary to develop a new method to better predict erodibility , the rate at which clay in a dyke erodes due to waves. By determining the erodibility of clay in a dyke, numerical models can be used to calculate the strength of a dyke under severe storm conditions with extreme waves.
Marine clay tested
In the ongoing research programme, this method has already been developed for clay from the Wadden Sea region. To verify whether it also applies to marine clay from other parts of the Netherlands, which differs in composition, the Brabantse Delta water authority and HWBP launched a new series of tests this spring together with Deltares, Fugro and Arcadis .
For this purpose, clay blocks were extracted from a dyke near Fort Sabina, south of the Volkerak sluices. The clay blocks were then transported to the Delta Flume at the Deltares campus in Delft, where a dyke was reconstructed using these blocks.
This dyke is subjected to extreme wave conditions in order to measure erodibility. Clay from an actual dyke was used because it contains a well-developed grass cover and has been exposed to weather conditions (such as cracks caused by swelling and shrinkage) and animal activity—all of which influence the erodibility of the clay.
“In our Delta Flume, we can simulate the high waves of extreme storms", says Suzanna Zwanenburg, project leader Delta Flume research and expert in coastal engineering and waves at Deltares.
"In reality, such conditions are very rare, but our dykes must be designed to withstand them. The Delta Flume is therefore essential for understanding the erosion process of clay and grass under these extreme conditions. Based on what we observe and measure in the Delta Flume, we can further develop calculation methods for wave-induced clay erosion. These then form the basis for dyke design and assessment.”
Calculation method
In addition to the tests in the Delta Flume, Fugro is examining a wide range of easily measurable properties of this clay. Based on these measurements and the erodibility observed in the Delta Flume, the partners are developing a method to predict the erodibility of clay dykes using a limited number of simple tests on clay properties.
This method makes it possible to determine how thick the grass-covered clay layer on a dyke needs to be to withstand wave attack during an extreme storm. This creates opportunities to make dykes greener and to use locally sourced materials. Using clay from the surrounding area generally reduces both costs and CO₂ emissions.
“As Brabantse Delta water authority, we manage 132 kilometres of primary flood defences and 186 kilometres of regional defences", says Anco Sneep, portfolio holder for water safety, waterways and innovation at Brabantse Delta. "With sea levels rising, we face a major upgrade challenge. To reinforce our dykes as smartly, efficiently and sustainably as possible, we need to develop knowledge about clay. The experiments in the Delta Flume show how strong the clay really is, they bring the models to life, as it were. This builds confidence in the calculation method we are developing together. Insight into clay erodibility is therefore essential for strong dykes and a water-safe future.”
Wadden Sea dykes
Deltares previously carried out research with the Noorderzijlvest water authority and Wetterskip Fryslân, focusing specifically on the strength of the grass-covered upper section of the seaward slope of Wadden Sea dykes . Tests were also conducted in the Deltares Delta Flume and calculations were performed using a numerical model.
These enable predictions of how thick the clay layer on the upper slope of a dyke should be and the height of the transition from hard revetment to grass cover. The results show that a smaller section of the dyke needs to be reinforced with stone than previously expected.