UT PhD candidate develops new facility for dike erosion research

Tuesday 29 September 2026

How long does a dike remain safe when waves overtop it and wash away its protective grass cover? University of Twente PhD candidate Niels van der Vegt will investigate this question using a new indoor Wave Overtopping Simulator (iWOS). Over the past year, Van der Vegt has been responsible for the detailed design of this unique research facility. Built in the Hydraulic Engineering Laboratory in Delft, the simulator makes it possible to study on a large scale and under controlled conditions how overtopping waves damage dikes.

During high water, waves overtopping a dike can tear away large sections of grass cover and eventually erode the underlying clay layer. Exactly how this erosion process develops is not yet sufficiently understood. This makes it difficult to accurately predict how long a damaged dike will remain safe.

Niels van der Vegt, a PhD candidate in the Marine and Fluvial Systems cluster within the Department of Civil Engineering and Management at the University of Twente, aims to change that. The development of the new simulator is an important part of his PhD research. Using the iWOS, he can measure the forces and flow patterns of overtopping water in detail and then link these measurements to the damage caused to a dike.

From field tests to the laboratory

Until now, tests involving overtopping waves have often been carried out outdoors on existing dikes. A large container filled with water is placed on top of the dike. When a gate is opened, a large volume of water rushes down the slope.

These field tests closely resemble real-life conditions, but they have one major drawback: it is almost impossible to see exactly what is happening inside the turbulent, churning mass of water. After many waves, the damage to the grass and clay becomes visible, but the internal structure of the flow and the forces causing the erosion are difficult to measure.

The idea for an indoor version came from Jord Warmink and Suzanne Hulscher (both University of Twente) and Bas Hofland (Delft University of Technology). Over the past year, Van der Vegt worked on the simulator’s detailed design. The result is a research environment that combines the scale of outdoor experiments with the control and measurement capabilities of a laboratory.

2,500 litres of water

The iWOS has been ready for use since September 2026. Its water reservoir is positioned close to the ceiling and reaches a maximum height of eight metres. It can hold 2,500 litres of water. During an experiment, the water flows down an artificial dike at speeds of up to ten metres per second.

The toe of the dike is made of glass, allowing researchers to measure what happens even within the large mass of foaming water. High-speed cameras, ultrasound, aeration sensors and pressure sensors are used to map the water’s velocity, turbulence and forces.

What happens once the grass is gone?

Van der Vegt is the first researcher to use the new facility. He will investigate flow patterns and hydraulic loads inside an erosion hole. His research focuses specifically on what happens after the dike’s strong grass cover has been washed away and the underlying clay layer is exposed.

A better understanding of how this clay layer erodes could help determine more accurately how much damage a dike can actually withstand. Grass-covered dikes may prove to be stronger than currently assumed in safety assessments. If this can be demonstrated, some dike reinforcement projects could potentially be postponed or carried out differently, while maintenance could be organised more efficiently.

Hofland also sees this as one of the main benefits of the new facility: “With these new data, we can link water flow to erosion. How long can a dike still withstand the water once the grass has been washed away? When does the dike fail? The research now getting underway will help us better assess how well our dikes continue to protect us during high water.”

Broader research into dike safety

Following Van der Vegt’s initial research, the iWOS can be used for a wide range of studies into dike safety. Researchers will be able to test different types of dike revetments and dike geometries, investigate transitions between materials, and test clay samples taken from existing dikes.

The iWOS facility was made possible within the Future FRM tech project, funded through the Dutch Research Council (NWO) Perspectief programme, together with STOWA, Rijkswaterstaat, the University of Twente and Delft University of Technology. The development of the facility forms part of Niels van der Vegt’s PhD research at the University of Twente. His research is supervised by researchers from the University of Twente, Delft University of Technology and Deltares. The facility was constructed with support from TU Delft DEMO and the laboratory technicians at the TU Delft Hydraulic Engineering Laboratory.

(Text: TU Delft and University of Twente)