Studying erosion under the Frisian Wadden Sea coast

Deltares has just revealed the first results from the study of sand erosion under the Frisian Wadden Sea coast.

The company is conducting this study with Wetterskip Fryslân, the Frisian water authority, and Fugro. The study is about the piping failure mechanism (sand erosion below dikes) on the Frisian Wadden Sea coast.

Piping is the formation of channels under a dike caused by seepage water when water levels are high. Previous studies have shown that tidal sand is less susceptible to piping than river sand.

According to Deltares, the first results are positive and they may help to reduce the scale of the Koehool-Lauwersmeer dike upgrade operation. The research is part of the Dutch Flood Risk Management Programme (HWBP).

Tidal sand at least twice as strong

The study was conducted on a natural deposit, which makes the trial unique. The approach consisted of raising the water pressure in the sand layer in stages.

The effect on the sand layer was then monitored in each stage. The first results are very promising. No piping was observed during the trial. The conclusion is that tidal sand is at least twice less susceptible to piping than river sand.

Looking ahead

In the coming months, Deltares and Fugro will continue to work on the study to determine the strength of tidal sand. Data from the trial will be analysed in detail and laboratory tests will be conducted.

A larger practical trial will then be launched in the Hedwige polder in Zeeland early next year.

The results will help to make better assessments of piping in a third of the dikes in the Netherlands, including the Wadden Sea dikes.

Photo: Deltares