First results from study of sand erosion under the Frisian Wadden Sea coast are positive

Published: 8 October 2020

Deltares has conducted a study with Wetterskip Fryslân, the Frisian water authority, and Fugro of the piping failure mechanism (sand erosion below dikes) on the Frisian Wadden Sea coast. 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).

Deltares teamed up with Wetterskip Fryslân and Fugro to study the strength of the tidal sand on the Frisian Wadden Sea coast and to see whether ‘piping’ was a possible threat. 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.

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.

Better assessment of piping in tidal sand

The results will help to make better assessments of piping in a third of the dikes in the Netherlands, including the Wadden Sea dikes. That means the results can be applied directly to the Koehool-Lauwersmeer dike upgrade project, which is currently in the exploratory phase.

National Flood Risk Management Programme

The study is part of the Dutch Flood Risk Management Programme in which the national government and the water authorities collaborate to protect the Netherlands against floods.