Final test in Delta Flume for Wadden Sea Dikes study programme

Published: 29 November 2019

High waves battered the Nordic rock in the Deltares Delta Flume on the afternoon of Wednesday, 27 November. Deltares was testing the strength of the non-bonded revetment on behalf of the Wadden Sea Dikes study programme (POV-W). The same programme has looked previously at bonded Nordic rock and the effect of transition structures (from stone to asphalt, for example) on stability.

Several visitors from the Dutch Flood Risk Management Programme (HWBP), the POV-W, the water authorities and some private bodies watched the final series of tests in the Delta Flume.

Background

The Wadden Sea Dikes study programme is a collaboration between the Hunze and Aa’s, Noorderzijlvest and Wetterskip Fryslan water authorities. They share responsibility for the Wadden Sea dike, of which various sections have been assessed. In recent years, in the context of the study programme, they have conducted twelve different studies looking at other ways of strengthening the dike and learning more about different dike revetment options, including rock revetments. The results are being used for the various dike-upgrade projects that have been completed or that are planned for the years to come.

Conservative

The current assessment methods for rock revetments are considered conservative by the management authorities. So it is suspected that, in the past, the design requirements for the revetments have been excessive and that dikes have been failed unnecessarily. Considerable savings could be made by thoroughly researching how strong the revetments actually are. For example because dikes are less likely to be failed or because they will cost less money to build.

Testing in practice

In order to arrive at concrete and valid results, three dike structures were rebuilt in the Delta Flume and tested with waves. During the process, accurate measurements were made with sensors. The Delta Flume trial for bonded Nordic rock was carried out in 2016. The Delta Flume trial for non-bonded Nordic rock and the effect of transition structures took place in the final months of 2019.

Mark Klein Breteler, research leader at Deltares: ‘Visitors are always impressed by the power of the waves. Even these heavy rocks – which weigh 30-150 kilogrammes and are carefully positioned by a professional contractor – eventually weaken and get washed off the dike. The Delta Flume is the ultimate tool for testing innovations or, as now, measuring the wave height at which an old stone revetment gives way.’

Millions already saved

Accurate testing allows you to confirm or disprove suspicions. ‘That is why the use of the Delta Flume is so important in our research’, says Ate Wijnstra, the project manager of the Wadden Sea Dikes study programme. ‘That already produced beneficial results for us in 2016. At that time, we tested the bonded Nordic rock in the flume and it was clear that this revetment could be left in place on the section of dike between Eemshaven and Delfzijl. That meant a saving of 23 million euros’. It is expected that the studies of the transitional structures and non-bonded Nordic rock will also produce savings. In January 2020, all the measurement data will be analysed and the results can be shared.

Fast progress

The POV-W study programme is being financed by the Dutch Flood Risk Management Programme, which is an alliance of all the Dutch water authorities and Rijkswaterstaat. Sea level rise and higher flow volumes in the rivers mean that 1,100 kilometres of dikes will have to be upgraded in the coming years. Innovation is essential to build dikes faster, more economically and better.