Wave and current data bolster nature-based Dutch flood defense Houtribdijk

How can waves, current and artificial sandbanks, in a fresh water system without tydal effects, better protect dykes?

Artificial sandbanks are being used to protect a dike (levee) in a non-tidal freshwater system, as part of a ground-breaking project the Netherlands. Nortek was asked to supply instruments capable of providing the highly accurate wave and current data required to ensure successful implementation and was also on hand to install the data telemetry system.

The dikes that protect the Netherlands from the impact of extreme weather in the North Sea, and flooding in river systems have taken a pummeling over the years. One of them, the 25 km-long Houtribdijk, had deteriorated to such an extent that it needed major repairs.

The established method of making it more secure would have been to bolster the barrier of rocks and other building materials already protecting the dike from the impact of water during storms. But the authorities opted for a different approach to maintain the dike, which separates two large artificial lakes, the IJsselmeer and Markermeer.

Taking inspiration from nature

Instead, they tried a more nature-based solution, by creating two 140 m wide artificial beaches on either side of the dike. The sloping contours of these beaches not only improve protection against waves and the elements, but also create a valuable habitat for coastal flora and fauna, and a new area for recreational activities.

Around 10 million m³ of sand needed to be deposited around the dike – equivalent to the amount of sand used to maintain the entire seashore of the Netherlands from erosion in a year. Following the reinforcement, the dike should be able to withstand storms up to an intensity likely to happen only once in every 10,000 years.

Extensive testing for unique application

The project is unique because, although such sand-based reinforcement has been used in coastal defenses against seawater, it has not been used before in non-tidal freshwater conditions.

Specialists from the EcoShape knowledge consortium initially conducted extensive research in a test area to see how the sand behaved.

The results of the test were used for techniques to realize the full-scale application. Henk Meuldijk is the Reinforcement Project Manager at Rijkswaterstaat.

He says: “It is really good that measurements were taken at the pilotsite for a number of years. We can see that the shape of the sand mass is relatively stable after two years. We have used this knowledge for the construction of the reinforcement as well as for our management and maintenance plan”.

Rijkswaterstaat forms part of the Dutch Ministry of Infrastructure and Water Management, handling the design, construction, management and maintenance of infrastructure facilities in the Netherlands. A research and monitoring program now running for the full, scaled-up solution is designed to ensure experience gained so far regarding the reinforcement of the Houtribdijk can be applied in the future. This program is being led by Rijkswaterstaat in collaboration with Delft University of Technology.

“The knowledge that we are gaining here will be very useful in the Netherlands and beyond,” Meuldijk says.

Video: Take a closer look at this video at how the dynamic behavior of sand in the Houtribdijk reinforcement is being monitored.

On-site monitoring needed for complex conditions

Desktop modeling could only help to a limited extent on the project, given the many forces at work and the complexities of the feedback between them. If the sand started shifting on test sections of the beach or holes started appearing in it, then the researchers needed to know exactly what forces were at play.

Detailed monitoring of the interaction between the water and sand was essential to ensure the defenses were fit for purpose. This is where Nortek’s expertise in current profiling and wave measurement came in.

“With traditional coastal protection works, you only deal with physical problems, such as currents, waves and rock forces. But with these nature-based solutions, you need a much greater degree of monitoring, because the outcomes are much harder to predict and you need to take into account ecological elements like flora and fauna,” says Sicco Kamminga, Managing Director at Nortek’s subsidiary in the Netherlands.

High precision, low power consumption solutions

Nortek provided Vector-stroommeters and Aquadopp-stroomprofilers, the latter operating in high-resolution mode for higher-precision measurements.

“Nortek was selected by means of a public tender procedure by the Dutch authorities, and was finally chosen because they offered the best quality for an acceptable price. Nortek suggested various additional elements of the total setup that led to a more robust and reliable monitoring system”, says Rinse Wilmink, Project Manager and Advisor on coastal flood risk management and morphology at Rijkswaterstaat.

Accuracy was of course vital, but at the same time energy consumption needed to be low, given that the instrument arrays were mounted on small platforms in shallow water, powered only by small solar panels.

“Low energy consumption was a key requirement, and our instruments consume very little power, even when working continuously”, says Rikke van der Grinten, Sales Engineer at Nortek.

Nortek was also asked to devise the telemetry system for data transfer from the instruments. Creating the capability to reliably transfer and present up to 1 GB a day of data was challenging, both technologically and in terms of the time available.

The client required Nortek to work with diverse instruments from multiple manufacturers. All of these instruments had to be integrated into one functioning system. This was a complex task. It even made it more challenging because Nortek’s engineers needed to get the job done in just two months, including the Christmas of 2018.

“Carrying out a project of this complexity in such a short timeframe was a big challenge, but two months after we took on the work, the systems were working”, says van der Grinten.

Nortek continues to support dike maintenance

The monitoring program has proven invaluable in determining how to maintain the dike more efficiently. These results show the degree to which the sandbanks will need to be maintained in coming years. The dike reinforcements – a mixture of the sandbanks, stone and asphalt – were completed in June 2020.