Flowery dike proves its sturdiness

One after another wave of water thunders down the dike. A test setup has been built at the marina of Wijchen, a village near Nijmegen, to test the strength of the dike. “We simulate a storm with waves of two meters that crash over the dike,” says Nils van Rooijen, an ecologist at Wageningen University.

It is a flowery dike that lies here. Now you mainly see a lot of grasses. “It is relatively monotonous green,” says Van Rooijen. Later in the year, dozens of herbs will set up and it will be a sea of ​​​​colors. “Knotweed, daisy, buttercup tuber, yarrow,” Van Rooijen cites as examples.

The idea is that such a flowery dike is stronger than a dike with a standard grass mixture, of which the Netherlands has quite a few. A standard grass mixture, mainly containing perennial ryegrass, roots 5 to 10 centimeters deep, explains Van Rooijen. But with dozens of types of grasses and herbs, a much more extensive and much deeper root system is created. It holds the clay and sand of the dike together much better. This makes the dike more resistant to waves and erosion.

And it benefits biodiversity. Van Rooijen: “On this dike you will find no less than 55 different types of plants on a piece of five by five meters.” In the background another wave of water gushes over the dike.

The trial is part of the program Future Dickes, which wants to determine the strength of flowery dikes. The project is coordinated by Radboud University in Nijmegen. In addition to Wageningen University, the Rivierenland Water Board and the Flood Protection Program of Rijkswaterstaat and the water boards are also participating. “The Netherlands has 17,000 kilometers of dikes,” says Van Rooijen. He hopes that flowery dikes will soon be installed in many more places. It could be a wonderful migratory route for insects, such as butterflies.

Storm Simulator

“We have now been working at this location for four days,” says Diederik Bijvoet, technology & innovation advisor at the Flood Protection Programme. He explains the design of the test. There is a storm stimulator on the dike, a device that has been specially developed to simulate a storm. Water is pumped up from the marina. It is poured out in waves over the inner slope, over a 4-metre-wide section that is lined with bulkheads. During the past few days, the amount of poured water has slowly increased.

At the bottom of the dike, the water hits a transverse wooden bulkhead to slow it down. Then it flows into a ditch. Bijvoet: “We want to determine how much the dike can still handle.”

He also says that flowery dikes are in principle stronger than grass dikes. “But you want to have it proven.” Moreover, says Bijvoet, there are many differences between dikes. The ratio of clay and sand can vary. The mixture of grasses and herbs that grow on it can vary. The plan is to develop protocols for the design and management of the different types of dikes.

Culture issue

The idea that flowery dikes are stronger than grass dikes is not new. Hans Sprangers, now a retired ecologist, already demonstrated this in his dissertation for which he obtained his PhD in 1999 at Wageningen University. “We then gave courses at the water boards to recognize herbs and grasses,” he says when asked. “And to remove a clod of soil from the dike and estimate the root penetration by eye.”

But the flowery dike never got a foothold after that. “I think Rijkswaterstaat found it too complicated,” says Sprangers. The idea was also that the maintenance of a flowery dike would be more expensive. It is also a cultural issue, says Sprangers. “It has long been an ideal to have a tight billiard cloth on your dike.”

Van Rooijen recognizes this. “In the past, every herb was sprayed away.” That picture is changing. “It’s a very different time.” The decline in biodiversity is receiving more attention. Just like global warming, and the question of whether our dikes are strong enough.

The same test was carried out five hundred meters further down the dike last week, on two strips of the dike. One strip is undamaged, “after 25 million liters of water that has been washed over it,” says Van Rooijen. In the other test strip, ‘the sod has been rolled up from the dike in a V-shape. There is damage, but no failure.”

Extensive root systems

Van Rooijen was surprised how well the dike has held up here. He drops to his knees and points to the stripped piece. You not only see a mix of clay and sand, but also extensive root systems. Van Rooijen distinguishes all kinds of carrots, thick, thin. “This taproot is of yarrow.”

We walk back to the storm simulator. The water will continue to slosh over the dike here for the next few days. At the end of this week, all equipment will move about five meters, and the next strip will be set up and tested. The experiments will then be over, but all the analyzes will then begin, says Van Rooijen. “And in May everything here is in bloom again.”