The dikes of the future will be more than just green turf

Rising sea levels, more frequent extreme precipitation and drier summers will put increasing demands on our dikes in the future. Over the next thirty years, 1,500 kilometres of dikes will need to be strengthened in the Netherlands alone. Ecologists from Radboud University and Wageningen University & Research claim that an increase in the number of flowers and herbs that are grown on dikes will already go a long way towards solving the problem.

In most cases, dikes are primarily strengthened by using extra soil to make them higher or wider. But the researchers believe that this could also be done more efficiently. Allowing more flowers and forb to grow on our dikes would have quite a beneficial effect. The dikes would not only become sturdier due to deeper rooting, but insects would also benefit from the broader plant diversity.

The Dutch dikes of today are dominated by green turf with relatively few herbs. Grassroots do not usually penetrate the soil as deeply, which means that the soil is not as well bound, while herbs often tend to have deeper roots. In addition, these grassy surfaces are not very resistant to increasingly prolonged periods of drought and, as a consequence, this also leads to less sturdy dikes. This calls for new and different types of vegetation for our future dikes.

Re-enacting a flood

"Over the past year, our ’ Future Dikes ’ research project has primarily focused on the current state of affairs," says Nils van Rooijen, who is a researcher at Wageningen University. "Where are most of these species-rich types of vegetation found on dikes, how is the soil structured in these dikes and how strong are the dikes themselves?"

In the forthcoming weeks, Van Rooijen and his colleagues will be investigating how flood-resistant different types of floral dikes in the Nijmegen area are. For this purpose, vast amounts of water will be poured over the crest of the dike until a hole forms in the topsoil. "We’ll be looking at the differences between the vegetation, soil composition and root growth in order to find out how erosion-resistant our biologically diverse dikes are," says Van Rooijen.

The perfect blend

Small dikes can also be found on the Radboud campus. PhD candidate Steven Huls uses these in his research as he searches to find the plant species that will form the best blend for a strong and resilient dike. Good root growth is therefore essential for stronger soil binding and drought resistance.

"We’re adding more and more types of grasses and flowers to the standard composition so that we can study the effect on root growth and proportions," says Huls. "We’ve selected the plant species on the basis of species that are commonly found on Dutch river dikes."

In spring 2023, the researchers will be publishing a report with their initial findings. Van Rooijen: "We combine all the data that we’ve collected from the trials that we’ve carried out. These are then added to a model that is used to help us deduce how strong floral dikes are. In the next few years, we’ll be taking a closer look at how these dikes should be managed. This means that the main question that we need to answer is: How can we make our strong dikes as biologically diverse as possible?"