Nature-based solutions: Dynamic dikes made from silt

The Netherlands, as a low-lying country, has a long history of flood protection. Dike improvement has always been high on the agenda. Current developments, such as sea-level rise, underline the need for such improvements.

Dikes can be reinforced and raised with concrete and asphalt, but also with clay and silt, as our ancestors have done for centuries. However, are such nature-based solutions sufficient to meet the accepted safety standards? And if so, how? Wageningen Marine Research is involved in various pilot experiments to investigate these questions.

Local silt

“Working with silt and clay has many advantages”, says Marinka van Puijenbroek, landscape and plant ecologist at Wageningen Marine Research. “It allows for the use of local materials, in this case from the port of Delfzijl or Lauwersoog. This eliminates the need to bring in hundreds of tons of material from elsewhere. In addition, harbours must be dredged regularly to prevent it from becoming too turbid and shallow. If you then use that sediment for dike reinforcement, you can kill two birds with one stone.”

The dredged sludge must first ‘ripen’ in a dedicated facility. This is an area where the sludge settles and the excess moisture evaporates, leaving a clay-like material that is suitable for strengthening dikes. “We are investigating to what extent plant growth can promote this ripening process”, says Van Puijenbroek. “Plant growth changes the soil in a way that accelerates the process. The clay from the ripening facility is already being used in the Wide Green Dike pilot project, near Delfzijl.”

Plant growth changes the soil in a way that accelerates the ripening process. This makes it more suitable for strenghtening dikes.
Marinka van Puijenbroek

Self-enforcing system

Another project is the so-called Growing Dike, which started in May 2023. “Usually, dike improvements involve the use of large quantities of clay in one huge effort, using large equipment”, says Van Puijenbroek. “Afterwards, the vegetation has to be reseeded. The entire process is time-consuming and leads to significant CO2 emissions. We are investigating an alternative approach: applying a thinner layer of silt more frequently, each time spraying a few centimetres of silt onto the dike. This would allow the vegetation to persist, and simply grow through the silt layer.”

We apply a thinner layer of silt more frequently. This allows the vegetation to persist, and simply grow through the silt layer.
Marinka van Puijenbroek

There are still many questions that Wageningen Marine Research aims to address. For example, which plant species to use, how much sludge to apply in order not to suffocate the plants, and whether the salt content of the sludge is a problem for the vegetation. “Other parties are investigating the flood risk management aspects”, says the researcher, “including Deltares, Witteveen+Bos and a number of Dutch water authorities. We have the impression that such a dike is not necessarily unstable. This is because the root cover of the plants grows upwards, creating a self-enforcing system.”