Mangroves for coastal protection
Experts from Deltares and Delft University of Technology, both Dutch research institutes, gradually increase the force on the mangrove trees to test how strong these are under extreme wave conditions and how they can help protect coastlines.
Mangroves play an important role in the protection of tropical coastal areas. Their large roots retain a lot of sediment and reduce wave actions. “Many claims are made about mangroves that provide increased flood safety. But hard proof is scarce. If we really want to include nature's protective role in our flood protection, measurements at full scale are indispensable. The scientific world is eagerly awaiting this," says Bas Hofland, hydraulic engineer at Delft University of Technology (TU Delft) and project leader of the 'WOODY' project.
Bregje van Wesenbeeck, ecologist and expert Building with Nature at Deltares and also working at TU Delft, has been studying the behaviour of vegetation under extreme hydraulic conditions for years. "Models that predict how trees behave in water are based on static sticks. However, under stormy conditions with high waves, trees bend." The wave attenuation may be different from what is thought, as trees move with the waves. To make the experiment happen, Van Wesenbeeck spent years cultivating tropical mangrove trees in a greenhouse at Deltares. These self-cultivated trees are now being used to study the real trees’ behaviour on a true scale.
Monitoring waves
Deltares’ Delta Flume, the world's most powerful wave flume, is being used to study how much wave power mangroves can absorb. The trees receive an ever-increasing amount of water and the waves swell. By monitoring speed and wave height, researchers keep a close eye on how the waves are attenuated.
Damping of waves by trees
They measure the load on the trees and the trees' movements. This allows them to estimate the wave attenuation. To this end, Hofland designed a new very cost-effective measurement method. "Instead of taking the whole tree out of context and hanging it in a system with external force gauges, we now use the tree itself. We measure the bending of the trunk directly, and with dedicated sensors we can determine the force on the tree." This method has already been used on trees on land. It is still exiting to see whether the sensors stay in place under the force of up to three-metre-high waves.
The WOODY project
The WOODY project, funded by NWO, started in 2020. The project investigates how trees such as willows and mangroves can reduce flood risk. Coastal trees reduce the impact of waves on dykes and increase local biodiversity.
