Wave attenuating willow forest – Noordwaard

Lessons Learned

A wave-reducing eco-dike leads to a sufficient wave reduction to lower dike height by 0.5 m or more according to the models. The width of the willow forest needed however is quite substantial, meaning that such a forest cannot be implemented everywhere due to lack of space. Furthermore, it needs to fit into existing plans for nature- and landscape development. Possibly adaptations could be made to optimizeĀ the dike, for example through the manner in which the trees are planted or through management. The research by Dekker and de Vries (2009) demonstrated that the main wave reduction is already realizedĀ in the first 20 m of the forest, which means that future designs could possibly have smaller dimensions. A point of attention is how storm proof willow trees are, which would be even more important in cases where the design contains no over dimensioning

he added value of the project lies in the innovative way of making use of ecosystem services (wave reduction) and multi functional use of space. It is the first time that vegetation is an integral part of a dike and that such a dike can be tested for safety according to an approved method.
For Water Boards this project offers a chance to look for multifunctional designs. Furthermore, it offers possibilities to include vegetation that is already present in front of a dike into safety testing. Existing vegetation could perhaps reduce wave height sufficiently to lead to a lower incoming wave. Therefore, a dike that would normally have to be raised, could still meet safety demands if vegetation is included. Such a design would contribute to goals for both nature development as safety.

The following lessons can be learned from this project:

  • This type of solution needs more space than a traditional solution, so it might not be feasible everywhere.
  • The first part of the forest, the part where the waves come in, has the strongest wave-reducing effect.
  • Limited field data is available of wave height reduction by a forest. The design is based on downscaled laboratory experiments in combination with modeling. Monitoring the health and growth of the trees and the effectiveness of the forest in wave reduction is important.
  • The design is focused on wave reduction but other positive relations to dike stability may exist and have to be studied in more detail.
  • The design process required the engagement of a group of experts from various disciplines to enable a sound combination of biological and engineering aspects.
  • The design process was dependent on close cooperation of various parties, driven by the urgent need to deliver an acceptable product that was supported by all, within a short time frame.
  • Legal issues have to be tackled. Testing protocols for dike safety including procedures to allow innovations have to be designed, proofed and constructed. The first step is already made in this project, where a safety testing scheme for the willow forest as integral part of the dike was set up.
  • An innovative dike design on the basis of eco-engineering using vegetation as a wave-reducing component is possible, practical and safe.
  • The design can be considered as a breakthrough in dike design. The willow forest has become integral part of the dike. As this is the first time, monitoring and research are needed to evaluate the effectiveness and to enable integration in future test protocols.