R&D highlights edition 2019

Flood risk 11 R&D Highlights 2019 W orldwide, organisations are promoting the protective power of ecosystems to reduce flood risks caused by hurricanes and storms. However, few actual tests have showcased the power of trees to provide protection from floods in extreme conditions. In Europe, willow forests are a typical type of vegetation on flood plains. They damp wind-induced waves, reducing the loads on embankments and dikes. Improving the quantitative data relating to this ecosystem service may not only reduce the cost of European river flood defence systems but also improve the conservation of floodplain and coastal vegetation, and enhance the natural values of these landscapes. The aim of this research is therefore to improve the predictability of wave dissipation by vegetation in extreme conditions and to work towards design criteria for vegetated foreshore and dike combinations. To that end, Deltares and its partners installed a 40-metre-long forest of 32 willow trees in the Delta Flume. These experiments evaluated how willow vegetation contributes to wave height reduction and wave run-up in varying conditions and with different vegetation characteristics. Tests were conducted with different water levels, wave heights, branch densities and in summer/winter conditions (with and without leaves). On average, wave damping was found to increase with increasing wave height and increasing tree biomass. Willows also proved to be quite resistant to storms with average wave heights of 1.5 metres and the breaking and removal of branches was very limited. The information collected will be used to enhance the reliability of numerical wave models. The Woods versus Waves consortium consists of a wide range of participants from industry, government, research institutes and NGOs. The partners at present are Deltares, Van Oord, Boskalis, Rijkswaterstaat, World Wide Fund for Nature, STOWA, NIOZ and Delft University of Technology. Additional experiments are planned for 2020 to look at how mangrove trees perform in extreme wave conditions. New partners are welcome to join the consortium for these tests. Contact: Bregje van Wesenbeeck, Bregje.vanWesenbeeck@deltares.nl , t +31 (0)6 5159 4127 Wiebe de Boer, Wiebe.deBoer@deltares.nl, t +31 (0)6 46911209 Guido Wolters, Guido.Wolters@deltares.nl, t +31 (0)6 31681567 Tests were conducted in the Delta Flume with a willow forest consisting of 32 trees to look at how trees can contribute to reducing wave height. The willows did not break and they reduced waves with heights of up to 1.5 metres significantly. The extent of damping depends largely on wave characteristics and on willow biomass. WOODS VERSUSWAVES: TESTING WILLOWS IN THE DELTA FLUME

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