Study proofs how nature can play a significant role in flood protection
A lot of claims are made worldwide about how nature can help to provide protection against flooding, typhoons and tsunamis. A paper published early February 2022 in Scientific Reports by scientists from Deltares, the Netherlands Institute for Marine Research (NIOZ) and Delft University of Technology, provides the first evidence about how we can best use trees for flood risk management.
Until now there have been few studies looking at how natural systems behave in extreme conditions like super storms. How well do trees break waves, and can they cope?
Deltares, Delft University of Technology and NIOZ conducted a trial in the Delta Flume in 2018 to see how a willow forest can attenuate waves. The main conclusions of their research called 'Wave attenuation through forests under extreme conditions' are that the trees can do an excellent job of attenuating waves up to wave heights of 2.5 metres. Without breaking. Trees are much more flexible than previously thought and bend well in response to the waves.
The Delta Flume is a 300 meter long man-made flume, located at Deltares Research Institute, with a wave generator that is capable of producing waves as tall as five meters.
Branches have more effect
With very high (>2.5 meters) and longer waves, however, the attenuation capacity of the trees lessens. The trees have most effect at medium-high water levels, when the wave passes through the middle of the canopy and therefore meets the largest tree surface.
Lead author and scientific director and coastal expert at Deltares Bregje van Wesenbeeck: “On the basis of this trial, we have acquired new insights into the factors that are important in predicting the attenuation properties of vegetation. For example, we now know that the leaves of willows have little effect, but that side branches actually have more effect than we always assumed”.
These properties are usually not included in the computer models used to estimate wave attenuation by vegetation.
Hard and soft solutions
The aim of the study is to establish a clear picture of the functions of natural systems to design better solutions for different coastlines and river areas around the world. Combinations of natural landscapes and hard defences are promising, according to the researchers, while they can make many places in the world a lot safer.
Van Wesenbeeck: ‘You can’t always just go around building dikes. Particularly in the context of climate change and sea level rise, we have to learn to work better with the natural systems that are already in place. This study is an important step towards establishing design rules for innovative approaches. For example, we can now make much better calculations of how long a forest in front of a dike should be to provide the desired wave attenuation.”
Tjeerd Bouma, a researcher with NIOZ, highlights how this research can contribute to additional challenges. “The further optimisation of the design rules in order to maximise not only protection against flooding but also biodiversity is an important challenge that we are addressing in our current field research.
Tropical regions
The study of willow trees in the Delta Flume was conducted and co-financed by Deltares, Delft University of technology, NIOZ, Boskalis, Van Oord, Rijkswaterstaat, World Wildlife Fund, VP Delta, TKI Delta Technology and the Netherlands Organisation for Scientific Research (NWO). The parties are preparing for follow-up research in the Delta Flume with mangrove trees, which are found on coasts in tropical regions. The mangroves are being grown now in a greenhouse at Deltares.
