Willow woods as breakwaters to improve flood protection

Published: 9 July 2015

A three-month ‘Building with nature’ study has been launched by Ecoshape, Deltares and Witteveen en Bos in the channel to the south of the Ridderkerkse Griend nature reserve in The Netherlands. The study will look at two types of willow woods - pollard willows and an osiery - to see whether it is possible to confirm what models and laboratory experiments have already indicated: can willow woods improve local flood risk management? In other words: are they good breakwaters? In late September, the willows will be taken out of the water and the results will be presented.

It is expected that this type of solution has the potential to make a genuine contribution, both in the Netherlands and in other countries, to improving flood protection.

Willow Trees in Ridderkerkse Griend

Willow woods as breakwaters to improve flood protection

‘Building with Nature’ assigns a central role to flood risk management against the backdrop of climate change, and to solutions that draw on nature. It is thought that using willow woods as breakwaters could mitigate the impact of the waves generated by shipping on the River Noord. The exact level of mitigation will be assessed: pressure sensors measure the height of the waves four times a second over the entire length of the section with willows. They also establish a picture of the extent to which the willows lower the waves, and therefore reduce the height and force of the wave impact, improving protection against floods.

Field research

Models and laboratory experiments have already shown the effect that willows have on waves but this is the first field study of willows. ‘We selected this location because the willows here can be located at some depth in the water. That means that we can take good measurements of how the willows work as breakwaters in extreme conditions, such as the submergence of the floodplains, when the water gets as far as the winter dike’, says Wouter van der Star, a project manager with Deltares.
The idea is that the results should produce guidelines. ‘On that basis, you can make calculations for other locations in the Netherlands and in other countries showing the potential of willow woods.’

Vegetation for flood risk management

Willows were used for flood risk management for the first time near Fort Steurgat in 2012 as part of the depolderisation of the Noordwaard. However, at that time, no measurements were made, and that is what we are doing now near Ridderkerk. Back in 2012, Deltares already identified different locations in the Netherlands where vegetation could play an important role in flood risk management.

Placement of the willow woods

Ecology

Building with  Nature solutions also add ecological value to an area. This trial will also be looking at the ecological impact of the two types of willow wood. It was decided to use osieries because these have traditionally been a common feature of this area. The willow shoots can be used for many purposes because they are so strong and flexible. The pollard willow is a refuge and nesting place for birds.

Willow Woods - a Building with Nature-proof

Willow Woods – a Building with Nature-proof

This trial was established as part of the ERDF project ‘Building with Nature: Ecoshape Knowledge Valorisation’, in which Deltares, Witteveen + Bos and the Ecoshape Foundation are trying out natural solutions for flood risk management in urban settings. The European Regional Development Fund (ERDF) is co-financing the project.