Tests on new type of floating reed marsh

Published: 20 November 2014

Deltares has tested a new type of floating willow mattress. It keeps the reed marsh that develops on top floating for longer than the ‘sinking’ reed marshes we have at present. The new type provides better wave attenuation than the older structure.

As a result, these floating reed marshes help to defend shores and therefore improve flood protection, ultimately generating savings for water management authorities. The wave attenuation of the new mattress was tested last week in a range of conditions and wave patterns in the Delta Flume at Deltares, The Netherlands.

Improving flood protection, biodiversity and water quality

Floating reed marshes are used throughout the world, primarily to improve biodiversity or water quality, but not for flood protection. Our initial attempt in 2010 to enhance flood protection using floating reed marshes partially failed because, in time, they sank. A new type of willow mattress was developed for the current trial. It is expected to keep floating for a long period. Plastic floats and willow rolls have been attached to the mattress. The floats stop the mattress from sinking and the rolls provide additional wave attenuation.

Fifty to one hundred metres long

Victor Beumer, an ecologist with Deltares and the project leader: ‘In a pilot setting the mattresses can be designed to be between fifty and one hundred metres long. They can be used in front of the shores of lakes where management agencies want to improve the marsh habitat and improve flood defences. But they can also be positioned in harbours, when there is enough room, to damp down waves generated by passing ships. We are also thinking about how to use them in rivers.’

Field application

Now that the trial has been a success, Rijkswaterstaat and Deltares hope that the new structure can be put into practice. Victor Beumer: ‘The Markermeer lake would be a nice location. A marsh habitat is created for birds and fish on and under the floating willow mattresses and, in addition, a quiet zone is created between the mattresses and the lake shore where aquatic vegetation can develop. The result will be a considerable improvement in local biodiversity. This will also be a good area to further test the wave attenuation and the resilience of the structure.’

Deltares organised the trial at the request of Rijkswaterstaat.