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Wave attenuating willow forest - Noordwaard, NL

Location: Noordwaard near Werkendam, the Netherlands
Date: 2009 - 2015
Involved parties: Deltares, WINN, Rijkswaterstaat Room for the River program, Project bureau Noordwaard, Waterschap Rivierenland

Technical Readiness Level: 9 (Full commercial application)
Environment: Lakes and Rivers
Keywords: wave reduction, willow forest, dike height, room for river, safety level

Building with Nature designTraditional design





The hybrid dike is integrated in the landscape because willow trees are common in the area. The armoured dike is now replaced by a lower clay covered dike resulting in a cheaper solution. The combination of a clay dike and willow plantation is generating additional natural and landscape values. In line with sustainability objectives of the Dutch government, production of biomass for energy as result of regular maintenance of the willow plantation is seen as a bonus of the project. It is expected that the new design will add to recreational value of the area for local residents.The dike has been optimized from a landscaping point of view. On the land side (right hand side of profile drawing) the slope is very gentle in order to provide an impression of a sloping meadow for the residents of Fort Steurgat. The traditional design on the side exposed to high water levels and wave impact is fortified with a concrete armoring layer to resist waves with a significant wave height (Hs) of 1.1 meter.

 

Abstract

Within the national Room for the River programme the polder Noordwaard was being prepared to function as an extra river branch of the Nieuwe Merwede river in times of high discharge. The aim was to lower water levels upstream of the new branch considerably, in order to protect the city of Gorinchem and the surrounding areas against flooding. 

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Initiation phase

It was concluded that a solution should be found that resulted in a lower dike height. This was a chance for innovative eco-engineering design alternatives. At the same time the additional construction cost of this multifunctional design should be minimal or should even result in lower costs than the traditional design.

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Planning and design

A study has been executed by Deltares (Dekker and de Vries, 2009, in Dutch) in order to research the possibilities of planting vegetation to sufficiently reduce the incoming wave height in order to enable adaptation of the design of the dike. The target was to lower the height of the dike to 4,5 m +NAP. This could lead to a smaller footprint width of the dike of ± 15 to 20 meter. On the basis of qualitative assessment of effectiveness, possible impact on hydraulics and ease of maintenance, one design was selected, consisting of the willow forest bordering the planned new dike transect.

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Construction

The project construction started in 2013 with planting the willow forest. The project was finished in 2015, by lowering the designated dike section along the river, thus connecting the polder Noordwaard to the Nieuwe Merwede river whenever it is flooded.

 

Operation and Maintenance

The Waterboard was involved in the project of Venema et al. (2013) in which the safety testing scheme was set up. This includes a monitoring program for the health of the willows and provides some guidelines for emergency situation, but not for the wave-reducing capacity in relation to the vegetation density, stem diameter and height.

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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 optimize 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 realized 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. 

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References

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