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 design||Traditional 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.|
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.
A new primary river dike is required in the North Eastern corner of the Noordwaard to protect the inhabitants at Fort Steurgat. During a 1/2000 year discharge event the average water depth in the polder will be 3 meter whereby, in combination with a severe storm, waves up to 1 meter high are expected near Fort Steurgat. A first 'traditional' dike design around Fort Steurgat resulted in a dike height of 5.5 meter above NAP, with concrete blocks as armouring layer, leading to protests from the local population. Therefore, the aim was to create an ecodynamic design that provided safety, additional values for nature and recreation and is durable and cost effective.
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.
Before the project implementation started, the Noordwaard area had several functions, mainly agriculture, tourism, nature, living and some industry. As a significant part of the polder (1600 out of 2000 ha) would become flood-prone, quite some discussions and additional measures were needed to get the support of the local stakeholders. The outcome was that large parts of the agricultural area will be transformed into nature areas, housing areas will need new protection measures and tourism will be enhanced. Every function has its own stakeholders with their own interests. For many stakeholders safety against flooding has a high (if not the highest) priority. Residents of the village of Werkendam need to be protected by a new dike, but for the people actually living right behind this new dike the blocking of their view and the change of landscape were important considerations, which also reflected in their house prices. The proposal to make a higher and broader dike led to a lot of protests from the residents of Fort Steurgat because they experienced this dike as a very massive object in their neighbourhood, decreasing their pleasure of living in the area. These reflections inspired the search for other solutions.
A design vision was made through an interactive design process of interest groups, (local)government, residents and farmers. The design had two starting points:
- At least 30 cm water level reduction at Gorinchem under design conditions (a 1/2000 per year flood event), to be maintained irrespective of future activities and functions of the Noordwaard.
- Construction and maintenance costs have to remain within the set budget.
Other aspects of interest were:
- Farmers must be able to continue their work. This relates to the frequency and the duration of inundation of their lands.
- For the present inhabitants of the Noordwaard possibilities shall be created to stay there.
Boundary conditions to the development of alternatives were:
- The characteristic natural landscape of the National Park Biesbosch, which borders the polder Noordwaard, shall be preserved and strengthened.
- The network of Natura2000 protected areas in connection with National Park Biesbosch shall be strengthened.
- The possibilities for recreation, recreational boating and canoeing, walking and cycling shall be used and enhanced, and recreational entrepreneurship shall be stimulated.
- The historical landmark Fort Steurgat shall be preserved.
- Design, exploitation and management of the area shall aim at sustaining these boundary conditions and starting points.
The design of the flood protection scheme for Werkendam and Fort Steurgat should focus on addressing the opposition by making the original dike design smaller. From a safety point of view, the total dike height is always composed of the design water level plus an addition to account for wave run-up and overtopping. Hence a reduction of wave height should lead to a reduction of dike height. In this specific case, the incoming wave height is considerable (1.2 m significant wave height) due to the long fetch (10 km) and the considerable water depth (3 m) under the design conditions. The aim was therefore to find a wave-reducing concept that would lead to a significant reduction of required total dike height.
During an expert meeting between the major stakeholders a number of possibilities was explored to develop a green wave-reducing dike for the protection of Fort Steurgat. Three conceptual designs were proposed:
- A platform with wave reducing vegetation (by planting a willow tree forest) next to the dike (figure from Dekker and De Vries, 2009).
- Vegetated platforms in an area along a newly created channel south-west of the Fort (figure from Dekker and De Vries, 2009).
- Wave reduction by a series of vegetated stream parallel banks (figure from Dekker and De Vries, 2009).
Dikes in the Netherlands are assessed every 6 years to check whether they meet the conditions laid down in the VTV (Voorschrift Toetsen op Veiligheid: National Protocol for Safety Assessment of Dikes). The primary function of a dike is to protect the hinterland from flooding. The VTV prescribes on which failure mechanisms a dike has to be tested, given the hydrodynamic design loads (laid down for each stretch of primary flood defence in a book issued for the occasion: the Hydraulic Boundary Conditions).
For innovative dike designs it is sometimes difficult to test the safety in conformity with the VTV, as specific rules for this new set-up remain to be formulated. On the other hand, innovative designs are based on knowledge of strength, slope, dike height. The VTV allows a way out by formulating exceptions in which testing for the required safety can be done using the best available knowledge. In this specific case, the VTV gives no rules to test the implementation of a willow forest as part of a dike design.
To ensure that the eco-friendly dike can be tested for safety, Venema et al., (2013) have designed a testing scheme providing guidelines on testing, monitoring and maintenance of the dike. This new safety testing scheme has been approved by ENW-Techniek, the national Expert group for Water Safety in the Netherlands, and could possibly serve as a basis for the implementation of such innovative designs into the VTV.
In order to get a fully accepted innovative design, relevant testing steps for vegetation as integral part of a dike will have to be designed and included into future new releases of the VTV.
Proof of concept
To determine the effect of a vegetation field on the wave conditions at the foot of the dike the numerical wave model SWAN is adapted by adding a vegetation module. The model results, in which vegetation was schematically represented, show that for the 1/2000 per year event the reduction of the wave height is between 60 to 80%. Therefore, waves up to 0.35 meter can be expected at the dike of Fort Steurgat. Details are described in the report of Dekker and de Vries (2009).
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.
From field and laboratory research it has become clear that vegetation can be an effective means of wave reduction (see, for instance, Coops et al., 1996, Mendez and Losada, 2004, or Lovstedt et al., 2010). Koch et al. (2009) describe the role of natural systems in relation to ecosystem services such as providing safety through wave reduction. This property of vegetation is utilized in the design via a wave-reducing willow tree forest in front of the dike. This means that the dike can be lower than in a traditional design to prevent overtopping. Furthermore, a clay cover is sufficient to resist the remaining wave attack, which means there is no need for a concrete revetment.
In the final design a continuous willow tree plantation in front of the dike provides 80% reduction of incoming wave height under 1/2000 per year storm conditions. This allows the dike crest to be 0.7 m lower than in the traditional design, without violating maximum overtopping limitations. The willow plantation is inspired by the centuries old tradition of willow culture for the production of brushwood, which is typical of this region. Thanks to this know-how, tree vitality can be guaranteed and long-term maintenance of the plantation can be defined and budgeted.
The decision for implementation of a willow tree plantation is based on several conditions. First of all the vegetation has to be (quite) rigid in order to withstand the waves. Also the vegetation must be able to survive several inundations per year, each lasting relatively long (weeks). Moreover, the vegetation must be able to grow in soil with high groundwater levels. It is recommended that a mix of species (i.e. Salix alba and Salix viminalis) are used mainly because a mix decreases the risk of a disease or pest affecting the entire forest thereby decreasing forest health. The willow species selected meet all the conditions and fit very well into the landscape, as they are domestic in the area. The willow forest is located on top of a bank rising 0.7 m above the normal ground level, in order for the willows not to be inundated too frequently or too long. The width of willow field is about 60-80 m and the forest extends along the dike of Fort Steurgat. The planting grid will be staggered, with a density of approximately 4 tree stems per m2, resulting in hundreds of branches per m2.
The lay-out of the design is optimized so as to have the largest width in the zone of highest expected wave impact. The width is enough to allow two-yearly cutting of trees (each year half of the forest) without influencing the required safety level.
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.
Firstly, the new dike has a wider footprint than the 'traditional' dike. In the Noordwaard situation, a wider footprint has possible consequences for the maximum discharge through the area under design conditions. This is a critical aspect of the design, because of the overarching goal to reduce water levels upstream
The design includes the implementation of a willow tree plantation. Working with this living element is beyond the expertise of traditional dike engineers and necessitates expert input of biologists. Biologists could even be involved during the design and construction of the dike. Planning of dike construction is more complex compared to a traditional design as a sufficiently dense tree plantation takes time to grow (3-4 years).
A monitoring program needs to be set up to assess the evolution of the willow forest and its wave-reduction capacity. This can be supported by a full-scale experiment under design conditions in a big laboratory wave flume. In the current phase of the project the monitoring program in terms of wave reduction has not been made explicit (2014). Monitoring and maintenance of the health of the willow forest to ensure sufficient wave reduction is described in Venema et al. (2013).
In case the willow forest does not meet the requirements, there is a contingency plan: the new dike is designed to withstand significant wave overtopping, so with sufficient installed polder drainage pumping capacity the area remains adequately protected.
Management and maintenance
Maintenance is more complicated than in the traditional situation. The dike proper is designed to be “maintenance free”, except for regular mowing of the grass. However, the willow plantation necessitates yearly maintenance to guarantee sufficient density and tree health. Bi-yearly cutting of willow branches, replanting and the removal of the grass and branches will be managed by the Waterboard. Some uncertainty remains on the sensitivity of the tree plantation to pests, diseases, ice-winters, forest fires and instability under extreme wave forces. Therefore, contingencies against failure are implemented, mainly in the over dimensioning of the design. Furthermore, in the testing report of the dike (Venema et al., 2013), guidelines on how to deal with these phenomena and their associated risks are presented to the eventual managers of the dike.
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.
The added value of the project lies in the innovative way of making use of ecosystem services (wave reduction) and multi functional use of space. It is the first time that vegetation is an integral part of a dike and that such a dike can be tested for safety according to an approved method.
For Water Boards this project offers a chance to look for multifunctional designs. Furthermore, it offers possibilities to include vegetation that is already present in front of a dike into safety testing. Existing vegetation could perhaps reduce wave height sufficiently to lead to a lower incoming wave. Therefore, a dike that would normally have to be raised, could still meet safety demands if vegetation is included. Such a design would contribute to goals for both nature development as safety.
The following lessons can be learned from this project:
- This type of solution needs more space than a traditional solution, so it might not be feasible everywhere.
- The first part of the forest, the part where the waves come in, has the strongest wave-reducing effect.
- Limited field data is available of wave height reduction by a forest. The design is based on downscaled laboratory experiments in combination with modeling. Monitoring the health and growth of the trees and the effectiveness of the forest in wave reduction is important.
- The design is focused on wave reduction but other positive relations to dike stability may exist and have to be studied in more detail.
- The design process required the engagement of a group of experts from various disciplines to enable a sound combination of biological and engineering aspects.
- The design process was dependent on close cooperation of various parties, driven by the urgent need to deliver an acceptable product that was supported by all, within a short time frame.
- Legal issues have to be tackled. Testing protocols for dike safety including procedures to allow innovations have to be designed, proofed and constructed. The first step is already made in this project, where a safety testing scheme for the willow forest as integral part of the dike was set up.
- An innovative dike design on the basis of eco-engineering using vegetation as a wave-reducing component is possible, practical and safe.
- The design can be considered as a breakthrough in dike design. The willow forest has become integral part of the dike. As this is the first time, monitoring and research are needed to evaluate the effectiveness and to enable integration in future test protocols.
- Projectbureau Noordwaard (2007). Ontwerpvisie Ontpoldering Noordwaard.
- Barbier, Edward B., Hacker, Sally D., Kennedy, Chris J., Koch, Evamaria W., Stier, Adrian C. and Silliman, Brian R. , The Value of Estuarine and Coastal Ecosystem Services (2011). Ecological Monographs, Vol. 81, No. 2, pp. 169-193, 2011
- Dekker, F and M.B. de Vries, (2009). Ontwerp groene golfremmende dijk Fort Steurgat bij Werkendam (in Dutch).
- Bureau Noordwaard (2008). Inrichtingsplan Ontpoldering Noordwaard.
- Coops, H., et al, 1996. Interactions between waves, bank erosion and
emergent vegetation: an experimental wave tank. Aquatic Botany 53 (1996) 187-198.
- Koch E.W., Barbier E.B., Silliman B.R., Reed D.J., Perillo G.M.E., Hacker S.D., Granek E.F., Primavera J.H., Muthiga N., Polasky S., Halpern B.S., Kennedy C.J., Kappel C.V., Wolanski E., 2009. Non-linearity in ecosystem services: temporal and spatial variability in coastal protection. Frontiers in ecology and the environment 7, 29-37.
- Lovstedt, C., et al, 2010. Wave Damping in Reed: Field Measurements and Mathematical Modeling. JOURNAL OF HYDRAULIC ENGINEERING © ASCE / APRIL 2010 / 223.
- Mendez, Fernando J. and Inigo J. Losada, 2004, An empirical model to estimate the propagation of random breaking and nonbreaking waves over vegetation fields, Coastal Engineering 51, pp. 103-118.
- Venema, Johannes E., Schelfhout, Harry A. and Van der Meulen, Myra D., 2013. Toetsmethode grienddijk Fort Steurgat, hoofdrapport en achtergrondrapport, Deltares report 1206002-000 (in Dutch - in preparation).
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