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Eastern Scheldt dike foreshore ecological upgrading, NL
In this project, the design of the foreshore strengthening has been enriched by placing armour stone in different shapes and depths. The variety of habitats improves the ecological conditions around the 'Zeelandbrug', one of the most important recreational diving sites of Northwest Europe.
At several locations along the Eastern and Western Scheldt, the dike foreshore needed to be strengthened in 2009. The tender requirements for this D&C contract to ensure dike stability were originally focused on economic aspects only. This lead to a straight, technical design, making use of the relatively cheap and technically effective material steel slag, a stone-like residue of steel production. Notwithstanding the fact that from the point of view of environmental protection law there was no necessity to adjust the technical design, the special ecological value of two locations in the Eastern Scheldt was recognized by the Contractor.
Following the 'Building with Nature' principles, the contractor had pro-actively indicated the possibilities to create additional value for underwater habitat development. After discussing the project with various stakeholders, the Ministry of Infrastructure and Environment (Rijkswaterstaat) recognised that the ecological value of the locations chosen could not be ignored.
The environmental impact of steel slag was widely challenged, mainly by action groups who claimed that the ecological effects of the large-scale application of this material, especially the effects on the flora and fauna living on this material, have not been studied completely. Furthermore, p ublic interest, especially of the foundation 'Stichting de Oosterschelde' and diving clubs, raised awareness of the importance of the diving spot at the Zeelandbrug.
After the contract was awarded, Van Oord Dredging and Marine Contractors and the Ministry of Infrastructure and the Environment investigated the options for nature conservation within the project context. Van Oord and the Ministry have joined forces with applied research institute Deltares and GiMaRIS consulting marine researchers to design and construct an underwater ecological landscape. Deltares and GiMaRIS organized a workshop for a large group of stakeholders, like several diving clubs, the aquaculture sector and even protest groups like the 'Stichting De Oosterschelde'.
Rijkswaterstaat agreed on the 'Ecodesign Structures' underwater landscape to enable fast ecological recovery and to enrich the ecosystem, as this was technically and economically feasible, and an ecologically responsible and effective solution.
Planning and Design Phase
Because the Eastern Scheldt is so famous for diving, the underwater life is well-documented. Many photographs record the former condition. Especially the Deltaworks provide an interesting habitat to organisms preferring a hard substrate. Due to its natural characteristics, the Eastern Scheldt provides for shelter and breeding conditions. Wave attack on the foreshore can be powerful. That is why the foreshore needed strengthening and soft sediment is not a suitable material to work with here. The 'Ecoreef' structures for ecological development can withstand the present design conditions. The project combines educational, ecological and touristic functions, i.e. people, planet and profit are served.
The design stimulates ecological functions. The diverse morphology creates a wide range of velocities. Velocity and morphology are important factors for the settlement of benthic organisms. Some can stand a high velocity better than others. Moreover, some organisms tend to avoid competition and rather adapt to another niche provided, which means that a range of different conditions is ecologically important. The material armour stone creates caves of different sizes and shapes, such that also prey-predator strategies vary widely. The tide accounts for a lot of variation in velocities and wet-dry transitions, and irregular paddles remain during low tide. Migration processes are also encouraged by the tide and the structure stimulates migration. Generally, a wide variety of niches is obtained, which enables the organisms to settle, breed, grow, hide and hunt.
The project can be seen as a no-regret, because the risk is low. The ecological condition of the Zeelandbrug with the traditional foreshore strengthening could hardly deteriorate. Yet, the goals for the 'Ecodesign' were ambitious: maximum biodiversity at minimum costs. Pilot research must reveal which shapes have the highest benefit/cost ratio in terms of ecological value. The design has to be economically and technically feasible, including maintenance and monitoring. Because the money spent will support the tourist sector, the costs can be seen as an investment.
Special engineering process
The engineering process was highly innovative not only because of the exceptional aim that had been identified, but also because of the way the various parties interacted. Ecologists from Deltares and GiMaRIS optimised the ecological aspect of the engineering design, while Van Oord kept a close eye on the technical feasibility. The Ministry looked at what could be accomplished within the relevant contract and budgetary constraints. Creative input from all four partners made it possible to optimise every aspect of the design, from biology to costs and technical realisation.
The primary aim of the underwater landscape 'Ecodesign Structures' is to enable a quick recovery of the underwater ecosystem after the dike strengthening works. The partners' approach to enriching the foreshore involved creating as much habitat variation as possible. The design called for a variety of different materials, gradients and shapes to create differences in height, hiding places, and variations in the exposure to and shelter from currents and waves. To ensure flexibility, the engineers came up with a modular system of building blocks consisting of round, criss-crossed and atoll-shaped piles of stones and linear elements, all with dimensions up to 23 m, and linear elements up to 60 m long. Combining these Building Solutions made it possible to achieve variety at a larger scale. In the final landscape plan, the combined modular system produced delineated spaces and areas that will gradually fill with fine sediment. The recovery and ecological value of these ‘Ecodesign Structures’ has been documented by voluntary divers. They report the presence of the rare lumpfish (Cyclopterus lumpus) and seahorse (Hippocampus hippocampus).
Filling delineated spaces
In the final landscape plan, the combined Building Solutions give rise to delineated spaces and areas that may gradually fill with fine sediment. Extra alternative materials will be introduced tentatively into these spaces to see whether they can be turned into valuable new habitats. In this way several options to stimulate ecological processes can be investigated and evaluated from different perspectives (financial, ecological, attractiveness to divers, short-term and long-term, etc.).
By involving the stakeholders and interdisciplinary experts a feasible design approach could be found. Technical and ecological feasibility could be taken into account right from the beginning. On top of this, tendering procedures had to be adjusted to enable this ecologically effective and feasible design to be implemented.
Van Oord was the contractor responsible for the execution of the project. The design was made digitally visible for the crane operator on the dredger, which enabled him to accurately place the armour stones bucket by bucket, despite the prevailing currents. After placement of the material, a bathymetric survey demonstrated that elements had successfully been brought into position.
Operation and maintenance
The technical design meets the requirements for safety, obviously the main objective of the dike and foreshore strengthening works. Flood safety provided by the structure can be assessed in the future following standard procedures. For the ecological aspects, maintenance requirements in the sense of engineering interventions is not necessary for the underwater landscape.
The risk of uncontrolled and undesired natural development is not an issue here, since nature development is the primary objective of the landscape. Of course it is interesting to compare the newly developed ecosystem with that in a conventional dike foot landscape. Rijkswaterstaat will include this in the monitoring campaigns out for the entire project. Furthermore, 'Stichting Anemoon' has started a voluntary monitoring campaign enabling divers to upload their photos for ecological monitoring. Divers will take photographs at the marking stones. A rapid recovery was observed as well as a high biodiversity. Also rare organisms appear to have settled within the landscape. Furthermore, to succeed with the project, several publications made the public aware of the principles of habitat enrichment and how the 'Ecodesign Structures' contribute to this and create added value.
Instead of ecosystem degradation due to a traditional design, new opportunities arise with the chosen design. If divers come from abroad to dive here, it must be possible for local entrepreneurs to make money out of this. Moreover, the concept can be exported to other countries.
In addition to the 'Ecodesign Structures', three steel structures have been placed at the planning site. They are meant to support the 'Sepia Project', a project to protect the cuttlefish that enter the Eastern Scheldt in spring to mate.
In response to the public interest to respect the diving spot, Rijkswaterstaat financed the pro-active design. Information panels make the public aware of the vulnerable ecosystem present under water. The larger the support group, the more prominent the issue will be on the political agenda.
The following lessons were learned from this project:
Related Building Solutions
Rich Enriching revetments