A new salt marsh for the Waddensea

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A new salt marsh for the Waddensea A study about the opportunities and conceptual design for a guiding dam constructed in the Waddensea near Holwerd Tijmen Hartung, Sebastiaan de Jongh, Bart

The towns population like many villages in the province is diminishing as a result of the aging citizens. To counteract this problem locals have created the initiative: Holwerd at Sea.

To assure that this canal won t fill with dredge, because of the tides, a water guiding dam is to be constructed.

This student project has been done to explore multiple ideas and options for this design. The result includes an inventarisation research, case studies and an concept design as inspiration.

The assignment was given however the research questions had to made by the students themselves.

Phases In phase one a lot of literature study has been done about the Waddensea and local features.

The information of this phase has been analysed and lays the foundation for phase two. Phase two was more specific to the actual dam.

The end result of both phases was a rapport containing the literature study, case studies, the designs and an advice about the local implementation.

Figure 1: Satellite image netherlands (Google) Figure 2: Satellite image Friesland (Google) Figure 3: conceptuel design Holwerd on Sea (Holwwerd on Sea.

1 A new salt marsh for the Waddensea A study about the opportunities and conceptual design for a guiding dam constructed in the Waddensea near Holwerd Tijmen Hartung, Sebastiaan de Jongh, Bart Hoogendoorn Introduction On the northern coast of Friesland, a province in the Netherlands (figure 1), lies a small former fishing village called Holwerd (figure 2). The towns population like many villages in the province is diminishing as a result of the aging citizens. To counteract this problem locals have created the initiative: Holwerd at Sea. This aspiring project aims to make Holwerd a new popular tourist destination. An important part of this is the creation of a lake and canal connected with the Waddensea. To assure that this canal won t fill with dredge, because of the tides, a water guiding dam is to be constructed. Since this dam is going to be built in a protected wildlife area the dam needs to have an environmental friendly design. This student project has been done to explore multiple ideas and options for this design. The result includes an inventarisation research, case studies and an concept design as inspiration. Material and Methods This student project had a timespan of two semesters,or about 19 weeks, with each semester having its own project phase. The assignment was given however the research questions had to made by the students themselves. The main research question was: How can the guiding dam be placed as ecological as possible into the Waddensea? Phases In phase one a lot of literature study has been done about the Waddensea and local features. Also some case studies has been done about sample projects like: the Mudmotor near Harlingen and the Kaichu-Doro road in Japan. The information of this phase has been analysed and lays the foundation for phase two. Phase two was more specific to the actual dam. The possible ecological adjustments based on the specified target species were elaborated and both the conceptual as the technical designs were made. The end result of both phases was a rapport containing the literature study, case studies, the designs and an advice about the local implementation. Beside the literature study and case studies the knowledge of experts has also been used. Figure 1: Satellite image netherlands (Google) Figure 2: Satellite image Friesland (Google) Figure 3: conceptuel design Holwerd on Sea (Holwwerd on Sea.nl) Ecological implementation and design Mussel beds The Waddensea is an area characrterized by the dynamic tide which influences essential ecological factors such as sandbanks, mussel beds and the unique salt marshes. The latter two of these are the motors of the high biodiversity the area can harbor. The mussel beds consisting from Myltilus edulis, the exoctic Crassostreas gigas and the endangered Ostrea edulis have an important function as building blocks of the ecosystem. When they settle they create grip, like the roots of plant keeping the otherwise loose sand together. While feeding they produce these so called pseudo-feces, this sludge like substance is very fertile and creates, unlike the sand, a firm soil for all kinds of plants to grow on. These plants in their turn attracts small fauna such as shrimps, snails and fish, which in their turn will attract bigger fish, thus forming a small ecosystem. The pseudo-feces spread up to 200 metres from the mussel bed providing a healthy base for a reef to develop. The effect created by these beds doesn t stop at marine levels. The coastal birds which reside in the area feed of these beds when low tide comes in and provide an important source of food. Due to intense fisheries many of these reefs have been removed over the years, for them to regrow in the area will take years without the help of mankind Design The conceptual design of the dam (figure 4) focuses on multiple zones. The zones are defined by their material, function and flood frequency. The most outer zone is the Waddensea, in this zone reefballs can be placed to stimulate the local ecosystem. At the salt marsh side of the dam, rubble will be used to create nesting places for birds. The target species per zone can be found in table 1. Table 1: Target species Salt marshes The salt marshes develop themselves due to the tide. During high tide the area is flooded and sediment from clay remains. This fertile clay makes the area habitable for salt loving plants. The sediment creates elevated places which coastal birds use to nest and use as a safe zone for high tide. Due to these uncommon circumstances the area is filled with plants rarely found elsewhere in the Netherlands, these plants themselves have an important function as host plants for insects. Salt marshes used to be widely present on the coastline, but are diminishing due to agricultural interventions. By surrounding them with dykes the process of them being flooded is halted. This causes them to dry out and become normal grasslands. Next to the loss of biodiversity this also diminishes the amount of areas where coastal birds can reside and raise their younglings. The latter is an issue amongst the coastal birds in general. Rising sea levels cause their nests to be flushed away and nest predation by species such as Vulpes Vulpes, Rattus norvegicus, different kind of martens and gulls cause the birds to raise less of their offspring. Dredge and sedimentation The closure of the water basins located in the Zuiderzee and Lauwersmeer resulted in a sedimentation hunger at the Waddensea due to changes in the morphological balance. To repair this balance more sand is required at the coast. The sandhunger causes the other coasts to erode at a higher rate. Seeing the speed of the coastal sedimentation shows there is still an imbalance in the area. The combination of rising sea levels increases the sedimentation in the area, this shows that the Waddensea is a real sedimentation catcher. Essential for the area is the ferry route to Ameland, a major tourist hotspot. Because of the local sedimentation this route is required to be dredged year round. Which is a very expensive activity costing the liable parties millions every year. Research indicates that this amount is increasing and will continue to do so over time (figure 3). As noted before, the main purpose of the guiding dam is to keep the canal from filling up with sediment, saving a lot of money in daily dredging costs. Figure 4 Conceptual design for the ecological adaptions applied on the dam (T. Hartung, S. de Jongh and B. Hoogendoorn) Materials Conclusion Soiltain tube For the realisation of the dam we use raw materials which are present in the Waddensea. The middle part of the dam is created by a soiltain tube. This tube is filled with sand, dredge and sludge from the Waddensea. These tubes are made of a geosynthetic material. By the re-use of these raw materials a financial advantage is created. Reefballs Reefballs are produced by eco-friendly concrete. In contrast to the usual concrete this concrete has a acidity which is equal to seawater. High levels of acidity are harmful to soil life. They will not use iron in the construction. Hillblock Hillblocks are made from concrete, unlike the normal concrete elements these blocks are made of 30% less concrete. The elements are shaped like an I leaving areas where flora and fauna can settle and reproduce. Spaces can be filled with sand, ground. In this study we have researched the possible environmental application of constructing a guiding dam in the Waddensea near Holwerd. The function of this dam, if realized, is to keep a possible canal dredge free. The concept design of the dam highlights two ideas. The first one is the increase of salt marshes at one side of the wall. Since salt marshes are an important but decreasing habitat in the Waddensea, the creation of them will have positive ecological effects. They are essential for the survival of coastal birds and host an variety of uncommon plant species, of which most are salt loving. The main characteristic of these marshes is that they can be flooded and thus very dynamic. Beside the salt marshes an artificial reef will be created, through the use of reefballs, to stimulate the marine life. The reefballs are easy to place and will offer substrate on which marine life can grow and develop to form an important building block in the food chain. With these ideas the negative impact on local ecosystems, by the construction and placement of the dam, will be reduced and mitigated. The dam itself will be made with sustainable materials and will also contain quarry stones to increase the nesting places for birds. Quarry stone On the side of the salt marsh, reused quarry stone will be placed. The robust appearance will break the wave force on the dam. Due to the natural structure and elevation it is a safe place for birds to nest. Figure 5 Concept for the technical design of the dam (T. Hartung, S. de Jongh and B. Hoogendoorn) References De ecologische geleidedam van Holwerd aan zee ; Tijmen Hartung, Sebastiaan de Jongh, Bart Hoogendoorn;

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