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How can we protect the southern salt marsh edge of the Boschplaat without losing too much of its natural dynamics? To find the answer to this question, a brushwood dam is being installed near the fourth duck decoy. Over the coming period, we will be investigating the effects of this brushwood dam.
On the southern side of the Boschplaat lies a large area outside the dikes: the Grië. As a link between land and sea, the area is vital for numerous plant and animal species. For the islanders, the Grië is just as important. For centuries, they have used this cultural landscape, with its alder hedges, to graze their livestock. Additionally, there are four historic duck decoys located on the Grië. In short, it is an area to be cherished. However, the Wadden Sea exerts a strong influence on the region: the power of the water causes the edges of the salt marsh to erode.
The Boschplaat on Terschelling is one of the largest contiguous nature reserves in the Netherlands.
To protect the Grië, a riprap dam was constructed several decades ago. It protects a large part of the salt marsh from erosion. However, as beneficial as this protection is, such a rigid boundary in the landscape also has its drawbacks. The riprap dam effectively reflects waves, meaning very little sediment settles in front of it. Because sediment does settle behind the dam—as water and silt can pass through the stones—a significant height difference has developed between the areas in front of and behind the dam. Furthermore, the dam alters water currents and creates an impassable barrier for aquatic life.
The riprap dam ends at the fourth duck decoy, or Rimkeskooi, where the Grië transitions into the Groede. On this part of the salt marsh edge, the Wadden Sea still had free rein. “We noticed that the salt marsh edge was eroding rapidly,” says Sandra Knop, management ranger at Staatsbosbeheer on Terschelling. “We had already reinforced the small dike that protects the duck decoy from the sea with clay. But that proved not to be a sustainable solution. The water essentially takes small bites out of the dike, causing it to deteriorate quickly.”
The deterioration of the salt marsh edge put the duck decoy at risk. And that was bad news, Sandra explains. “The Rimkeskooi has great cultural-historical value for Terschelling. Today, research is conducted there into wild ducks and zoonoses—animal diseases that can be transmitted to humans—which also gives the decoy scientific value. If we do nothing, this place will eventually disappear into the sea. That would be a terrible shame.”
This created a dilemma: how do you protect the valuable Grië and Rimkeskooi areas without placing too many constraints on the natural processes that shape the landscape? And how do you ensure that the natural transition from dunes and salt marshes to the Wadden Sea remains as undisturbed as possible?
"Extending the rock revetment was not an option. So we started looking for a more nature-based solution," says Dolf Visser. Dolf is the Natura 2000 Project Manager for the Wadden Islands at the Province of Fryslân and a member of the Boschplaat programme team. The Province of Fryslân leads the Wadden Sea Salt Marsh Edge programme within the Boschplaat Vision.
The solution was found in the form of a brushwood barrier extending from the existing rock revetment. The barrier consists of two rows of wooden posts with pruned branches from the island woven between them. The idea is that the brushwood barrier dissipates wave energy before the waves reach the salt marsh edge, while being less rigid and intrusive than a stone structure. It also allows water to pass through more freely than a rock revetment.
This is the first time a brushwood barrier has been used on Terschelling for shoreline protection. For that reason, a pilot section will be installed first. "We will build a brushwood barrier approximately 100 to 150 metres long, extending just beyond the duck decoy. This will protect the duck decoy while allowing us to study how effectively the brushwood barrier performs."
The prunings used for the brushwood hedge.
The brushwood hedge is currently being constructed. To minimize disturbance to nature, the management path leading to the duck decoy is being used to transport materials. Work is also only carried out at low tide. Sandra explains why: “The salt marsh is a high-tide refuge. This means that birds come here to rest when the tide is high. We do not work during those times. At low tide, the birds head back out to the Wadden Sea, and we can resume work on the salt marsh.”
Monitoring begins after construction. Does the brushwood hedge actually provide sufficient protection for the salt marsh edge? How much silt is deposited on the salt marsh behind the hedge? And how well does the hedge hold up in salt water? “These are all questions we hope to find the answers to in the near future,” says Dolf. He explains that the height of the salt marsh will be studied using GPS measurement points. The condition of the salt marsh edge and the brushwood hedge will be tracked with the help of a drone.
“I am pleased that we reached this solution together with the province,” says Sandra. The forest ranger is curious to see how the brushwood hedge will perform. “We naturally hope that the hedge will provide protection while still allowing for natural dynamics. We will discover over the coming period whether that is the case in practice.”
