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Marine Engineering Saves Griend Island

Griend isle

Between Harlingen and Terschelling in the northern Netherlands, the hook-shaped island of Griend is an extremely valuable nature reserve, but it has been increasingly affected by erosion.

Boskalis project manager Johan Miedema said, “The Wadden Sea area is a UNESCO World Heritage site, and it is one of the most beautiful nature reserves in the Netherlands. But it is also a treacherous environment because the mud flats are submerged by the tide twice a day.

“Measures to save and stabilise Griend focused on building a foreshore,” he noted, “but the work had to be done quickly while keeping the disruption of nature to a minimum.

“We had to find creative solutions, and those included housing all our colleagues on dry land rather than in an accommodation vessel. In turn, it meant taking them to work in small boats and disembarking on a car connected to a bulldozer. But even then, they sometimes had to walk to work up to 2 km across the mud flats.”

Scope of Work

To save the island, Boskalis held consultations with the Dutch Directorate General for Public Works and Water Management (Rijkswaterstaat) and with the Dutch Society for the Preservation of Nature (Natuurmonumenten). As a result, a foreshore consisting of 250,000 m³ of sand on the southwestern part of the island was decided on.

This plateau, which covers about 18 ha, is semicircular and some of the sand used to build it was obtained by removing the top layer of a row of dunes on the north side of the island.

The remaining sand was excavated by cutter suction dredger Seine from the Blauwe Slenk area, the storage location for sand mined during maintenance dredging of the Wadden Sea shipping channels. It was then pumped through a 4.5 km pipeline with both floating and submerged sections.

As Miedema pointed out, building such a long pipeline is not the most obvious solution for a short-term project like this. But it was one of the creative solutions the company needed to find.

“We adopted this approach because it is far less disruptive than bringing in sand using trailing suction hopper dredgers,” he explained. “It was a complex logistical operation: we had to pass through small locks and shallow channels to get all the pipeline sections into place.

“We also had to keep an eye on high and low tides at all times,” he added. “That made positioning of the submerged pipeline a challenge, too. Time wasn’t on our side either. We had to work flat out day and night to get the job done before the start of the storm season.”

Mini Sand Motor

Excavators, shovels, bulldozers, and cranes were used to construct the foreshore, then shellfish banks 100 m long and 50 cm high were created at eight sites.

“It is expected that the combination of shells and sand from the top layer will be a good place for sea grass to grow and make the plateau stronger,” Miedema said.

“We have also made a 50 m-wide opening on the northern side of the island that allows the sea to flow in more often and to deposit more silt in the large salt marsh on the southern side. That silt will settle and harden in the years to come, providing the island with more robust coastal protection.

“The sand that is pumped in will spread along the coast of the island over time, like a small sand motor [similar in concept to the 1 km² sand motor created off South Holland to continually nourish the Delfland Coast].

“We expect this saltmarsh to eventually form the western edge of the island. The idea is that Griend will move about 7 m a year in a southeasterly direction, just like it used to.”

“Natuurmonumenten awarded us this project mainly because of all the measures we proposed to minimise any disruption of nature,” Miedema concluded. “For example, as well as walking to work, we also replaced the reversing alarms on our machines with air blowers and used green lights.

“All in all, it was quite a puzzle to get the job done quickly and properly. But that makes the result all the more satisfying.”

A Natura 2000 Site

Located about 12 km southwest of the island of Terschelling, to which it belongs in administrative terms, Griend is a Natura 2000 site and thus part of the European Union network of core breeding and resting areas for rare and threatened species – as well as some rare natural habitat types – stretching across all 28 countries.

Listed under both the Birds and Habitats directives, Natura 2000’s aim is to ensure the long-term survival of Europe’s most valuable and threatened species and habitats.

Griend used to be a lot larger and was inhabited for many years. Today, it is no longer open to the public to prevent any disturbance of the bird colonies.

The island hosts the largest tern colony in western Europe: more than 10,000 pairs breed there annually. It is also a breeding place for 50,000 birds, including common terns, oystercatchers, redshanks, and numerous ducks and gulls. Additionally, thousands of migratory birds stop at the island on their journey from Siberia to Africa, and it is a resting place for grey seals.

Original article: Griend Island