Dredging sludge to restore nature    


The new Marker Wadden


Dutch nature conservation organisation Natuurmonumenten has begun restoring the Markermeer – one of the largest freshwater lakes in western Europe – by constructing islands, marshes and mud flats (Dutch: wadden) from the sediments that have accumulated there in recent decades. Over time, these so-called Marker Wadden will transform into a unique ecosystem, boosting biodiversity in the Netherlands. Boskalis Dredging & Marine Experts are using sludge from the lake to build the islands.

The construction of the 26 km long Houtrib dike between the towns of Enkhuizen and Lelystad in 1976 has divided the IJsselmeer into two sections. The southern section has been given a new name: Markermeer. Originally the idea was to reclaim land from the lake and construct a large polder, called Markerwaard. For economic, ecological and recreational reasons, and because of strong opposition, the government finally decided not to impolder the lake. However, the separation from the larger IJsselmeer has led to a disturbance of the balance of nature. The lake is surrounded by dams and dikes, and therefore most natural banks, shallow places and life at the bottom have disappeared, which means there is hardly enough food left for fish and consequently migratory birds. In 2012, Natuurmonumenten came up with a plan to restore nature by constructing a number of nature islands along the Houtrib dike. They were called Marker Wadden. The idea has been developed  in cooperation with the Dutch Directorate-General for public works and water management (Rijkswaterstaat). For now, the plan is to realise five new islands with a total surface area of eight hectares by 2020. Later, more islands might follow to form an archipelago of about 10,000 hectares.

Nature and recreation

Dredging company Boskalis began  executing the project in March, 2017. Six months later state secretary Martijn van Dam (Ministry of Economic Affairs) set foot on the first island, with a surface area of 250 hectares. This main island, surrounded by beaches and shallow waters,  should become a breeding ground and a resting place for birds. Only this island will provide special facilities for tourists, such as a marina, a visitors centre, hiking trails and a lookout tower.

“I would like to think that birds, fish, shellfish, and water plants will feel at home here, once the project is finished”

At their office in the town of Papendrecht, Corné Appelo (Boskalis), who managed the project for the first six months, shows an aerial view of the island, highlighting the contours of the different compartments it is made of. “Using crane vessels, we have constructed a stone edge at the side of the island that is most vulnerable to storms. We have also built two harbour dams, using stone. The edges of the compartments are made of sand dams (on which the hike trails will be built), and filled with a mixture of clay, sand, and peat. Once the desired height has been reached – on average, the Markermeer is 4 metres deep – and seeds have been sown, vegetation starts and nature will take its own course. I would like to think that birds, fish, shellfish, and water plants will feel at home here, once the project is finished.” 

Swirling sludge

To create beaches, build sand dams and fill up compartments, Boskalis uses the ‘Edax’. This 85 metre long, 9,500 kW cutter suction dredger has proven its value before, particularly during the construction of the second Maasvlakte (a new port and supporting infrastructure near Rotterdam), and the 35 km long bypass of the Suez Canal in Egypt.  The ‘Edax’ has dredged the eight metres thick upper layer of clay and peat from the bottom of the Markermeer 24/7, and pumped these natural resources through floating pipers to their final destination, followed by deeper layers of sand.

The ‘Edax’ will also be used to do the dredging for the other islands. Because these islands are in the lee of the main island, they don’t need to be protected by stone edges. The dredging will be done in a unique way. The top layer of the Markermeer bottom consists of an approximately 20 centimetres thick blanket of sludge. Swirling sludge not only gives the water a blue-greyish colour, but threatens most of the living creatures and plants in the lake. The plan is to use the sludge for filling up the compartments on the future nature islands.


Due to the fact that using sludge to build an island still is experimental, a compartment with  a surface area of 10 hectares on the western half of the main island, will be used as a testing ground. Bart van Asperen, who is the current project manager, explains: “We will dig a 6 metres deep, and 30 to 50 metres wide groove in the bottom of the Markermeer. As there are no tidal influences, the sludge in the groove will start to move and swirl before these sediments settle. So we let nature do our job. The sludge – which is as thick as yogurt – in the groove will thicken because of its own weight. Afterwards it can be used to fill up the testing ground, and finally to raise the compartments on the new island. All islands will naturally take out sludge from the lake, resulting in increasingly clean water.”

Mr Van Asperen adds that the sludge wetlands, where reed will grow, must be accessible to geese. “These birds must be able to move around freely.”


Boskalis has linked a second experiment to the project. During the build of the main island, the ‘Edax’ used biofuels, made of a mixture of gas oil, and residual products of a Finnish paper and pulp mill. This experiment is part of a two year Boskalis pilot program for biofuels, in collaboration with engine manufacturer Wärtsilä and sustainable fuel producer Goodfuels. Van Asperen: “We did not have to adapt our engines. It was a perfect test, without encountering any problems.”

Appelo adds that the use of biofuels seamlessly fits with the Marker Wadden project approach. “Biofuels are still in their infancy, and we would like to be frontrunners in applying them.”

“We did not have to adapt our engines. It was a perfect test, without encountering any problems”

Source: John Ekkelboom, Maritiem Nederland


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