The new dyke along Dollard is made of salt clay. You have to tamp it well

As he walks along the seawall, along Dollard in Groningen, his appearance suddenly changes. The dyke becomes somewhat less steep and runs a little further into the adjacent estuary. It is a completely new concept that is being tested here, says Wouter van der Star, researcher at the Deltares knowledge institute, as he tries to warm up his booted feet.

It is bitterly cold and the air is humid. Next to him stands Henk van Norel, specialist in flood protection and safety at the Hunze en Aas water board. He explains that over a length of 750 metres, an extra metre-thick layer of clay has been laid on the existing dyke to adapt to higher water levels and waves, which are the result of climate change. If this so-called wide, green dyke proves to provide sufficient strength and protection, it will be extended to 12.5 kilometers. And perhaps the concept can be used elsewhere in the Netherlands or abroad.

Van Norel says that the traditional approach here has been “turned on its head”. Normally, the design of the dyke is leading. “You make sure that the materials used, such as clay, meet the requirements of the design.” Now it’s the other way around. Leading in this case is the clay. The design has been adapted accordingly.

The fact that the clay is leading is because it has been specifically chosen to use clay close to it. From the Eems-Dollard area itself. It is related to a problem that has been going on here since the 90s. In parts of the area, the water has become much more cloudy because a lot more silt has entered it, due to a combination of factors. For example, since 1994, less sludge has been extracted for land reclamation. In addition, millions of tons of silt are dredged from harbors and waterways to keep them navigable, and that silt is returned to designated sites elsewhere in the estuary.

The clay should not be too dry, because then you will get lumps and cracks

Henk van Norel water board Hunze and Aa’s

All that extra silt suppresses the vertical mixing of oxygen in the water. This makes it more difficult for many organisms. The biomass has decreased in large parts of the Ems-Dollard area since the 1990s and is significantly lower than in the rest of the Wadden Sea.

After persistent protests from nature organizations, two ministries (LNV and I&W), the Rijkswaterstaat, provinces, municipalities, nature organizations and business drew up a major plan in 2015, Eems-Dollard 2050. A year later, all sorts of projects started – the magazine Landscape devoted a special issue to it two years ago. Nature restoration is part of the plan. The area, especially Delfzijl, also needs to be upgraded and given a better look to reverse population decline and economic decline. Solutions are also being sought for the excess sludge. And innovative dike concepts are being tested. The last two intentions have led to the wide green dyke.

Also read: Self-made drowned land in the Ems water

Breeding ground for avocets

The big question in this project was whether the spent saline sludge would prove suitable as crucibles? Because its composition and properties were unknown. The sludge now used comes from three places, Van Norel says, clapping his gloved hands together to expel the cold water. From the harbor canal in Delfzijl, from the natural polder Breebaart and from the salt meadow a little further on. Van der Star points into the distance. “See that puddle with the island in the middle?” Salt marshes have been excavated around the island in ribbon form. The tape was then immersed in water. The island has now developed into a breeding ground for avocets, says Van der Star. “Foxes can’t get to it.”

The two types of sludge and the saltmarsh were then dried on site, separated from each other. Large rectangular sections were built in 2018 on the seaward side of the dike where we walk. The material has been poured into it. It’s gone now, but the outlines of the boxes are still visible. The mud was stirred up at regular intervals, says Van Norel. “To speed up drying.” If you let it take its natural course, drying can easily take ten years, he says. “Here we succeeded in two years.” The removal of the water – through thickening and evaporation – went well, says Van Norel. However, the sludge lost its salt and organic matter less quickly.

Sake Elzinga’s picture

Bulldozer with crawler track

A full-scale test dike has been built at Deltares in Delft with the salty clay, in the large trench where they can mimic each wave. Waves two meters high pounded the test dike for hours. “The clay turned out to be less strong than standard clay,” says Van der Star. The original design of the dyke has been adapted accordingly, taking into account a probability of flooding once every 3,000 years. For example, the angle of inclination was slightly softened. “It makes you less susceptible to erosion.” Van Norel adds: “Finally, an additional layer of clay of at least 2.5 meters was placed on top of the existing dike.” It happened in 2022.

The three types of clay – the harbor mud, the polder mud and the salt marsh clay – have been applied to separate pieces. Decisive in this was the ‘good tamping’ of the clay, says Van Norel. A tracked bulldozer drove over the clay to compact it. It is close to. “The clay must not be too dry, because then you get lumps and cracks. But it must not be too wet either, because then it will behave spongy, and the clay cannot compress properly.”

In this way, we can hopefully let the coast grow along with the rising sea level

Walter van der Star Participant

The new dyke part is innovative in several respects. Sea dikes are often reinforced by removing the top layer of asphalt, applying an additional layer of sand or clay, and then placing a new layer of asphalt on top, with a tarry top layer, often bitumen. “But it doesn’t fit in with the landscape here,” says Van Norel. Along the Dollard nature reserve, the existing dyke is covered with grass – it also does not contain asphalt. “It saved us a lot of work that we applied the clay directly to the existing dike here,” says Van Norel. The fact that the clay comes close also saves a lot of transport, adds Van der Star. And then CO2emissions.”

But they keep talking about the broad, green dykes. Why green? Van der Star points to strips we pass on the dyke slope. No standard grass is sown, but a herbaceous mixture. “You don’t see much of them yet, but those species root at different depths.” It should better protect you against drying out and erosion.

Nature of compensation

The new dyke section is 24 meters wider than the adjacent sections and continues into the estuary – part of a protected nature reserve. Should it be compensated with nature elsewhere? “It wasn’t necessary for this pilot project,” says Van Norel. But if the experiment turns out to be successful and we extend the dike to 12.5 kilometers, then yes.”

Whether the concept will be successful remains to be seen in the coming years. The dyke can withstand a sea rise of half a metre, says Van Norel. “At least it’s enough for the next fifty years.”

There are also ideas for installing so-called culverts through the dike, adjustable connections that allow the tide to enter the dike at set times. The idea is that silt also comes with the water. Van der Star: “Hopefully we can let the coast grow along with the rising sea level and we can keep the northern part of the Netherlands going for a long time to come.”

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