The sea level is rising and the soil is sinking. That is why at Bierum in North Groningen, experiments are being carried out with a dike under which the sea can flow. It is precisely by allowing flooding that the land behind can grow along with the rise in sea level, or so the idea is. Is this the solution for the coastal provinces?
Bierum is located on the Eems-Dollard, which is connected to the Wadden Sea. A small dike section is now being used to test various new ideas. For example, the dike is being reinforced with dredging sludge and other vegetation is being tested.
But the most remarkable thing is that the dike is permeable. “Small openings are made in it,” Peter van Dijken of the province of Groningen tells NU.nl.
Those openings are culverts: long tubes with a valve in them. If those valves are open, the sea can flood the land behind at high tide.
Land sinks, while the sea rises
The openings seem to conflict with the original idea of the dike, which is supposed to prevent flooding. But in addition to safety, the permeable dike must also offer a solution to another problem that dikes themselves cause: soil subsidence.
The Netherlands has been largely formed by flooding of the sea and rivers. Every high tide and every high tide brought clay and sand with it. For example, the Netherlands was raised layer by layer and for centuries the land remained higher than the (average) level of the water.
Since we have dikes, the opposite has happened: the land behind the dikes is sinking deeper and deeper. Clay sinks in, and peat dries out and evaporates. The problem of subsidence is exacerbated by sea level rise, which is accelerating.
A dike can stop the water, but not the salt
The polders behind the dikes are becoming increasingly deeper, more vulnerable and more expensive. Pumping stations have to pump harder. That costs more energy and the risk of accidents increases. Moreover, these pumping stations eventually draw the salty seawater under the dike through the polders. This puts an end to existing agriculture.
Then we might as well let that sea in every now and then. This also offers perspective for agriculture: fresh marine clay is rich in lime and other minerals. “Farmers are queuing up to raise their fields,” says Van Dijken.
This partly concerns new forms of agriculture, such as ‘salt cultivation’ of potatoes and vegetables such as sea kale. Space has even been made for shrimp fishing in the Dollarddijk. And sheep can graze on flooded grassland, for example.
Dollard has too much silt, other coasts too little
At the Dollarddijk, the divers have to kill two birds with one stone. The sinking land can make good use of the fresh layers of silt, but it should also help to reduce the silt concentrations in the seawater. As a result of diking and dredging work in the Dollard, these are so high that they suffocate the underwater life.
At Bierum, the permeable dike is therefore not a bad idea. But it is only 750 meters long, and the section behind it is narrow. What about when we zoom out? The Netherlands has a total of 523 kilometers of seacoasts, with large polder areas behind them. The same problem also occurs further inland, along river dikes.
Looking at sea level rise, you would prefer to apply this on a large scale, says professor of physical geography Maarten Kleinhans of Utrecht University. “But it can’t be done everywhere, and it doesn’t make the same sense everywhere.”
No solution for the whole of the Netherlands
You need places with fluctuating water levels and sufficient silt, says Kleinhans. According to Kleinhans, the most suitable is the coast along the Eems-Dollard in Groningen and the Western Scheldt in Zeeland. These are places where a river (the Ems and the Scheldt, respectively) flows into the sea. Fresh and salt water mix there, there is a lot of silt in the water and the tidal range is high.
Further along the coast you can let the sea in, but the accretion will be much slower, and probably will not be able to keep up with the ever faster sea level rise.
“So this can help to strengthen a few weak spots here and there along the coast,” says Kleinhans. But we need a solution for the entire coastal system. And the faster the sea level rises, the more difficult that will be.
Export knowledge abroad
Van Dijken says that there is a lot of international interest. For example, delegations from other European delta areas will visit Bierum next year, including a delegation from Venice and the estuaries of the Rhône in France and the Ebro in Spain.
“They are also experimenting with this in Bangladesh,” says Kleinhans. And he thinks it’s also interesting for the Mississippi delta. There the water has even more free rein to raise the land.
For example, the permeable dike in Bierum can provide valuable knowledge for all kinds of countries. But we do not (yet) have a conclusive solution for the long-term future of our own country.
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