“Open up!” shouts Marjan Kreijns, director of the open-air laboratory The Green Village (Tu Delft). A few seconds later, eight shower heads suspended from the frame of a party tent spray a heavy downpour onto a strip of pavement made up of different types of clinker bricks. “This is a T200 storm, like the one in Limburg,” says Kreijns. T200 means that the chance of such a heavy rain shower in the Netherlands is estimated at once every two hundred years. “But we can also simulate a prolonged drizzle here.”
The heavy, prolonged rain that fell last week in Limburg and the neighboring parts of Belgium and Germany was unexpected. “Until last week we thought: we can simulate it, but it is not happening in the Netherlands for the time being,” says Kreijns. “Now we have seen that it does occur and our research is suddenly extra important.”
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Green solutions for extreme weather due to climate change are being researched in practice at The Green Village on the campus of Delft University of Technology. Think of ways to quickly drain water from a street and store it in the ground, or of a method to have natural minerals filter the water for reuse – drought is also part of climate change. “This is often the first place in the Netherlands to test outdoors. But of course it doesn’t always rain, so we made this rain shower”, says Kreijns.
Storage for rainwater
The water from the rain shower pours onto grey, red and beige cobblestones with sometimes square or oval holes. The different stones are placed next to each other in strips. Their task is to let the water through as quickly as possible to a layer that actually retains the water, here consisting of ‘Drainmix’: waste from the PreZero (formerly Suez) incinerator in Rotterdam. Between these leftover pieces of glass and stone there is space that can be filled by the water. ‘BufferBlocks’, another solution for retaining water, is further down the road. These are concrete blocks with a large hole in the middle, which are placed in a row next to each other under the pavement. Because the holes come into contact with each other, they together form one space in which the rainwater can be stored.
The intention is that by combining water-permeable stones with a water-retaining layer underneath, the water does not remain in the street and the sewerage is less polluted. The water can drain from this buffer into the groundwater. Or evaporating through joints and thus also provide cooling. The solutions tested here are just starting to enter the market. “BufferBlocks are already located under the square at the entrance of Diergaarde Blijdorp,” says Kreijns.
Due to the hilly landscape, the extreme rainfall in Limburg had also become too much for the water storage place under the pavement, Kreijns thinks, “but in a heavy rain shower it can make a difference.” Under the shower heads – which alternately simulated drizzle and mega showers – it is now quite wet, but no large puddles are formed. The storage under the pavement and the gutter on the side, together prevented the street from being flooded.
If the street can’t handle the water, it must be stopped. In Limburg sandbags were used for this. “Old-fashioned”, says Kreijns. “It takes a lot of time to put the bags there – you don’t have that during a calamity – and now the Defense has to come and get them again. Moreover, the sand is polluted by the rising sewage water and must be disposed of as waste.”
Kreijns knows something better about this, which she shows just outside The Green Village. She gets on the bike. In a meadow south of the campus, ‘temporary flood defences’ are being investigated. A replacement for the sandbag, which can be put down in no time. So there are the BoxBarrier: plastic bins, which can be connected with an intermediate piece to a desired dike length. Filled with water, one tank would be equivalent to about fifty sandbags.
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The meadow is divided by dikes into several polders. The largest – in the middle – is filled with water from the ditch next to the meadow. Kreijns can have that water flow into another polder of his choice, in order to test whether a mobile dike can withstand a flood.
She sends the water to the Tubebarrier: a mobile dike that looks like a plastic trunk about ten meters long and about seventy centimeters high, held up with a kind of bent tent pole. The trunk would be good for six hundred sandbags. First, the water comes on top of a plastic flap that rests on the ground at the water’s edge of the trunk. The weight of the water keeps it in place. The water then runs in through small holes, giving the flood defense its strength. The water, as it were, stops itself.
The trunk and plastic bins have proven themselves and the entrepreneurs who invented them are now selling and scaling up. As a result, Kreijns followed the flooding with a feeling of impotence. “I almost threw the solutions we have here in a trailer to help.”
But because the entrepreneurs are still in the start-up phase, they were unable to have enough containers or meters of trunk made at once. “I hope that the water boards and government have now been shaken up and purchase our solutions so that they are ready in case of emergency,” says Raymond Hofer, director of BoxBarrier, pointing to his plastic bins that are in another polder.
If the water does manage to get past the flood defences, there is only one thing scientists can do: know when the situation becomes unsafe. Kreijns: “Research into the flood disaster of 1953 showed that most deaths were caused by collapsed houses, and not by drowning.”
In order to investigate when there is a danger of collapse, a piece of facade has been erected in a polder at the back of the meadow. Previously, a blind facade and a facade with a door were tested, now there is a part with a window. “Water puts enormous pressure on a facade, here we investigate when cracks start to appear.” The research set-up cannot be compared one-on-one with a house, but it is more realistic than inside a lab, where the researchers applied forces to the wall with an airbag.
As a victim, can you still do something if everything is under water? Kreijns: “It is better to open your front door during a strong flood to reduce the pressure on the facade.”
A version of this article also appeared in NRC Handelsblad on 24 July 2021
A version of this article also appeared in NRC in the morning of July 24, 2021