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How can we prepare for extreme flooding?

July 19, 2021

As the floodwaters recede following the devastating deluge in western Europe, survivors have begun the long, difficult process of cleaning up and rebuilding. But what can be done to reduce future risks?

Jutta Schnelleckes sits in the living room of her apartment, which was completely destroyed by the flood in Bad Neuenahr
Thousands of people have seen their homes destroyed by last week's floodsImage: Thomas Lohnes/Getty Images

Extreme flooding in western Germany and neighboring Belgium has killed nearly 200 people and left dozens still missing. With recovery operations still underway, talk has now turned to how communities can prepare for the extreme weather events expected to become more likely with climate change.

On a visit to the disaster area, Armin Laschet, the conservative state premier of the hard-hit German state of North Rhine-Westphalia, said more needed to be done to protect people from the effects of the climate crisis. "We will be faced with such events over and over, and that means we need to speed up climate protection measures, on European, federal and global levels," he said.

DW speaks to residents in Erftstadt

But, for experts like Lamia Messari-Becker, a civil engineering professor focused on sustainable building and design at the University of Siegen, now is not the time to talk about vague climate change adaptation.

"Now is the time for engineers," she told DW. "We need real help, ideas, solutions. We can't rely on the normal processes or procedures now. We are dealing with an exceptional situation."

The German government has already announced a financial aid package to help support the huge task of rebuilding private properties and public infrastructure such as roads, highways, and communication and power networks.

People stand near a collapsed older home in Ahrweiler
Centuries-old homes were not built to withstand such volumes of waterImage: Harald Tittel/dpa/picture alliance

Building back better

When it comes to adapting buildings to withstand floodwaters, Messari-Becker drew parallels with earthquake-resistant architecture. In such buildings, the depth of the foundation, structural design and building materials are specifically chosen to be able to handle extreme flooding. Many collapsed homes in the worst-affected regions were hundreds of years old, built around a wood frame not able to withstand masses of water.

"That's exactly how we have to operate here, when we're dealing with such amounts of water," she said. "We need to reinforce basements so that they can also fill up with water and people can quickly get to safety. It's also about the reinforcing measures needed for outer walls, for roofs."

A map of the flooded regions

Boris Lehmann, a professor in hydraulic engineering at the Technical University of Darmstadt, said measures such as retention valves on sewage connections, which prevent floodwaters from backing up into homes, and waterproofing windows and doors on the lower levels of buildings were also essential.

"Our damage evaluations show that private precautionary measures can significantly reduce flood damage," said Annegret Thieken, a professor who focuses on natural hazards research at the University of Potsdam. She also pointed out the need to secure potentially destructive elements like fuel tanks used to heat homes.

"Fuel oil can penetrate deep into the masonry and also damage neighboring buildings," she said. "In severe cases, oil damage can make buildings uninhabitable. Flood proofing can prevent oil tanks from heaving up, reducing damage to buildings and the natural environment."

An aerial view shows people as they inspect the emptying of the damaged Steinbach hydrolic dam in Euskirchen
Filled to the brim, the Steinbach dam in Euskirchen is still at riskImage: Sebastien Bozon/Getty Images/AFP

Weatherproofing cities

It's not enough to just focus on buildings. Cities and other urban areas need to think about controlling the water before it has a chance to flood basements in the first place, by reinforcing reservoirs and dams that can help absorb sudden surges.

Last week's floods showed that small streams in narrow valleys, where the water doesn't have much room to spread out — like in the devastated Ahr region south of Bonn — can turn into deadly torrents within hours. In such places, said Messari-Becker, dams and dikes need to be raised and expanded to better protect cities from high water levels.

Floods in Belgium

She warned, however, that this won't be cheap — simply extending a dike, for example, can cost at least €1 million ($1.2 million) per kilometer. "And the narrower a valley is, the more costly these measures are," she said.

"In order to effectively protect infrastructure against such extreme events, the current design of our water management and hydraulic engineering systems are not sufficient — as the current dire consequences have shown," Lehmann said. Experts have stressed the urgency of future-proofing aging infrastructure over the next decade.

But, Lehmann pointed out, we can't just expect better building measures to solve all our problems. "From a technical, economic and practical point of view, it's not possible to completely reassess, reconstruct and thus protect all elements of our built environment and infrastructure due to such extreme weather events," he said.

The skyline of Dresden along the Elbe River showing a green flood plain
After devastating floods in eastern Germany, the city of Dresden expanded flood plains Image: Sylvio Dittrich/imago images

Working with nature, not against it

That's where planners and engineers will have to find ways to work with the natural world, rather than trying to control it. Wherever possible, said Messari-Becker, waterways should be allowed to flow as nature intended, and not be altered or straightened. Doing so concentrates and further accelerates the volumes of water during a flood event, she said.

Instead of confining rivers, levees should be moved back to make space for flood plains — wide open green spaces which can serve as overflow reservoirs during floods. Such places were expanded along the Elbe River in eastern Germany, following several destructive flooding events in the early 2000s.

Another approach is to make urban areas more permeable, so that water is more easily absorbed over a wider area and not concentrated in specific spots. According to the Federal Environment Agency, 45% Germany's residential and traffic areas have already been covered with concrete or asphalt. As a result, water can't naturally seep into the ground, leading to overflowing sewage systems and an increased risk of flooding.

The town of Leichlingen, southeast of Düsseldorf, has been hit by severe flooding several times in recent years, including last week. To ease the stress on their water management, they are aiming to make use of a new planning model known as a "sponge city." 

The idea is to channel rainwater from roofs, squares and streets into grass-covered ditches at the side of the road. Excess water would then be allowed to drain away naturally and add to the local groundwater, reducing the load on water management infrastructure. Backup cisterns would also be installed to collect overflow and could be used to water the city's green spaces.

A car flooded almost to the roof
People need to know how to react in extreme flooding eventsImage: David Young/dpa/picture alliance

Preparing people for the worst

Improving infrastructure and water management systems won't help if people don't know how to react when faced with a wall of water. Which is why Lehmann, the hydraulic engineering expert at the Technical University of Darmstadt, stressed the need for an increased public awareness.

"Especially in the case of flash floods caused by extreme weather, there's not just a lot of water — there's also a great deal of floating debris, garbage and other things moving with the water," he said, adding that people who go into these waters risk drowning and being crushed. He said ongoing education campaigns were necessary to teach the public how to react in extreme situations — for example, how to escape from a car caught up in a current.

"'Run away from the water and get to safety as quickly as possible' — we should start teaching such rules of conduct as early as elementary school," he said. "In the case of emergency, it can save lives."

In the extreme case, people will have to reconsider where they're living in the first place. Instead of rebuilding in the same location some might be forced to go for higher ground, away from potentially dangerous flood zones. Some areas might no longer be tenable.

But, Messari-Becker said, if the necessary investments in protection measures are made quickly and effectively now, it might not be too late.