Flash floods engulfing cities and washing away entire communities are yet another reminder of the climate crisis. But are we bound to live with the ever-present danger of deadly downpours? And how can cities prepare for the next big disaster? We take a closer look in this edition of Down to Earth.
Catastrophic floods are the reflection of our changing climate. As temperatures rise, more water gets evaporated and stored in the atmosphere, unleashing torrential rains and wreaking havoc along the way. In Europe alone, the maximum amount of daily rainfall has increased by 3 percent to 19 percent. Such weather events are becoming more extreme but also more common, climate change having made deadly floods in the continent up to nine times more likely.
'Rebuild with the water, not against it'
On the night of October 2, 2020, the small village of Saint-Martin-Vésubie in the south of France was almost entirely swept away by lava-like mud flows of rocks and debris.
Xavier Pelletier, the deputy prefect in charge of reconstruction, remembers that night, when 600 millimetres of rain fell in just 12 hours, the equivalent of 600 litres of water per square metre. "That’s huge!" he says. "In terms of damage, this is the biggest disaster to hit France since the Second World War."
Fast-forward to today and the reconstruction process has only just begun. Dykes have been reinforced, riverbeds kept as wide as possible, and certain areas bordering the river are no longer suitable for housing.
Pelletier says that if there's one lesson to learned from this disaster, it's that of learning to live with the risk of flooding: "We need to rebuild with the water, not against it."
Protecting Paris at all costs?
Built on the River Seine, Paris and its suburbs are at risk of flooding. The devastating floods that hit the City of Lights in 1910 may be a distant memory, but the deluges of 2016 and 2018 are fresh on people's minds. The stakes are particularly high for Paris, the capital being France’s economic heart. To prepare for the next big one, the city has built four reservoirs capable of storing 800 million cubic metres of water. A fifth reservoir is in the works and could provide 55 million cubic metres of additional storage.
"Essentially, we empty the Seine of some its water," explains Frederic Darsaut, who's in charge of the project. The reservoir will fill up in 96 hours and once the storm has passed, the water will return to the Seine.
"It’s clever. The system dates back to the floods that hit Paris in 1910," he adds.
As a whole, the five reservoirs could limit the amount of flood water by almost one metre and avoid millions of euros in damages.
Waterproof cities
To face the threat of flooding, cities will also need to get a massive revamp, starting with the pavements. When it rains heavily, water falls directly onto waterproof concrete. The sewer system is supposed to capture all this water, but it can easily get overwhelmed during rainstorms, transforming our streets into rivers. Could permeable concrete be the answer?
French cement maker Lafarge has created a type of concrete capable of absorbing large quantities of water. It is made of pebbles and a cement paste that binds the grains together, leaving small cavities inside. Just like a piece of cheese filled with holes or a sponge, the water will trickle through it.
According to Sébastien Renard, a development manager at Lafarge, this type of concrete can "absorb about 100 litres of water per square metre, per second".
