Could such a phenomenon occur one day in Bordeaux?

A flash flood, torrents of water and mud in the streets, residents surprised in their cars and a catastrophic human toll. Could the extreme climatic event of “cold drop”, where the equivalent of a year's rain fell in a few hours in Valencia, in Spain, occur in Bordeaux, in Gironde? What would be the consequences? Could we observe, one day, the same images as in Spain?

From a meteorological point of view, such a quantity of rain falling in such a short time in Bordeaux has never happened. That doesn't mean it won't happen, especially with global warming bringing more localized, frequent and violent storm cells. In June 2021, exceptional amounts of rain during a storm (80 liters of water per m² in less than 12 hours) quickly flooded the town of Saint-Germain-du-Puch, in Libournais, around twenty kilometers away. from Bordeaux. “If such a quantity were to fall on Bordeaux, the current water evacuation network would not be able to support it,” assures Nicolas Dion, president of Endeo Environnement, a Bordeaux consulting firm specializing in hydrogeology and hydrology.

The danger of localized torrential rains

However, images of torrents of water in the streets like in Paiporta, near Valencia, seem unlikely on the left bank of Bordeaux, because there are not as many differences in height. “The most exposed sectors would rather be the right bank, such as the lower part of Cenon, Bouliac or Floirac, where there could be a higher runoff coefficient,” he specifies. The topography and developments in Bordeaux are also not the same as in the conurbation of Valencia, where the diversion of the river, the Turia, has preserved the city center but has worsened the situation for the municipalities further south. There is no work of this type in Bordeaux.

The management of flood crises linked to exceptional torrential rains, however, remains poorly documented in Bordeaux. Hydrological studies with different scenarios are underway, in a context where public authorities have recently become aware that soil sealing is an aggravating factor.

There is a real issue about Bordeaux land, with a lack of land available to design prevention works”

“There is a significant sanitation network installed in the town [Ramses, NDLR]with new retention basins like in Nansouty. But at the same time, there is a densification of habitats and roads, which encourage runoff. There is a real issue about Bordeaux land, with a lack of land available to design prevention works,” according to Nicolas Dion, who works with several municipalities on the subject.

Known river-sea floods

In Bordeaux, the most frequent and documented risk is above all that of a river-sea flood. That is to say a flood of the Garonne linked to a significant river flow, a high tidal coefficient (often the case at the March equinox), intense winds and high atmospheric pressure. According to the Flood Risk Prevention Plan (PPRI) of the Bordeaux conurbation, certain districts of Bordeaux such as the right bank, Bacalan and Chartrons can be flooded up to 5.5 meters/NGF (above the general level in France). The towns of Bègles, Bassens, Bruges and Ambarès-et-Lagrave are also affected.

“This modeling is, however, based on ten-year and hundred-year floods, as we have already experienced. But with climate change, these data could be much higher,” explains Virginie Perromat-Malikité, risk culture and resilience study manager for the French Association for the Prevention of Natural and Technological Disasters (AFPCNT).

“In the event of a river-sea flood, the rise in water levels would be quite slow, over several hours, with a forecast system (Vigicrue) which would make it possible to alert residents, in addition to the Municipal Protection Plan (PCS). It is not as brutal a flood as in Spain or what happens in the south-east of France. » The city center of Bordeaux would rather be spared, as in Valence, thanks to existing works.

Buffer zones

On the other hand, certain small municipalities located outside the urban area, which are buffer zones, such as those in the South-Médoc or to the south of the metropolis of Bordeaux, would be very exposed. The town of Macau, located in the middle of the Garonne flood zone, would be one of the first affected, as would Labarde, Ludon-Médoc, Parempuyre and Blanquefort. A submersible dike was also built on the left bank (with a height of 4.35 m/NGF). It prevents river-sea flooding to a certain extent, to prevent excess water from flowing into the much more densely populated Bordeaux area.

“We can protect ourselves, but not too much, because the water must overflow here in the event of a major flood in Bordeaux. This is anticipated thanks to the RIG tool (Gironde flood reference), in particular with scenarios of erasing the dike [rupture non réparée, NDLR]. Residents are aware of the risks,” assures Chrystel Colmont-Digneau, mayor of Macau.

We can protect ourselves, but not too much, because the water must overflow here in the event of a major flood in Bordeaux”

“What we have not yet measured is the torrential rains, which affect residents near small rivers rather than those on the banks of the Garonne. In two or three hours, we can be flooded downstream with torrents of water coming down from the ditches, whereas before, the water took between twelve and fifteen hours to drain,” she warns. A municipal rainwater management plan has been launched but still remains theoretical.

The worst scenario would be exceptional torrential rain combined with a significant river-maritime phenomenon. The Garonne would swell and prevent the flow of water from the evacuation network, already saturated. In this exceptional case, the rise in water levels would be rapid and the risk would become major for the entire Bordeaux metropolitan area and the municipalities located in the buffer zones. This is what happened during the “flood of the century” of March 1930, causing the death of around 200 people throughout the South-West.