375 municipalities were recognized as being in a state of natural disaster in France after the floods of October which hit the South and Center-East of the country hard. And again, it was only one of the disastrous episodes of a particularly rainy year 2024. Paris remained spared, with only a few metro stations flooded by rainwater. But the threat of a historic flood like that of 1910 hovers above the capital. This is the Metropolis of Greater Parisan intercommunity of 131 municipalities and 7.2 million inhabitants, which has been in charge since 2018 of the Management of Aquatic Environments and Flood Prevention (known as GEMAPI). Didier Gonzales, mayor of Villeneuve-le-Roi and GEMAPI advisor, explains the action plan to contain the Seine.
Capital: What assessment do you draw from last October, which was particularly rainy?
Didier Gonzales : We must distinguish two things. Some communities were flooded due to runoff caused by heavy rainfall. This is a phenomenon that can be combated by de-waterproofing the soil and creating temporary storage areas in the roads, for example. And there are floods due to river flooding, which is completely different. They occur when the level of the watercourse rises suddenly and ends up overflowing. On this level, Paris was spared last October. I see this as proof of the effectiveness of our centralized management well upstream of the Seine, in collaboration with the river unions. But we are looking to improve the system in the event, one day, that all the tributaries find themselves with a flood peak at the same time. This is what happened during the historic floods of 1910.
© Metropolis of Greater Paris
– The Greater Paris Metropolis has 131 municipalities and 7.2 million inhabitants.
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What does your prevention system consist of?
First of all, there is the public establishment Seine Grand Lacs which manages four large reservoir lakes: Der, Orient, Amance-Temple and Pannecière, in the Grand Est and in Burgundy-Franche-Comté. They make it possible to regulate the flow of the Marne, the Seine, the Aube and the Yonne by retaining water during floods. They are able to store 830 million cubic meters and could reduce the level of the Seine by 70 cm in the event of a 100-year flood in inner Paris.
You also encouraged farmers to accept flooding…
Yes, we sign agreements with chambers of agriculture so that farmers accept that their land is over-flooded during floods over a fairly short period of time, in return for compensation of 1000 euros per hectare. These are called flood expansion zones (ZEC). And it's a big help to get through the peaks. We also manage 100 kilometers of dike structures along the Seine to prevent water penetration. Each department had its own protection system with jacks, boards and plaster or even sandbags. We are in the process of unifying everything.
And when will this large retention basin project come into operation?
The inauguration is planned for February 2025. It is a huge basin, in Seine-et-Marne, which will allow us to act directly on the Seine. It will be able to store 10 million cubic meters. Enough to reduce the flood level by up to 15 cm downstream. We financed it to the tune of 39 million euros out of a total budget of 114 million euros. If the exceptional conditions of 1910 were to recur today, we would be much better equipped to face them. But above all I want to insist on the need to have a global vision. It's not just about preventing floods, we also need to better manage aquatic environments. This involves maintaining and monitoring waterways all year round. This will also allow us to prevent them from becoming dry in the summer as much as possible.
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