Tunnels, basin… Nîmes invests in anti-flood infrastructure to limit damage

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Dominique Dussenne has one worry: ending up with his house completely flooded. In Nîmes (Gard), this already happened in 2015 in his neighborhood. So, to protect himself, the town hall offered him equipment. “I put this there, I swell”he explains to us. A cofferdam that attaches to the front door frame. Waterproof protection, security for this Nîmes: “For me, in any case, it reassures me. When it rains too hard or whatever and we are really given warnings in the Gard, we put that in place, systematically”he assures.

And he is not the only resident who is concerned about it. In Nîmes, half of the population lives in a flood zone. Forty years of rain, the streams that cross the city swell, the rivers overflow their banks and overflow. Like on October 3, 1988, torrential rains engulfed the city in just a few hours. The toll is heavy: 11 dead, 45,000 victims and trauma that remains. Since then, floods have been recurring, like in 2005 or 2014.

Today, the municipality is pulling out all the stops. She decided to dig a gigantic tunnel under the city to protect herself. A first in France, with one objective: to evacuate rainwater through the basement as quickly as possible. “We are between two and six meters underground. We are passing under part of Nîmes”explains Guillaume Huyghues-Despointes, works director – Entreprise Razel-Bec. Within two years, the pipe will be operational. “We are inside a tunnel of 3.30 meters in interior diameter. The objective of these tunnels is simply to pass the water which today, during the Cévenols episodes, passes through the streets. Simply to pass it under the city”he emphasizes.

This tunnel aims to improve an already existing system. Today, in the event of heavy flooding, the water flows into underground galleries called “gifts”. Their role? Capture rainwater to evacuate it further away, outside the city. But their capacity has become insufficient. The water saturates the galleries, rises and floods the streets. The work aims to expand these galleries. To make it a huge tunnel of more than a kilometer, it must multiply the water flow capacity by 10. Cost of the operation: 52 million euros.

A colossal investment, accompanied on the heights of the city by the digging of an enormous quarry. Here, the largest retention basin in Nîmes is taking shape. “We are going down to the bottom of a hole that is more than 30 meters deep and more than 500 cubic meters of hydraulic storage, that is to say the equivalent of almost 200 Olympic swimming pools”indicates Jean-Luc Nuel, head of the Flood Prevention Service for the Nîmes Metropolis.

In this basin, rainwater will be stored, enough to protect more than 34,000 residents. “We have this large tank which brings back all the runoff water from the entire basin and which fills the quarry. And so, once the event is over, we have this volume of water which we pump out very gradually to be able to discharge it into the natural environment”he continues.

Five more years of work before this basin is operational.