Grenoble now less flood-prone? | ECHOSCIENCES

Grenoble is one of the rare metropolises, perhaps the only one, to have four “chances” of being flooded: by its rivers, the Drac and the Isère, by the rivers coming from Belledonne (the Sonner) , plateaus (the Verderet), and by “water table floods” (water table rise).

These risks have led to improvements which have reduced them and that of Isère has just been greatly reduced. The return of a flood like that of November 2, 1859, the last to have submerged the city, estimated at 2,000 m3 / second, would be better controlled today.

The 1859 flood

Since 2009, the SYndicat Mixte des Bassins Hydrauliques de l’Isère (SYMBHI), has led upstream of Grenoble, to the limit of the department, improvements aimed at restoring “a controlled life” to the overflows of the river, rather than ‘to try to contain the water between the dikes at all costs.

Protection is ensured by Controlled Flood Fields (CIC), which consist, by finely calibrating the height of the dikes, in diverting the surplus water towards agricultural areas, natural spaces, and, thus, in clipping the peak. flood. Sixteen CICs, capable of storing 35 million m3, preserve Grenoble and its surroundings against major floods with ten-year, thirty-year or two-hundred-year return periods.

At the same time, improvements were made in the river, on the banks and in the plain. In the river, the work consisted of calibrating the bed (leveling the banks), in order to allow the passage of a thirty-year flood, above which the discharge to the CICs begins. On the banks, the work consisted in reinforcing the dikes (insertion of sheet piles), in reworking their coverings, in order to allow the circulation of soft modes.

In the plain, the removal of the dikes made it possible to reconstitute “alluvial forests”, flooded during weak floods. Likewise, the water bodies, created by the extraction of materials, are now integrated into the fight against floods. Completed by major replanting, the work is also helping to restore the continuity of biological corridors, while ensuring the protection of infrastructures such as the railway or the highway. All in all, innovative developments in more than one respect which testify to a strong public commitment in terms of safety and environmental quality.