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Ecological restoration of rivers

The example of the Mérantaise

In the department of Essonne, in Gif sur Yvette, renaturation work along almost two kilometres of the Mérantaise river has been completed. This work will promote ecological continuity as well as combating flood risk. It was the frequent floods caused by the Mérantaise river, a tributary of the Yvette, that inspired the project. In 15 years, the city of Gif-sur-Yvette has experienced five natural disasters.
Today, thanks to this rehabilitation, the river can now absorb a fifty-year flood.


A left-bank tributary of the Yvette, the Mérantaise was identified as a still largely pristine river. It was also classified as a biological reservoir and still hosted functioning populations of brown trout (Salmo trutta). The downstream section of the Mérantaise was partitioned by numerous hydraulic structures, with a resulting negative effect on ecological continuity. Concerned about the impact of these works on the overall functioning of the river, the SIAHVY (Intermunicipal Association for the Hydraulic Development of the Yvette Valley) decided to implement an ambitious operation to restore ecological continuity. Egis provided full EPCM services along with Sepia GC (for the geotechnical side).

  • Six hydraulic structures, sluices in particular, were removed
  • 9 metres in height difference were put right.
  • the bed of the Mérantaise was placed back at the bottom of the valley
  • forestry work including felling and pruning accompanied the creation of a wet meadow bordered by groves of ash and willow (beneficial to all: walkers, animals, plants). We are now expecting to see the return of animals who favour these ecosystems, such as the wagtail, the Sedge Warbler, the Green Frog, and more.

With the rehabilitation and expansion of the stilling basin, the river can now absorb a fifty-year flood.