How lessons learned from preliminary field observations of pioneer spontaneous vegetation can inform civil engineering projects: what are the revegetation and stabilization potentials of the Sélune Riverbanks prior to dam removal?

Abstract

The restoration of ecological connectivity along the Sélune River (Normandy, France) involves the removal of two tall hydroelectric dams (36 m and 16 m high), a project without precedent in Europe. In the pre-removal phase, the reservoir's slow draining (2014–2018) allowed to conduct a showcase engineering project focused on sediment management in order to avoide excessive delivery to the river's downstream sections. Progressive spontaneous revegetation of the Sélune Riverbanks, as sediments were exposed, has been surveyed since 2015 in accordance with the modalities for conducting civil engineering works aiming to stabilize sediments. The lessons learnt can inform the ecological restoration approaches to be implemented. A great diversity and a habitat mosaic characterize the spontaneous vegetation colonizing an old reservoir (90 years old). Ecological functions provided by this vegetation highlight an opportunity to consider the options of a full and fast revegetation and a sediment stabilization over several dozen hectares. However, the season when the water level decreases affects patterns of this initial development. Moreover, riverbank profiling works that were executed very belatedly negatively affect the plant community's trajectory. The relevance of considering preliminary field observations along reservoirs can help limit profiling works, which goes against the advantages obtained after 2,5 years of passive ecological restoration.