The Buëch is a gravel-bed braided river draining the Southern French Prealps, most of its catchment being included in the Hautes-Alpes department. It is one of the major alpine tributary to the Durance River, with a confluence at the city of Sisteron. The climate is Mediterranean, with a mean annual rainfall of ~800 mm. Heavy rainfalls typically occur during autumn, and secondary during spring. The hydrological regime is also influenced by a moderate spring snowmelt.
The study reach is located close to the city of Serres, downstream from the EDF dam of Saint-Sauveur, at an elevation of 640 m above sea level. This reach drains a 836-km² upland catchment with a maximum elevation of 2709 m (Pic de Bure in the Dévoluy Massif). The hydrological regime of the reach is impacted by the Saint-Sauveur dam, which diverted more than 75% of the natural flow; the guaranteed flow downstream from the dam does not exceed 2.5 m3/s. Although the Saint-Sauveur dam is equipped with 3 flood gates, allowing some sediment transport continuity during floods, most of the coarse sediments are trapped in the proximal part of the reservoir. Therefore, most of the sediment supply of the restoration reach comes from bank erosion downstream from the dam, since no major tributaries are joining the reach upstream from the Torrent de Channe.
The dying-off braided corridor of the Buëch River near Saint-Sauveur dam illustrated by aerial photographs comparison (1956-2006) (©IGN)
The restoration project
The restoration project of the degraded reach downstream from the dam includes an important operation of artificial gravel replenishment of 44 000 m3, implemented in September 2016. Replenished gravels were directly dredged from the alluvial fan of the Buëch forming into the proximal part of the St Sauveur reservoir. The increase of sediment supply is expected to induce a raise of the bed-level (e.g. increase of sediment storage), and the spontaneous development of macroforms like riffle-pool-bars, typical of braided river patterns.
A dedicated monitoring program is active to capture the geomorphic and biological responses of the degraded reach to the artificial gravel recharge.