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.
Header image: Artificial gravel replenishment downstream from Saint-Sauveur dam ; 44 000 m3 of coarse sediment has been reinjected in September 2016 (©EDF)
General relief map of the Buëch catchment, main physical features at the restoration site, the EDF Saint-Sauveur dam and the “hungry water” impacted reach downstream from the dam (©Irstea).
Like most of alpine braided rivers in France, the Buëch has been highly impacted by intensive gravel mining since the late 1960s (Gautier, 1994: Liébault et al., 2013). A recent investigation about the bedload transport management of the river provides an estimate of 9.7 Mm3 (Hydrétudes, 2013). In addition to gravel mining, the bedload transport continuity has been strongly impacted by the construction of the Saint-Sauveur dam (1990-1991), the last hydropower dam deployed in the French Alps; the construction necessitates the dredging of 600000m3 of sediment.
As early as 1993, only few months after the dam commissioning, the formation of an entrenched single-thread channel in the former braided corridor downstream from the dam was observed, highlighting the strong and rapid geomorphic effect of the dam (Gautier, 1993). Upstream from the Saint-Sauveur dam, the Buëch River is also locally regulated by embankments.
These human alterations of the sediment regime resulted in important channel responses, like active channel narrowing, attested by historical aerial photographs (Fig. 4), and channel degradation, as attested by the historical long profile of 1908 (Liébault et al., 2013). A shift from a braided to a wandering pattern can be clearly observed along several reaches, including the study reach. Downstream from the dam, the incision reaches 3 m, and propagates downstream (Hydrétudes, 2013).
The dying-off braided corridor of the Buëch River near Saint-Sauveur dam illustrated by aerial photographs comparison (1956-2006) (©IGN)
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.