Gravel replenishment and active-channel widening for braided-river restoration: The case of the Upper Drac River (France)

Massive gravel replenishment combined with active-channel widening could theoretically improve the morphological recovery of altered braided rivers but this restoration strategy was not yet tested in the field. A recent braided restoration project based on this principle was set up to restore a 4.2 km long reach in the Upper Drac River (French Alps) using 355.000 m(3) of gravels to rise the bed level and to design a 100-m wide trapezoidal cross-section. The aim of this paper is to capture the morphological trajectory after restoration in order to evaluate efficiency and sustainability of this strategy. A Before and After Control Impact monitoring design has been used by combining a repetitive topographic survey (using airborne LiDAR data and terrestrial topographic surveys along cross-sections), an assessment of bedload supply to the restored reach using sediment tracing and active-layer surveys, and a systematic analysis of historical aerial photographs. In a particular context of low hydrological forcing after restoration, the restored reach adjusts with local braiding reference that highlights the efficiency of the restoration strategy. Despite this spontaneous braiding recovery, scouring processes are observed locally along the restored reach and the sustainability remains uncertain even if a good connection to sediment sources was observed. Feedbacks make it possible to propose recommendations to river managers who plan to use similar strategy of braided river restoration. This field study demonstrated for the first time that sediment replenishment combined with channel widening can be an efficient solution for the spontaneous recovery of braiding conditions in altered alpine gravel-bed rivers. (C) 2020 Published by Elsevier B.V.

Automated summary

This study investigates the effectiveness of using gravel replenishment and channel widening for restoring altered river morphologies. The results show that, in a context of low hydrological forcing, the restored reach gradually adjusts to local braiding characteristics, but still faces local scouring and sustainability issues.