Restoring bedload dynamics in sediment-starved rivers: Gravel reintroduction design, monitoring, and lessons learned from two French rivers
Résumé
Adding artificially bed material to the channel—a practice termed sediment augmentation or sediment reintroduction—is increasingly applied to mitigate adverse effects of human pressures on gravel-bed rivers, e.g. damming, gravel mining or channelization that cause riverbed degradation and ecological alterations. These issues are critical in France where several rivers have been trained over the last two centuries, disturbing both sediment fluxes and hydrological regimes. The European Water Framework Directive (2000/60/EU) required for water bodies to achieve good status by 2015. This paper presents gravel reintroductions conducted on the Ain and the Rhine rivers with the purpose to restore bedload dynamics and ecosystem quality downstream of dams which block bedload transport. Since 2005, 95,000 m3 of gravels have been supplied to the lower Ain valley to mitigate a 15,000 m3 y.-1 sediment deficit along a 25-km-long reach below the Allement dam. Geomorphic monitoring was implemented on some pilot sites to assess the effects of gravel input. The Rhine River experiment benefited from the Ain feedback. In 2010, a 25,000 m3 gravel reintroduction test was carried out into a 50-km-long by-passed reach downstream of the Kembs dam. Changes in channel morphology and potential environmental risks were assessed based on six successive surveys (before-after gravel reintroduction, and after Q2 and Q15 floods) coupling airborne imagery, topo-bathymetric surveys, radio-frequency particle tracking and channel substrate sampling. Monitoring and evaluation of such local gravel reintroductions allowed large scale and sustainable restoration plans to be outlined on the Ain and the Rhine rivers. Reflections are in progress on: (i) the most appropriate sediment source related to the restoration strategy, e.g. coupling gravel reintroduction with channel widening, former channel dredging or floodplain lowering, (ii) the modalities of sediment reintroduction (volume, grain size, location for gravel input), and (iii) the monitoring program to implement (suitable evaluation indicators) in an adaptive management way.