This study was aimed at untangling the relative impacts of successive phases of human modifications on changes in bedload transport along a 430 km-long river reach: the Rhône River from Motz dam to the sea. We used a 1D hydraulic model to solve for water lines across a range of discharges and all along the reach. Next, using grain sizes measured in the channel, we estimate flow competence and mean annual bedload transport capacities using the Recking (2013) bedload transport equation. In addition, we used the Generalized Threshold Model to estimate the relative fine and coarse fractions of the load. Bedload transport estimates were carried out under present-day hydraulic conditions and compared to estimates based on model runs simulating an unimpeded flow regime and using grain sizes measured in bars as a proxy for conditions prior to armouring. Our results show that present-day bedload transport along the Rhône is significantly fragmented by multiple closely spaced dams. Mean annual bedload capacity varies between 2500 and 16,300 m3/year over all the reaches, with an average of 4700 m3/year. Results of the GTM analysis suggest that this load is composed of 89% fines. We find bed sediment mobility to be very low in most reaches, and that potentially mobile sediments are finer than the median grain size in the riverbed even at high flows. Our results suggest that bedload capacities were 25-35 times higher prior to bed armouring and flow modifications; dams had an impact 2-3 times more important on transport capacities than channel embankments, and bed armouring was foremost a response to channel embankments. Based on an analysis of the ratio of sediment yields to transport capacities, we propose a conceptual scheme illustrating how bedload supply, channel morphology, and surface texture coevolved in the Rhône over the past century and half.
Keywords: Bedload; Dams; Rhône river; River embankment; Sediment transport.
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