Effects of continuous embankments and successive run-of-the-river dams on bedload transport capacities along the Rhône River, France

Abstract

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 m 3 /year over all the reaches, with an average of 4700 m 3 /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.

Legacy sediments in a European context: The example of infrastructure-induced sediments on the Rhône River

“…connectivity status index ≥ 95%) in Europe. While the consequences of such river manipulations have often been studied in terms of main channel adjustment (e.g., incision, narrowing, armouring, stabilization: Surian and Rinaldi, 2003 ;Zawiejska and Wyżga, 2010 ;Arnaud et al, 2015 ;Habersack et al, 2016 ;Downs et Piégay, 2019;Vázquez-Tarrío et al, 2019), their potential role in creating new floodplain features due to legacy sedimentation has been less investigated, and the characteristics of these sediments have rarely been studied.…”

Long-term river management legacies strongly alter riparian forest attributes and constrain restoration strategies along a large, multi-use river

“…Conversely, a few very large pioneer trees dominated the dike field stands, indicating a lack of flood disturbance and subsequent regeneration on these surfaces (Vázquez-Tarrío et al, 2019), along with an early maturation of these stands to their maximum potential biomass. Thus, despite large variations in surface age among dike field sites (i.e., range 22-135 years), large pioneer trees already dominated the youngest surfaces (Janssen et al, 2020), indicating high growth rates of Salicacea species in these resource-rich environments (Karrenberg et al, 2002).…”

“…Of these, sixteen dams feature diversion canals, excavated in parallel to the natural river course, creating secondary bypass channels in these reaches (total length = 162 km, Lamouroux et al, 2015). The diversion dams distribute the largest part of the discharge into the canals, while the bypassed reaches receive only a minimum baseflow discharge (i.e., residual flow is 10 to 30% of the mean annual flow arriving at the upstream diversion dams) (Vázquez-Tarrío et al, 2019).…”

Quantifying the Variability in Flow Competence and Streambed Mobility with Water Discharge in a Gravel-Bed Channel: River Esva, NW Spain

“…In summary, there are three main advantages of the GTM model compared to previous approaches to flow competence: (1) its flexibility, which allows to model a large range of particle entrainment and streambed stability conditions; (2) the model is well-rooted into our current knowledge on sediment transport mechanics and considers some situations that were not adequately grasped by previous flow competence models; i.e., the change in entrainment conditions with armour break up; and (3) while most of the previous flow competence models focused mainly on computing the mobilized grain sizes, the GTM model also allows the estimation of the mobile fraction in each size range which is at least as important as predicting the maximum transported grain in terms of evaluating streambed disturbance (interesting, for example in many ecological applications). In spite of all these advantages, there have been no previous attempts to take advantage of the GTM model for the characterization of streambed mobility in specific field cases (Vázquez-Tarrío et al (2019) [55] being the only exception to the best of our knowledge).…”