Particle transport in gravel‐bed rivers: Revisiting passive tracer data

Abstract

Data from tracer experiments were compiled and analysed in order to explore the role of geomorphological, hydrological and sedimentological constraints on fluvial gravel transport in gravel‐bed rivers. A large data set from 217 transport episodes of tagged stones were compiled from 33 scientific papers. Our analyses showed that while magnitude of peak discharge is a major control on gravel transport and mobility, tracer travel distances show some scale dependence on the morphological configuration of the channel. Our results also highlight differences in the way tracers are displaced between step–pool and riffle and pool channels. The riffle–pool sequence seems to be a more efficient trap for travelling gravels than the step–pool pair. In addition, in step–pool channels there are clear differences in tracer transport between observations of first displacements after tracer seeding (unconstrained‐stone conditions), and second and subsequent observations of tracer displacements (constrained‐stone conditions). The comparison between tracer experiments under constrained conditions and those under unconstrained conditions also highlights the importance of bed state and structures in gravel mobility. The results of this study confirm that sediment transport in gravel‐bed rivers is a complex process, whereby sedimentological and geomorphological controls are superimposed on the hydraulic forcing. © 2018 John Wiley & Sons, Ltd.

Controls Over Particle Motion and Resting Times of Coarse Bed Load Transport in a Glacier‐Fed Mountain Stream

“…Once small particles are dislodged, they are transported for long distances, before coming to rest. This is consistent with what is shown in Figure 10 and with the outcomes of recent works (e.g., Vázquez‐Tarrío et al, 2019). The value obtained for c is higher than the average distance between steps in Estero Morales, which is around 10 m. The spacing between pools is a relevant geomorphic dimension of step‐pool sequences, given that it is closely related to step formation (Curran, 2007) and flow resistance (Giménez‐Curto & Corniero, 2006).…”

“…The average value of dimensionless displacement for binned classes of dimensionless particles sizes decreases for coarser sediments. Although considerable scatter is observed, the size‐dependent trend is confirmed, and the field data are plotted around previously derived empirical formulas (i.e., Church & Hassan, 1992; Vázquez‐Tarrío et al, 2019). However, the empirical relationship is not statistically significant ( R 2 = 0.095; Figure 8).…”

“…The filled dots represent the average values of dimensionless displacements for binned classes of dimensionless particles sizes. Empirical equations of Church and Hassan (1992) and Vázquez‐Tarrío et al (2019) are plotted as well.…”

Virtual Velocity Approach for Estimating Bed Material Transport in Gravel‐Bed Rivers: Key Factors and Significance

“…Improvements could be achieved adopting the procedure developed by Klösch and Habersack () to calculate unsteady virtual velocity from repeated surveys of tracer positions. Another virtual velocity calculation weakness is due to the exclusive use of tracers starting at the bed surface, which move more easily than the average grains in the active layer mobilized during full transport (Vázquez‐Tarrío et al, ). As explained by Ferguson and Hoey (), if tracers are not fully mixed into the bed, the calculated virtual velocity can be overestimated leading to full transport flux overestimate of high‐magnitude events.…”

“…As explained by Ferguson and Hoey (), if tracers are not fully mixed into the bed, the calculated virtual velocity can be overestimated leading to full transport flux overestimate of high‐magnitude events. Some improvements will be possible considering long‐term tracer dispersion (Haschenburger, ) and relation between morphology and tracers mobility (Papangelakis & Hassan, ; Vázquez‐Tarrío et al, ).…”