The active layer in gravel‐bed rivers: An empirical appraisal

Abstract

The vertical position of the streambed–water boundary fluctuates during the course of sediment transport episodes, due to particle entrainment/deposition and bedform migration, amongst other hydraulic and bedload mechanisms. These vertical oscillations define a topmost stratum of the streambed (i.e. the ‘active layer or active depth’), which usually represents the main source of particles entrained during long and high‐magnitude bedload transport episodes. The vertical extent of this layer is hence a capital parameter for the quantification of bedload volumes and a major driver of stream ecology in gravel‐bed rivers. However, knowledge on how the active depth scales to flow strength and the nature of the different controls on the relation between the flow strength and the active depth is still scarce. In this paper we present a meta‐analysis over active depth data coming from ~130 transport episodes extracted from a series of published field studies. We also incorporate our own field data for the rivers Ebro and Muga (unpublished), both in the Iberian Peninsula. We explore the database searching for the influence of flow strength, grain size, streambed mobility and channel morphology on the vertical extent of the active layer. A multivariate statistical analysis (stepwise multiple regression) confirms that the set of selected variables explains a significant amount of variance in the compiled variables. The analysis shows a positive scaling between active depth and flow strength. We have also identified some links between the active depth and particle travel distances. However, these relations are also largely modulated by other fluvial drivers, such as the grain size of the bed surface and the dominant channel macro‐bedforms, with remarkable differences between plane‐bed, step‐pool and riffle‐pool channels. © 2020 John Wiley & Sons, Ltd.

Flow Strength and Bedload Sediment Travel Distance in Gravel Bed Rivers

“…The predicted active layer depths (shown in Figure 8 and Tables –) are within the range measured in other studies using tracers, scour chains, and indirect estimates (Vázques‐Tarrío et al., 2021), demonstrating that the approach yields reasonable estimates of maximum burial depths. The different size classes tend to yield similar estimates of $${d}_{a}$$ for any given event, with the result that the error bars are usually small relative to the range of the figure, and increases confidence that the method is consistent.…”

The contribution of grain sorting to the dynamics of the bedload active layer

“…We consider a selection of scour–fill depth data compiled by Vázquez‐Tarrío et al (2020). We used riffle‐pool morphologies.…”

“…Comparison between scour depth computed (a) with Equations (), () and () and (b) with Equation () and the measured values (Vázquez‐Tarrío et al, 2020). [Colour figure can be viewed at wileyonlinelibrary.com]…”

A multi-spatiotemporal scale strategy to evaluate factors controlling pebble mobility and its interactions with bedforms in a lowland gravel-bed river

“…In this respect, Haschenburger (2011a) showed, for example, that large floods accelerate the rate at which the vertical mixing of surface tracers occurs. Recently, Vázquez-Tarrío et al (2021) highlighted that, at the event scale, the ratio between the Shields stress for the peak flow and the critical Shields stress is one of the main drivers of the active layer's depth. By comparing the virtual velocity (i.e., total distance divided by the total duration of competent events) of tracers 1 year and 10 years following their injection, we could indirectly gain insight into the intensity of their mixing process, the idea being that the smaller the difference, the weaker is the mixing.…”