In this paper, we present a detailed investigation of the size, scale and dynamics of macro-turbulent flow structures in gravel-bed rivers. We used an array of seven electromagnetic current meters with high resolution in both space and time to measure the streamwise velocity fluctuations in a gravel-bed river. The array was deployed successively in various configurations in order to quantify the vertical, lateral and longitudinal extent of the flow structures and to estimate their advecting velocities. To depict the spatial and temporal properties of the flow structures, we used space–time velocity matrices, space–time correlation analysis and coherent-structure detection schemes. The results show that the large-scale turbulent flow structures in a gravel-bed river occupy the entire depth of the flow and that they are elongated and narrow. The length of the structures is 3 to 5 times the flow depth while the width is between 0.5 and 1 times flow depth. In spite of the high roughness of the bed, these values are similar to those reported in the literature for laboratory experiments on large-scale turbulent flow structures. The dynamics of the large-scale turbulent flow structures investigated using flow visualization highlight the interactions between the outer flow region and the near-bed region. Our evidence suggests that large-scale flow incursions trigger ejections in the near-bed region that can develop into megabursts that can reach the water surface.

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In gravel-bed rivers, bed sedimentary structures exert a significant influence on the stability of the bed, particle motion and stream dynamics. A detailed description of the development of sedimentary structures is essential to define the overall activity of sediment transport. This paper aims to improve our understanding of the dynamics of sedimentary structures at the reach scale using a field approach. The objectives of the study are (1) to describe the processes of sedimentary structures development from a manipulated bed surface and (2) to show the effect of keystones on the development of the sedimentary structures by monitoring the displacement of individual tagged particles. We investigated the dynamics of sedimentary structures in three 15 m long reaches of a small gravel-bed river characterized by moderate slope and high relative roughness. The bed material is composed of a range of gravel sizes with sparse boulders that remain immobile during most flows. We have disrupted the bed surface material of the first reach to create a disorganized bed surface. Two reaches where the bed material remained undisturbed were used as control sections. Detailed topographic maps and passive transponders were used to describe the formation processes of sedimentary structures. The initial manipulations of the bed led to a more fragmented surface in the experimental section. However, sedimentary structures developed very quickly, with an equilibrium bed surface developing after only two sediment transport events of low magnitude discharge. This rapid adjustment of the bed is associated with the development of key sedimentary structures anchored by the large boulders. At the reach scale, these structures increased the probability of congested areas in the displacement of bed material. In both control sections, sedimentary structures were maintained during the experiment. Results Morphological changesImmediately after the bed manipulations, the surface of the experimental section appears to be highly segmented and disjointed (Figure 4). Small zones with elevation values higher or lower than average are numerous and dispersed over Development of sedimentary structures in a gravel-bed river