On the integration of turbulent flow structures within the dynamics of a gravel-bed river reach

On the integration of turbulent flow structures within the dynamics of a gravel-bed river reach. Understanding the dynamics of a river involves knowledge on the interactions between flow, sediment transport and bedform development at a range of scales. This requires the characterisation of flow structures and of the flow organisation at the reach scale. Three types of flow structures have been commonly described in flows over gravel-bed rivers: the bursting motions in the near-bed region (ejections), the shedding motions from the larger protruding particles and the large-scale flow structures which develop in the outer region of the flow. We describe the processes that give birth to these flow structures along with the possible interactions between them. As an example, the passage of large-scale flow structures changes the dynamics of the separation of flow in the wake of an obstacle, thus affecting the manifestation of shedding motions. These processes and interactions are then combined into an illustration of the complex organisation of flow at the scale of a river reach. Although speculative, this illustration highlights that (1) the scales and locations in space of the structures are closely related to the scale and distribution of the roughness elements, (2) the flow is organized into clear zones of production and dissipation of flow structures, and (3) the organization of the flow presents a strong structural anisotropy with complex interactions between the flow structures. This essay outlines that the interactions between flow structures are as important as the presence of the structures themselves in the description of turbulent flows over gravel-bed rivers.