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 gr avel-bed river reach. Understanding the dynamics of a river involves knowle dge on the interactions between flow, sediment transport and bedform develo pment at a range of scales. This requires the characterisation of flow stru ctures and of the flow organisation at the reach scale. Three types of flow structures have been commonly described in flows over gravel-bed rivers: t he bursting motions in the near-bed region (ejections), the shedding motion s from the larger protruding particles and the large-scale flow structures which develop in the outer region of the flow. We describe the processes th at give birth to these flow structures along with the possible interactions between them. As an example, the passage of large-scale flow structures ch anges the dynamics of the separation of flow in the wake of an obstacle, th us affecting the manifestation of shedding motions. These processes and int eractions are then combined into an illustration of the complex organisatio n of flow at the scale of a river reach. Although speculative, this illustr ation highlights that (1) the scales and locations in space of the structur es are closely related to the scale and distribution of the roughness eleme nts, (2) the flow is organized into clear zones of production and dissipati on of flow structures, and (3) the organization of the flow presents a stro ng structural anisotropy with complex interactions between the flow structu res. This essay outlines that the interactions between flow structures are as important as the presence of the structures themselves in the descriptio n of turbulent flows over gravel-bed rivers.