Effects of the spatial flow variability induced by an array of boulders of low relative submergence on bedload transport

Abstract: Macro-roughness elements of low relative submergence typically encountered in mountain streams affect flow dynamics and bedload transport at different scales. At a reach scale, they dissipate a significant amount of flow energy, diminishing that available for moving the sediment grains; and at a local scale, they produce substantial spatial variability of mean velocity and Reynolds stresses, flow separation and recirculation, and generate coherent structures in the vicinity of them. However, such spatial flow variations are ignored with the commonly used reach-averaged shear stress in bedload transport estimations, even when an effective shear stress is used instead of the calculated from the depth-slope product. Moreover, because bedload transport is a nonlinear phenomenon related to flow variables, spatial flow variations can introduce a wide spatial difference in bedload fluxes; thus, using reach-averaged flow quantities can lead to large errors on bedload transport estimations. 

I will present my Ph.D. research which is focused on evaluating how bedload transport is influenced by the spatial flow variability induced by an array of immobile boulders of low relative submergence placed on a rough bed. I will also be discussing the methods that I am using which consist of performing Large Eddy Simulations of the turbulent flow coupled with the Discrete Element Method to compute the individual motion of spherical grains that compose the bed. Therefore, the trajectory of each particle is calculated based on the forces acting on them, which include fluid-particle interactions and particle-particle interactions implemented though a collision model.

Biography: Magdalena Barros Alcalde is a civil engineer with a major in hydraulic engineering from the Pontificia Universidad Católica de Chile. She entered the PhD program in engineering sciences in 2020 at the same university and is working with Prof. Cristián Escauriaza on turbulent flows and sediment transport. Since October 2022 sha has been a visiting student at Fluid Mechanics and Acoustics Laboratory in Lyon working with Prof. Ivana Vinkovic.