2022 CSDMS meeting-082
Riverbank stability and cross-stream sediment diffusion governed by turbulence
Christian Gonzalez, Arizona State University Tempe Arizona, United States. email@example.com
Mark Schmeeckle, Arizona State University Tempe Arizona, United States. firstname.lastname@example.org
Natural channels are continuously changing their shape, where meanders and other complex configurations appear (e.g. bars, braided rivers, inner confluences, etc.). Channel evolution is strongly determined by the interactions occurring between its banks and the flow. These interactions also determine when a channel stabilizes, i.e. when its width remains constant. Current literature explains the stabilization of channels by the attainment of the equilibrium between sediment diffusion and gravity forces. However, the role of other potentially relevant processes is uncertain and needs to be addressed. Among them are secondary currents close to the banks and the spatial distribution of turbulence. Furthermore, the transition to steady-state banks is not fully understood. We explored these issues aiming to provide a better understanding of bank erosion and channel stability. To do this, we simulated a flatbed channel under 8 conditions, with Shields parameter spanning from 0.03 to 1.78. These simulations solved a 3D turbulent flow by carrying out Large-Eddy Simulations (LES) and the particles’ motion through a Discrete Element Method (DEM). We observed streamwise-aligned vortices appearing close to the banks, which were associated with high levels of TKE and shear stress, as well as flow spanwise velocity fluctuations. These fluctuations were mainly sweeps and ejections, which helped to dislodge sediments from the banks. Once detached, sediments could travel downstream. The role of the turbulence was also observed by separating the diffusive and advective components of the transport, where the initial bank erosion was dominated mainly by the former. Indeed, turbulence roughly explained 90% of sediment flux under erosion and bedload transport conditions. We conclude turbulent events increase shear stress close to the banks, promoting entrainment. Once the flow has transferred enough momentum to sediments, flow mean-velocity and fluctuations decrease. In this manner, shear stress decreases as the channel width increases. Eventually, shear stress reaches the threshold for transport close to the banks. Here, channel stabilization occurs. Notwithstanding that, stresses in the center of the channel are high enough to continue transporting sediments.