Understanding Plunging Flows Mechanisms in The Fraser River Using a Large Eddy Simulation (LES) Model

Understanding incision mechanisms and processes in bedrock rivers are essential to study fluvial landscape evolution. Previous research studies in the Frasier River have suggested that entrainment mechanisms are governed by plunging flows, responsible for erosion at the riverbed and banks, that could potentially lead to massive landslides. Plunging flows are dominated by large scale velocity inversions possibly caused by anisotropic turbulence and gravity currents. Thus, numerical models capable to resolve turbulence could lead to a better understanding of entrainment and depositional processes in canyon-bound rivers. In this study, an eddy-resolving model, at the laboratory scale, has been developed and applied to understand macro-turbulence and complex fluid dynamics, such as, plunging flows and eddy coherent structures, that could occur in the Fraser River along the Black and Alexandra Canyons. The computational domain was constructed from previous experiments of laboratory flume with dimensions of 4.8 m long, 0.2 m wide and 0.2 m deep with an adjustable side wall to accommodate different widths, using hexagonal grid mesh. The LES technique was employed to resolve turbulence above a spatial filter, and an incompressible multiphase solver was applied to simulate the water and air motion interactions. The boundary conditions of the LES model consisted of non-slip bed, slip side walls, and top wall and the inlet with turbulence from Divergence-Free Synthetic Eddy Method (DFSEM). The multiphase fluid model followed a similar approach for bed and slide walls, but the channel inlet was defined as a fixed velocity and the top face with an air pressure represents two-phase motion. The expected results aim to contribute to understanding plunging flows that are conducive to sediment transport and bed evolution change in bedrock canyon rivers. In particular, the study reveals lateral constrictions control the channel width and steep slopes increase the strength of plunging flows. Additionally, the higher velocities generated due to lateral constriction could lead to velocity inversions. At the same time, the higher discharges intensified plunging flows compared to moderate flows. This hydro-morphodynamic eddy-resolving model helps to understand entrainment mechanisms in bedrock rivers.