3D numerical simulations of the bed evolution at an open-channel junction in flood conditions

Abstract

Open-channel junctions pose a risk associated with flooding and erosion. However, despite the numerous studies concerning the channel junctions, the bed and flow field evolutions at junctions for flood conditions have yet to be investigated. To address this research gap, this paper investigates the evolutions of the bed and flow field during a flood event at a right-angled open-channel junction utilizing three-dimensional (3D) computational fluid dynamics (CFD) modeling. The transient Reynolds-averaged Navier–Stokes (RANS) equations with the renormalized group (RNG) k-ϵ as the turbulence closure for the flow field, the Meyer-Peter and Muller equation for bed-load sediment transport rate, the convection–diffusion equation for the sediment concentration distribution, and the sediment continuity equation for the bed evolution are used in the numerical model. The model simulates the characteristics bed morphology at the junction with reasonable accuracy compared with the available laboratory data. Three scenarios (Scenarios 1, 2, and 3) were considered by applying the triangular flood wave at the inlet of the tributary channel, main channel, and both channels, respectively. The quasi-equilibrium deformed bed channel junction with steady inflow conditions of the validation case was considered as the initial condition for the flood flow simulations. The simulated results show that the junction bed is deformed considerably due to the significant increase in bed shear stress during a flood event. The deepest scour area gradually grows wider during the passage of the flood wave. The maximum value of the scour increased by 24.4%, 14.6%, and 31.2% in Scenarios 1, 2, and 3, respectively. The findings of the study improve the understanding of the junction hydrodynamics and bed morphological features for flood conditions.