Numerical assessments of bed morphological evolution in mountain river confluences under effects of hydro‐morphological factors

Abstract

Understanding hydro‐morphological characteristics in mountain river confluences is significant in flood disaster mitigation and mountain river restoration, which is still poorly explored. This study employed a 2D hydro‐morphological model based on the finite element method to simulate the major bed morphological features in a mountainous‐type flume confluence and evaluate the effects of main controlling factors on these features. The modelling results show that the dominant bed morphological features in mountain river confluences can be well produced by the 2D model. Five controlling factors including downstream water level (H), branch discharge ratio (Qr), main channel sediment supply ratio (ψ), tributary slope (St) and confluence angle (α) reflecting different flow‐sediment‐geometry scenarios were adopted into a sensitive study. The upstream reach of the main channel is controlled by H and ψ and their decreases lead to bed degradation. The increase in Qr, St, and α mainly results in bed aggradation and steepening in the tributary, attaining a higher sediment transport rate. This contrasts the existing experimental results, due to the difference in sediment feeding. The size of the bank‐attached bar is promoted by the increase in Qr, St, and α and the decrease in H and ψ. A quantitative analysis shows that the water surface slope well correlates with various dimensions of the scour hole and bar morphology (correlation = 0.58–0.8). Meanwhile, the bank‐attached bar displays a strong self‐similarity (correlation = 0.84–0.98) regarding its geometric dimensions. This study provides new insights into the existing knowledge of the hydro‐sediment‐morphodynamics of mountain river confluences obtained from laboratory studies and field investigations.

Numerical Simulation of Confluence Flow in a Degraded Bed Under Different Turbulence Models with a Comparison of Rigid Lid and Volume of Fluid

“…This study demonstrates the feasibility of high-resolution LES for large-scale rivers by considering the detailed bathymetry and submerged hydraulic structures [26]. [31] using the experimental data collected by Yuan et al, (2016) to develop a numerical model in OpenFOAM. In this study, flow ratio and downstream Froude number of two geometries were compared with experimental results by Yuan (2016) in a flume with a wider downstream than the upstream main and tributary channels.…”

“…A 2D hydro-morphological model based on the finite element method was employed by Yan et al (2022) [31] to simulate the bed morphological features in a mountainous-type flume confluence and to investigate the main controlling factors on these features. The results illustrated that the bed morphological features in mountain river confluences can be simulated by the 2-D model ((Mflow_02 imbedded in the iRIC open-source software) is employed (MFlow_02, 2018)).…”

Numerical assessment of hydro‐morphodynamics affected by altered upstream discharge in the Dujiangyan reach of the Min River, China

“…The issue of whether 2D models are capable of effectively simulating hydro‐morphodynamics in complex river channels continues to be a matter of debate, as some simulation results have been found to be accurate, whilst others are not (Alho & Mäkinen, 2010; Guan et al, 2016; Kasvi et al, 2015; Lane, 1998). In fact, the simulation performance can be attributed to numerous factors, and the parameterization of helical flow performs well in other areas, which were away from the banks (Sloff & Mosselman, 2012; Yan et al, 2022). Second, although the initial size distribution of the sediment is prescribed, the numerical model cannot solve sediment sorting processes, which are crucial for the formation of scour holes and bank‐attached bars in the confluence reach (Guillén‐Ludeña et al, 2016; Leite Ribeiro et al, 2012).…”