Experimental Investigation of Flow Characteristics in a Confluent Channel with Bed Elevation Difference

Abstract

Changes of the river morphology at the river confluence became a big issue nowadays. The large-scale dredging at the main streams as a part of the Four Major River Restoration Project led to the bed elevation difference between the tributary and main streams. This may induce headward erosion and the stronger secondary flow motion at the confluence. Dredging to secure water induces headward erosion and sudden increase of the tributary discharge takes the detention at the confluence. The most previous studies of experiments were performed without considering the bed elevation difference between main and tributary channels. The purpose of this study is to investigate confluent flow by analyzing flow mechanism at junction with bed elevation difference. Velocity components at confluence with bed elevation difference were measured in the hydraulic model experiment. The confluent channel was designed with data from the Nakdong River in Korea. In this experiment, three dimensional velocity components were measured using micro-ADV, and they were analyzed in depth to reveal flow and turbulent characteristics at the confluence. The measured u-v vector fields show the flow characteristics of the confluence near surface. Comparing the u-v vector fields with the discharge ratio, the bed elevation difference and the confluent angle, the lateral momentum of the tributary flow entity was found to be an important factor to induce the separation zone. As the lateral momentum of the tributary flow entity is greater, the entity angle is larger and the separation zone is wider. By analyzing the v-w vector fields and the vorticity contours, how the surface water of the tributary affects the evolution of the secondary flow. Because of the bed elevation difference, the surface momentum of the tributary flow force to rotate the upper part water of the main channel. The surface water of the tributary is approaching the right side wall of the main channel and the clockwise circulation starts. This flow mechanism can be defined as the down flow. The down flow results in the evolution of the secondary flow. The turbulent kinetic energy (TKE) and the Reynolds shear stress were calculated at confluence. Both the turbulent kinetic energy and the Reynolds shear stress developed at the confluence and dissipated at the downstream. The analysis of the turbulent kinetic energy and the Reynolds shear stress at the confluence showed that, the region of turbulent motion is bigger with the greater discharge ratio and the increased bed elevation difference. The regression analysis of the relative turbulent kinetic energy revealed that, among many hydraulic and geometric parameters, both the confluence angle and velocity ratio were main factors to the TKE. As these factors increase, then TKE increase monotomically.