Author(s): Yuchen Zheng; Yihong Chen; Jiawei Lin; Saiyu Yuan
Keywords: Large Wood; Channel Confluence; Separation Zone; Laboratory Experiment
Abstract: Knowledge of motions of large wood near river confluences is important for prediction of wood distribution in a river network, and wood distribution is a key factor affecting flow, sediment transport and riverbed evolution. The separation zone near the confluence of a river network is a usual gathering place for large wood. In this study, a series of laboratory experiments were done to determine the motion responses of large wood to the hydrodynamic forces near the separation zone of channel confluence. A single wood was released at the tributary upstream and its motion and trajectory near the separation zone was recorded by infrared thermal imaging. Different sizes of woods (with different diameters and lengths) and discharge ratios of flow confluence (ratio of the tributary discharge to the total flow discharge) were considered. Each case was repeated for 30 times. The results showed there were two main motion patterns of wood which entered the separation zone: (1) the wood touched the sharp corner wall, and rotated at a certain angle due to the friction of the wall and entered directly into the upstream part of the separation zone; (2) the wood did not touch the sharp corner wall, and travelled along the boundary of the separation zone; and then it rotated due to the strong flow shear and entered into the downstream part of the separation zone. In addition, the number of woods entering the separation zone increased when the confluence ratio increased. The wood with larger length had a higher probability of entering the separation zone. These observations provide important knowledge on motion of large wood near river confluences and are beneficial for disaster prevention and control related to the wood blocking in the river network.