PIANC Smart Rivers 2022

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Abstract : *(250~1000 words)

The tidal reaches are characterized by unsteady and non-uniform flow (UNF), which is significantly different from the commonly assumed steady and uniform flow (SUF) in hydraulics. The SUF shows invariant temporal and spatial flow characteristics, and thus flow acceleration is absent. However, for the UNF, the variation of flow velocity and depth in both temporal and spatial scales causes the loss of flow energy, and thus increases the flow resistance. In order to clarify the variation of flow resistance and its influencing factors in tidal reaches, this study investigates the flow resistance characteristics under UNF conditions. In this study, a typical tidal section of the Lower Yangtze River (LYR) – Kouanzhi Waterway (KW) – was selected as the study area, where the temporal variation of water surface along the river course at different tide levels, the bathymetry of multiple cross-sections, the distribution of cross-sectional flow velocity and its temporal variation were measured in detail. Based on these field measurement data, the contribution terms to the flow resistance were calculated and evaluated, by decomposing the momentum equation. The calculated contributing terms to the flow resistance include water surface gradient, local acceleration term, and convective acceleration term. It is found that the local acceleration and convective acceleration have a significant effect on the total flow resistance at specific time periods due to the fluctuation of the water surface which is not considered in UNF conditions. The contribution of local acceleration and convective acceleration to the total energy slope was found to be much more significant than those obtained in previous studies, which is attributed to the topographic characteristics of the KW. The results show that the local acceleration term is more significant than the convective acceleration term except when the water surface slope is close to zero, and its contribution is significant throughout the high tide and the initial low tide periods. The above research results are of great reference significance for the flow resistance issues in river dynamics analysis and numerical simulation of tidal reaches.

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