Investigating tidal river dynamics in a longitudinally varying channel geometry

Abstract

Examining the flow dynamics in a tidal river, particularly downstream, is of great importance to consider the substantial interaction between river runoff and tidal waves. Previous studies have investigated flow dynamics in tidal rivers mainly concentrated on the lateral structure of the estuarine dynamics, with limited consideration of the impact of geometrical variation of the tidal channel, which has an enormous effect along the longitudinal direction. The aim of this study is to offer a comprehensive understanding of the dynamics of a tidal river with longitudinally varying channel geometry. To accomplish the aforementioned aims, this study provides a spatiotemporal analysis of water current and density for a fortnight period of a mesotidal estuarine channel that encompasses both curved and straight channels in the streamwise direction. Moreover, a quantitative analysis of the governing mechanisms for estuarine dynamics was performed to determine the alteration of driving mechanisms due to channel geometry variation. The longitudinal velocity profile indicates the existence of tidal asymmetry where the ebb currents are dominant over flood currents. The asymmetry, which is induced by the interaction between the channel curvature and density gradient, is observed to be more enhanced in the curved channel than in the straight channel. Moreover, the across-channel bathymetry difference between the curved and straight channels leads to different patterns of lateral flow in those channels. Additionally, the longitudinal and lateral velocities averaged for the spring/neap tides reveal that the channel with a higher curvature degree has greater residual circulation. On the other hand, the periodically varying density distribution denotes the existence of tidal straining, which is slightly modified by the variation in geometry. The notable findings of this study are as follows: (i) the variability in channel geometries introduces different responses of tidal straining and tidal stirring; (ii) the river effect (river-induced shear) in the curved channel substantially increases significantly compared to the straight channel over a certain threshold of river runoff. We believe that the findings of this study deepen our understanding of tidal river dynamics in a longitudinally varying channel geometry.