Abstract: The North Branch estuary of the Yangtze River is a typical tide-dominated estuary with bifurcation. The hydrodynamics and diversion processes are the major factors in sediment transport and geomorphology evolution of the tide-dominated estuary. This paper is based on data from the bottom tripod system and a cross-section survey of the North Branch in April 2018, whereby the dominant flow of each channel is calculated. The results show that during the spring tide cycle, the Santiao Port and the Guyuansha south waterway are flood-dominated and the net tidal current flows landward, whereas the Guyuansha north waterway is ebb-dominated, and the net flow has a seaward direction. Moreover, a high-resolution numerical FVCOM model is used to simulate the diversion process of the North Branch estuary. The results suggest that: the cross-sectional area of Guyuansha south waterway is much bigger than that of the north waterway, and the propagation direction of the lateral tide is consistent with the orientation of the Guyuansha south waterway, which mainly has onshore movement through the Guyuansha south waterway. The spatial distribution of the flood tide flow is significantly uneven in these two waterways. Meanwhile, tidal current from the Guyuansha south waterway crosses the shallow shoal and reaches the north waterway. It produces a net flow with the seaward direction through the north waterway. The flood-tide and ebb-tide diversion ratio of the Guyuansha north waterway are 29.7% and 47.2%, respectively, during the spring tide period and 41.6% and 43.1%, respectively, during the neap tide period. The residual patterns indicate that the tidal current enters the estuary from the south waterway and exits through the north waterway. These indicators and diversion characteristics can help predict estuarine sediment transport and geomorphological evolution processes within an individual bifurcated channel.

Key words: the North Branch estuary, tide-dominated, bifurcation, flow diversion ratio, FVCOM