Hydrodynamic Characteristics of Undular Tidal Bores in the Qiantang River Based on Field Observations

Understanding the undular tidal bores in the Qiantang River is essential for effective river management and maintenance. While breaking tidal bores have been studied extensively, reports on undular tidal bores in the Qiantang River remain limited. Furthermore, observed data on undular tidal bores fulfilling the requirements of short measurement time intervals, and spring, medium, and neap tide coverage, and providing detailed data for the global vertical stratification of flow velocity are quite limited. Based on field observations at Qige in the Qiantang estuary, we analyzed the characteristics of undular tidal bores. The results showed that the flooding amplitude (a) of the first wave is always larger than its ebbing amplitude (b). Moreover, the vertical distribution of the maximum flood velocity exhibites three shapes, influenced by the tidal range, while that of the maximum ebb velocity exhibites a single shape. During the initial phase of the flood tide in the spring and medium tides, the upper water body experiences multiple oscillating changes along the flow direction, corresponding to the alternating process of the crest and trough of the tide level upon the arrival of the tidal bore. The tidal range is a crucial parameter in tidal bore hydrodynamics. By establishing the relationship between hydrodynamic parameters and tidal range, other hydrodynamic parameters, such as the tidal bore height, maximum flood depth-averaged velocity, maximum flood stratified velocity at the measurement points, and duration of the flood tide current, can be effectively predicted, thereby providing an important reference for river management and maintenance.