为探究强潮河口弯道的环流规律,并评估SCHISM模型在该类复杂水流模拟中的适用性与计算效率,基于SCHISM建立钱塘江河口闻家堰河段三维水动力数学模型,模拟洪水期及潮水期弯道的纵向流速、水位横比降及环流强度的时空分布,并分析模型计算效率。SCHISM对强潮河口弯道水流模拟具有较好的适用性和高效性;弯道横比降在涨潮期变化剧烈且存在双峰现象,落潮期变化相对平缓,最大横比降远小于涨潮期;环流强度空间分布呈弯顶大、两端小的特征;一般情况下弯道横比降和环流强度主要由潮水控制。SCHISM可有效模拟强潮河口复杂弯道环流特性。研究成果可为类似河口弯道的防洪与治理提供参考。

[Objective] This study aims to reveal the spatiotemporal distribution patterns and controlling mechanisms of bend circulation in a macrotidal estuarine environment. The specific objectives are as follows: (1) to conduct an in-depth analysis of the dynamic variations in longitudinal velocity, transverse water surface slope, and circulation intensity in the bend during flood and tidal periods; (2) to clarify the controlling role of strong tidal dynamics in the formation and evolution of bend circulation and to reveal its differences from those in conventional rivers. [Methods] Based on the Semi-implicit Cross-scale Hydroscience Integrated System Model (SCHISM) framework, a large-scale three-dimensional hydrodynamic model covering the Qiantang River Estuary from the hydropower station to Ganpu was established. The spatiotemporal distributions of longitudinal velocity, transverse water surface slope, and circulation intensity in the bend were simulated during both flood and tidal periods. The model employed an unstructured grid, with local refinement in the Wenyan bend to accurately resolve the channel topography and flow structures. Model driving conditions included astronomical tidal levels at the offshore open boundary and river discharge at the upstream boundary. Recent synchronous field measurements obtained in the Wenyan reach during spring tide and flood periods, including water level, flow velocity, and flow direction, were used for model calibration and validation. [Results] (1) The SCHISM model showed high applicability and remarkable computational efficiency for this complex application. Under the same parallel computing environment, its computational speed was nearly two orders of magnitude higher than that of conventional explicit Euler-type models (approximately 88 times faster). (2) The transverse water surface slope in the bend varied drastically during the flood tide acceleration stage, and a distinct bimodal pattern was identified for the first time. This behavior was markedly different from the unimodal or gradual variation patterns commonly observed in estuaries with moderate tidal ranges or in unidirectional rivers. In contrast, the variation in the transverse slope during ebb tide was much milder. Quantitative analysis showed that the maximum transverse slope during flood tide reached 5.3 times that during ebb tide, indicating pronounced tidal asymmetry. (3) During both flood and tidal periods, the bend circulation intensity exhibited a spatial pattern characterized by larger values near the bend apex and smaller values toward both ends. The core zone of maximum circulation intensity was consistently located at the cross-section on the downstream side of the bend apex. During the flood period, the maximum circulation intensity was approximately 0.08-0.12. Notably, the maximum circulation intensity during the flood tide of the autumn spring tide (0.09) was slightly greater than that during the 50-year return-period flood and was much greater than that during ebb tide. [Conclusion] (1) The SCHISM model is a powerful and efficient tool for simulating complex hydrodynamic processes in macrotidal estuaries. Its ultrafast computational capability (nearly 100 times faster than explicit models) represents a major technical advantage and has great potential for scientific and engineering applications requiring numerous simulation scenarios. (2) The hydrodynamic phenomena in bends of macrotidal estuaries are highly distinctive. The newly identified bimodal transverse slope pattern during flood tide, together with the pronounced tidal asymmetry, provides an important extension to conventional bend hydrodynamic theory and highlights the fundamental differences between macrotidal estuaries and ordinary rivers. (3) Tidal dynamics are an important factor controlling the generation and development of bend circulation, and their influence may even exceed that of extreme flood events.