Impact of tidal shear fronts on terrigenous sediment transport in the Yellow River Mouth: observations and a synthesis

Observational and modeling studies have been conducted to investigate the impact of a reversed tidal shear front (TSF) on the transport of freshwater and sediment in the Yellow River Mouth (YRM). The TSF duration is only about 40% of a tidal cycle; however, sediment transport across the frontal zone does not become more significant when the front disappears. This suggests that the mechanism of cross-frontal sediment transport remains unclear. In this study, synchronous observations of water and sediment along the river mouth profiles at the YRM in 1995, 2003, and 2019 were used to investigate the long-term variation characteristics, mechanisms, and impacts of the TSF on cross-frontal sediment transport. The results indicate that in addition to the reversed TSF, there are also homogenous TSF during flood and ebb tides, with the ebb homogenous TSF being more significant due to tidal asymmetry in the YRM. The ebb homogenous TSF is located between shallow shoals and the slopes of the subaqueous delta front and lasts for about 3 hours around the maximum ebb. Its maximum shear strength is similar to that of the reversed TSF, and the formation mechanism is also similar. The presence of the ebb homogenous TSF reduces the efficiency of cross-frontal sediment transport during the ebb. The reversed and homogenous TSF alternately block sediment moving towards the sea, causing some freshwater and sediment transported across the front during the ebb to be carried back by the flood current. As a result, terrigenous sediment cannot be transported across the frontal zone to the central Bohai Sea by tidal currents solely. Over the past two decades of low discharge, with the Yellow River implementing artificial water and sediment regulation, the homogenous TSF during the ebb tide has become more and more important and cannot be neglected for cross-frontal sediment transport under normal conditions. Furthermore, from a long-term perspective, the shear strength of the TSF is positively correlated with the distance of river mouth siltation into the sea. Additionally, salinity and turbidity gradients are also positively correlated with the shear strength in the estuary. This indicates that the blocking of cross-frontal sediment transport by the TSF is a key process in river-sea interaction in the YRM.

已有观测与模拟研究围绕反向潮汐切变锋（reversed tidal shear front, TSF）对黄河口（Yellow River Mouth, YRM）淡水与泥沙输运的影响展开了系统探讨。该潮汐切变锋的持续时长仅约占潮汐周期的40%，但锋面消散时，跨锋面泥沙输运（cross-frontal sediment transport）并未出现显著增强，这表明跨锋面泥沙输运的核心机制仍未明晰。本研究采用1995年、2003年及2019年在黄河口河道剖面获取的同步水沙观测数据，对潮汐切变锋的长期变化特征、作用机制及其对跨锋面泥沙输运的影响展开了研究。研究结果表明，除反向潮汐切变锋外，黄河口在涨潮与落潮阶段还存在均匀型潮汐切变锋（homogenous tidal shear front）；受黄河口潮汐不对称性（tidal asymmetry）影响，落潮均匀型潮汐切变锋的特征更为显著。落潮均匀型潮汐切变锋分布于浅滩与水下三角洲前缘坡（subaqueous delta front）之间，在最大落潮时刻前后持续约3小时，其最大切变强度与反向潮汐切变锋相当，形成机制也较为相似。落潮均匀型潮汐切变锋的存在会降低落潮阶段跨锋面泥沙输运的效率。反向与均匀型潮汐切变锋交替阻滞泥沙向海输运，使得落潮期间跨锋面输运的部分淡水与泥沙会被涨潮流裹挟回流。最终，陆源泥沙（terrigenous sediment）无法仅依靠潮流输运至渤海中部海域。在过去二十年的低径流量时期，随着黄河实施人工水沙调控（artificial water and sediment regulation），落潮均匀型潮汐切变锋的作用愈发凸显，常规工况下其对跨锋面泥沙输运的影响已不容忽视。此外，从长期视角来看，潮汐切变锋的切变强度与河口泥沙淤积向海延伸的距离呈显著正相关；河口区域的盐度与浊度梯度同样与切变强度呈正相关。这表明潮汐切变锋对跨锋面泥沙输运的阻滞作用，是黄河口河海相互作用（river-sea interaction）中的关键过程。