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.