Heightening of the South Dam Field Sand-Blocking Dike in the North Channel of the Yangtze River Estuary and evolution of fluid mud in the deep-water navigation channel

To clarify the regulatory effect of the heightening project of the South Dam Field Sand-Blocking Dike in the North Channel of the Yangtze estuary on the characteristics of fluid mud in the deep-water channel and the actual effectiveness of sediment reduction, this paper conducts an in-depth analysis of the multi-dimensional characteristics of fluid mud before and after the project. It concludes that the amount of fluid mud in the deep-water channel has significantly decreased after the project, thereby verifying the sediment reduction effect of the project from the perspective of fluid mud. The research findings are as follows: (1) During normal weather and neap tides, the thickness, volume, and spatial distribution of fluid mud have all significantly decreased after the project. Specifically, the average reduction in fluid mud volume reached 550,000 cubic meters, the average thickness of fluid mud decreased by 0.21 meters, and the extent of fluid mud distribution (with a thickness greater than 0.5 meters) shortened by 5.30 kilometers. This substantial reduction indicates that the heightening of the sand-blocking dike has effectively suppressed the accumulation and spread of fluid mud under stable hydrological conditions, which is of great importance for maintaining the normal water depth of the deep-water channel. (2) During abnormal weather events, under the same cumulative wave energy conditions, both the volume and the amount of fluid mud incorporated into undisturbed sediment after wind events were clearly smaller following the heightening project than before. Abnormal weather, such as strong winds, often causes intense disturbance to the water body, leading to resuspension and redistribution of bottom sediment, which in turn affects the formation and development of fluid mud. The results show that the project can still play a positive role in controlling fluid mud even under extreme weather conditions, further confirming the stability and reliability of its regulatory effect. (3) After the project, the peak thickness of fluid mud decreased by 0.50 meters. For fluid mud with a thickness greater than 0.5 meters, the average thickness after the project was reduced by 0.21 meters, the distribution extent shrank by 5.46 kilometers, and the occurrence frequency decreased by 29%. The reduction in peak thickness indicates that the project has effectively mitigated the intensity of fluid mud accumulation in key areas, while the decrease in occurrence frequency reflects that the overall probability of fluid mud hazards in the channel has been significantly lowered, which helps reduce both the difficulty and frequency of channel maintenance. (4) The reduction in thickness, distribution extent, and occurrence frequency of large-thickness fluid mud was more pronounced. Large-thickness fluid mud is typically the main factor causing severe siltation in the deep-water channel, as it is less susceptible to scouring by water flow and tends to accumulate over long periods, thereby posing a greater threat to the navigation capacity of the channel. The more evident improvement in this aspect clearly demonstrates that the heightening project of the sand-blocking dike has achieved a targeted control effect on the key issues of fluid mud siltation. In summary, this study systematically reveals the changes in fluid mud characteristics before and after the heightening project of the South Dam Field Sand-Blocking Dike and fully confirms the positive role of the project in reducing fluid mud siltation in the deep-water channel. The research findings provide a key basis for the long-term stable maintenance of the deep-water channel in the Yangtze estuary, support rational planning of subsequent estuary regulation projects, optimize engineering design schemes, reduce channel maintenance costs, and further ensure the safety of regional water transport and the stable development of the local economy. At the same time, the study offers valuable reference experience for similar estuary and coastal engineering practices regarding fluid mud control and sedimentation reduction.