Settling velocity of fine sediment in quiescent water with varying sediment concentration and salinity

The settling velocity of cohesive sediment is a critical parameter in estuarine and coastal engineering. Its accurate determination is essential for numerical modeling, as it governs the deposition of fine-grained sediments and the subsequent geomorphological evolution of estuaries. Flocculation plays a significant role in the dynamics of these sediments. In this study, we employed a newly designed settling cylinder to analyze fine sediment and seawater samples collected from the Yangtze Estuary. Over 200 settling tests were conducted to investigate the settling velocity under varying conditions. The results reveal a highly complex, nonlinear interaction between suspended sediment concentration (SSC) and salinity in governing the settling velocity. An isolines map of settling velocity exhibits two distinct ridges, each indicative of a different settling-enhancement mechanism: one dominated by increasing SSC and the other by increasing salinity. The critical flocculation concentration, beyond which hindered settling occurs, is found to decrease with increasing salinity. Conversely, the critical flocculation salinity—marking the transition from flocculation-enhanced settling to settling retardation—increases with SSC until the hindered settling regime begins. A conceptual analogy model is proposed to account for aggregation process and an explicit formula to predict the settling velocity, incorporating SSC and salinity as key factors. The parameters in this formula are expressed as power functions of surface coverage parameter, which is related to salinity. Predictions from the formula show close agreement with the experimental data, effectively capturing the settling behavior across both the flocculation-accelerated and hindered settling phases. But this work only provides insights and a potential framework for future work, rather than a universally validated predictive model.