Navigation-Induced Suspended Resuspension at a River Confluence: Phase Lag and Amplitude Attenuation

Vessel movement at river confluences can erode bottom and bank sediments. While vessel-generated flows are known to resuspend significant amounts of sediment, limited data exist on the timing and mechanisms of this process. Suspended sediment concentration (SSC) responds to vessel-induced flow changes with a measurable phase lag and amplitude attenuation. This study quantitatively describes these phenomena using laboratory results from a 90° river confluence model, incorporating high-resolution SSC and three-dimensional velocity measurements. The results show that SSC consistently lags behind vessel-generated flows, with the lag increasing with height above the bed. Near-bed SSC typically equilibrates within one to two wave periods, whereas lag times near the water surface are longer due to persistent turbulence injection from vessel-induced underflow. A strong correlation (R² ~ 80%) was observed between SSC and turbulent kinetic energy (TKE), highlighting that SSC increases with rising TKE. The magnitude of sediment resuspension also depends on sediment availability, particularly at sediment bar formations. Sediment transport was predominantly directed toward the bankside, with occasional weak transport toward the channel center, influenced by wave groups and low-frequency drawdown timing. A wave-averaged suspended-load model was used to quantify the SSC lag relative to vessel-generated flows. Incorporating a decay rate of 0.06 s⁻¹ for antecedent waves significantly improved suspended-load predictions downstream of the confluence. Applying this decay rate across five additional sections reduced mean absolute error by 1.5 to 2 times compared to the unmodified model. The entrainment constant in the model varied slightly with water depth.