Optimization of Threshold Velocity Values for Sediment Transport at the Outer Bank of a 180-Degree Bend with Emergent Vegetation

The interaction between curvature-induced flow and vegetation plays a crucial role in regulating threshold velocity, influencing sediment transport dynamics. This experimental study investigates the effects of flow velocity and turbulence, induced by both emergent vegetation and curvature-driven flow, on the threshold of sediment motion around a vegetated patch. Using an Acoustic Doppler Velocimeter (ADV), a total of 504 velocity profiles were collected under vegetated and non-vegetated conditions, considering a range of vegetation densities (φ = 0.001–0.0099) in both a straight channel and a 180-degree bend. The results indicate that vegetation modifies turbulent kinetic energy (TKE) and velocity gradients, thereby enhancing sediment mobility. Specifically, vegetation significantly reduces maximum velocity by up to 37%, shifting the flow core to the center and enhancing TKE by up to 30 times. This analysis shows that channel curvature contributes a maximum of 34% and 17% to turbulent kinetic energy in the first and second halves of a 180-degree bend, respectively. Turbulence from the bed and vegetation accounts for 50% in straight paths, while in curved paths, it reaches 37% and 32% in the first half and 48% and 42% in the second half of a 180-degree bend. This study proposes a model for turbulent kinetic energy (kt) that incorporates velocity threshold constraints, validated through controlled laboratory experiments, highlighting the role of near-bed turbulence in modulating sediment transport. Furthermore, the findings demonstrate that sediment motion initiation is governed by both mean flow velocity and TKE, leading to the introduction of a novel criterion for assessing initial sediment transport conditions in curved and vegetated flows.