Applicability of Friction Velocity Estimation Methods in Sediment-Deposit-Induced Non-Uniform Open-Channel Flow

ABSTRACT Wenchuan earthquake–induced landslides generated large volumes of sediment that formed lateral sediment deposits in mountainous river channels, significantly altering flow structures and channel evolution. Under such strongly non-uniform flow conditions, accurate estimation of bed shear stress remains challenging. Because bed shear stress is difficult to measure directly in natural rivers, friction velocity is commonly used as an indirect proxy. This study conducted fixed-bed flume experiments with representative riverbank sediment deposits to evaluate six classical friction-velocity estimation methods under uniform-flow conditions and four methods—the Reynolds shear stress (RSS), turbulent kinetic energy (TKE), vertical turbulent kinetic energy (TKE w′), and logarithmic law methods—under sediment-deposit conditions. The results show that estimated friction velocity is more closely associated with turbulence intensity than with longitudinal mean flow velocity. Turbulence-based methods (RSS, TKE, and TKE w′) showed more consistent responses than the logarithmic law method under strongly non-uniform flow conditions, with the TKE method exhibiting the most stable performance across different flow zones. This study clarifies the applicability and limitations of classical friction velocity estimation methods under different degrees of flow non-uniformity, providing guidance for estimating bed shear stress and conducting sediment-transport studies.