Suspended Sediment Grains Transport in Open-Channel Flow: A Flume-Based Validation of Mixing Length Theory.

Abstract

The transport of suspended sediment in low-concentration flows is fundamentally 
governed by flow turbulence generated through basal shear stress. Traditional 
interpretations of sediment suspension and turbulent mixing have largely relied 
on the classical mixing-length turbulence theory developed by Prandtl, von 
Kármán, and subsequent researchers. Although more sophisticated turbulence 
models have emerged over recent decades, the mixing-length approach remains 
widely employed in hydraulic engineering, fluvial geomorphology, and 
sedimentology because of its conceptual simplicity, computational efficiency, and 
practical applicability. In particular, the theory continues to play an important 
role in the reconstruction and interpretation of ancient sedimentary 
environments. Despite its extensive application, the predictive capability of 
mixing-length theory has not been comprehensively reassessed using modern 
high-resolution flow measurement techniques. This study therefore evaluates 
the validity of the classical turbulence-suspension framework through a series 
of controlled open-channel flume experiments utilizing advanced Acoustic 
Doppler Velocimeter (ADV) technology. High-frequency velocity measurements 
were acquired to characterize turbulent flow structures and to assess the 
correspondence between observed flow behaviour and theoretical predictions. 
The experimental results demonstrate a strong agreement between measured 
and model-predicted velocity profiles, indicating that mixing-length theory 
provides a reliable representation of the time-averaged vertical velocity 
distribution in turbulent open-channel flows. The findings further confirm the 
continued usefulness of the theory for estimating basal shear stress and 
predicting suspended sediment transport under low-concentration conditions. 
Furthermore, analysis of the velocity data suggests that a von Kármán constant 
of approximately 0.29 yields a more accurate description of low-concentration 
sand-laden flows than the conventionally adopted value of 0.41. These results 
provide new experimental support for the continued application of classical 
turbulence theory while highlighting the need for revised parameterization in 
sediment-laden flow systems.