An Investigation into the Applicability of Friction Velocity Estimation Methods for the Channel with A Deposit Body

TL;DR

The paper tackles the challenge of estimating bed shear stress in rivers obstructed by a deposit body, where non-uniform flow alters near-bed friction. It employs a generalized flume setup to test the applicability of classical uniform-flow friction-velocity models, including RSS, TKE, TKE w', and the Logarithmic Law, against measurements under both uniform and deposit-affected conditions. The main finding is that fluctuation-based approaches (RSS$_{single}$, TKE, and TKE w') robustly capture the friction characteristics in non-uniform flows, with TKE providing the most comprehensive turbulence-aware description, while the Logarithmic Law can fail in recovery regions. This work advances understanding of friction velocity estimation in deposit-influenced rivers and informs sediment transport and river disaster simulations by highlighting the turbulence-driven nature of bed friction in complex flow fields.

Abstract

This study investigates how the deposit body influences friction characteristics by altering local flow fields, which is closely related to bed shear stress. Using generalized flume experiments, the study assesses the applicability of classical uniform flow friction models in deposit body river sections, revealing frictional changes induced by flow field non-uniformity. Initially, based on \( \frac{u_*}{\sqrt{\overline{w'^2}}} = 0.85 \sim 1.15 \) under uniform flow conditions as the judgment basis, the reliability of the classical model is verified. Four models are then applied to estimate near-bed friction velocity in deposit body sections. Results show a significant alignment between the longitudinal velocity gradient and the peak friction velocity derived from the turbulent kinetic energy method (TKE). Dimensional analysis of friction indicators reveals that: (a) friction velocity is primarily influenced by turbulence intensity, with constricted and narrowed sections resembling uniform flow, while the expansion section forms a peak; (b) models incorporating flow field fluctuations (TKE, Vertical Turbulence Kinetic Energy (TKE w'), Reynolds shear stress method (RSS)) effectively capture the impact of non-uniform flow fields on friction characteristics; (c) when energy states are low or when deposit body proportions are large, the deposit body's resistance ratio increases, and peak friction velocity rises. This study provides theoretical insights into friction estimation and sediment transport in non-uniform flow fields of deposit bodies.

Figures (8)

• Figure 1: (a) Photograph of the flume; (b) Three-dimensional schematic diagram of deposit body models((1)M1:$b/B$=0.5, $S$=45°; (2)M2:$b/B$=0.4, $S$=45°; (3)M3:$b/B$=0.3, $S$=45°); (c) Schematic illustration of the flume(The deposit in the figure is M1); (d)The plan view of the flume section with measurement points( The red hollow points indicate measurement locations under uniform flow conditions, while the black solid points correspond to measurement locations under deposit body conditions.)
• Figure 2: Estimation of Near-Bed Friction Velocity under Uniform Flow Conditions
• Figure 3: Percentage Distribution of Data Points (%) for $\frac{u_*}{\sqrt{\overline{w'^2}}} = 0.85 \sim 1.15$
• Figure 4: Presents a schematic representation of the river flow zonation within the deposit body.
• Figure 5: Velocity vectors and longitudinal velocity contour map for experimental group B3 (cm/s)