Uncertainty analysis for direct bed shear stress measurements using shear plates

Abstract

Accurate estimation of bed shear stress is crucial for understanding sediment transport and morphodynamic processes in fluvial and environmental flows. Shear plates offer a direct measurement approach that avoids the restrictive flow assumptions of many indirect methods. However, their broader use has been limited by misconceptions and concerns about measurement uncertainty. This study addresses these challenges by applying uncertainty analysis based on the Guide to the Expression of Uncertainty in Measurement (GUM), specifically adapted to quantify and evaluate the reliability of shear plate measurements. Experiments were conducted under three hydraulically complex conditions: propeller-induced jets, flow through rigid emergent vegetation, and flexible vegetation over three-dimensional bedforms. Results show that shear plates consistently deliver low relative uncertainties, with over 70% of measurements below 10% and 44% below 5%. The dominant sources of uncertainty were associated with experimental conditions, particularly flow non-uniformity and the need to estimate vegetation drag, while contributions from the measurement instrumentation itself remained minor. Comparisons were made with the traditional gravity method, an indirect approach that estimates bed shear stress based on the energy slope within a control volume. Shear plates provided more accurate and reliable estimates, particularly in vegetated flows where local gradients compromise slope-based calculations. This study demonstrates the practical value of applying the GUM framework in experimental hydraulics, promoting greater confidence in shear plate measurements.

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