Jornal de Geologia e Geofísica
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Sediment Grains Entrainment: Comparing Bed Shear Stress Estimation Methods

Lucky O Imagbe*

Sediment bed shear stress is well known to effectively contribute to the entrainment of sediment grains from parent bed as well as in the initiation of grain motion into suspension flow. Direct measurement of the magnitude of bed shear stress in varying field conditions has always presented difficult challenges hence it is estimated from observations and analysis of flow velocity fluctuations above the flow bed.

Reports of some recent researches have shown poorer agreement in estimates of bed shear stresses from existing multiple sources including the bed slope, log profile, Reynolds stress distribution, turbulent kinetic energy and this raises questions with regards to the estimates validity. Estimates obtained for bed shear stresses from multiple methods are expected to show considerable agreement using similar sets of flow data.

This paper reports the investigation carried out to determine the consistency and possible correlation in bed shear stress estimates or otherwise obtained from multiple estimation methods, using an unusually large flow fluctuating velocity dataset obtained from a laboratory flume tank experiment instrumented with a three-component (3-C) Acoustic Doppler Velocimeter (ADV).

The analysis of results and comparison of estimates from three methods suggest a significant consistency in estimates especially estimates using the log profile and Reynolds stress methods. However, estimates from the bed slope methods seemed to be relatively higher with up to 26% as in experimental case 5, when compared to the other two methods. These research findings further affirm the reliability of existing methods of bed shear stress evaluation especially the log profile and Reynolds stress.