Estimation of bed shear stress using the turbulent kinetic energy approach—A comparison of annular flume and field data

Abstract

Annular flumes are often used to quantify the interactions between hydrodynamics, biological activity and sediment dynamics. Therefore it is essential that experimental laboratory flume systems adequately replicate natural conditions occurring in the field. This paper applies the turbulent kinetic energy (TKE) approach to the determination of bed shear stresses ( τ 0) on natural sediments from several sites in southern England and briefly discusses the advantages of this method compared to alternatives (log-profile) in the context of a combined flume and field study. Sediments studied varied in roughness length from 0.0013–1.18×10 −3 m (drag coefficient 0.0017–0.0204) and exhibited a non-linear increase in bed shear stress with increasing current velocity. The slope of this relationship increased also with increasing bed roughness. Several ‘smooth’ sediments (roughness length <0.0036×10 −3 m, C D <0.0024) showed very similar relationships between bed shear stress and current velocity, enabling estimation of bed shear stress from velocity measurements alone. Results from flume studies showed the suitability of open cell polyurethane foam to act as a surrogate smooth sediment in laboratory investigations. Data presented here for natural sediments shows that comparison of bed shear stress versus current velocity in a laboratory annular flume with data for the same sediments in the field reveal very similar relationships, highlighting the suitability of laboratory annular flume systems as platforms to study critical bed shear stress and critical erosion velocity of natural sediments under near-natural conditions.