Abstract

Influence of bed roughness on near-bed flow characteristics is of considerable importance, especially in turbulent flows over rough beds. This experimental study is devoted to insight various turbulent flow characteristics such as velocity distributions, Reynolds shear stresses (RSS) and velocity power spectra at the groove, lee and crest level of carrier fluid in an immobile gravel bed stream. Uniform round shape gravels were used in five layers in a laboratory flume. Nortek’s Vectrino four-receiver Acoustic Doppler velocimeter (ADV) probe was used to capture the instantaneous three-dimensional velocity components at groove. The interference between incident and reflected pulses in the ADV data measured in the near-bed flow zone produced spikes. Spikes were detected and filtered with a spike removal algorithm based on the acceleration threshold method. The velocity power spectra provides a satisfactory fit with the Kolmogorov “–5/3 scaling-law” in the inertial subrange. Velocity distributions below the roughness crest level fairly follows a third-degree polynomial series.

Key words: Bed roughness, gravel-bed stream, turbulence, Reynolds shear stresses, inertial sub-range.