Comparison of flow turbulence over a sand bed and gravel bed channel

Experiments were performed to examine the variations in flow turbulence with respect to sand and gravel bed channel. The 3D instantaneous velocity of water is measured by acoustic doppler velocimeter (ADV) at the test section, which will provide an important data related to the flow turbulence. The parameters of turbulence measured that the vertical profiles of longitudinal velocity in flows with a sand bed channel at the vicinity of the bed surface are higher by 2–6% than flow subjected to gravel bed channel, while away from the boundary surface, it decreases by 5–10%. The variations of Reynolds shear stress increase by 35–50% with gravel bed channel, indicating higher exchange of flow energy towards the boundary and vice versa. The higher value is obtained for shear velocities by 58.5% and the von Karman's constant with gravel bed channel. The gravel bed channel influenced the turbulence intensities with higher magnitude in the streamwise and vertical direction. The present study also analysed the flow anisotropy, correlations coefficient and turbulence diffusivity and compared the results. The outcomes of the current work are beneficial for civil and hydraulic engineers, since the data of turbulence will advance the management of bed materials present in the river. HIGHLIGHTS Water velocity is higher at the near bed in a sand bed channel.; A gravel bed channel increases the velocity fluctuations.; Flow momentum exchange is higher in a gravel bed channel.; Riverbeds are usually composed of sand and gravel, therefore analyzing the turbulent characteristics of flow is important.; There are benefits for the river engineer since the turbulence governs the morphological changes in the riverbeds.