Drag Forces on Sediment Particles in Unidirectional Water Flows

The fluctuating drag forces acting on spherical roughness elements comprising the bed of an open-channel flow have been recorded along with synchronous measurements of the surrounding velocity field using stereoscopic particle image velocimetry. Pre-multiplied spectra of drag force fluctuations were found to have bi-modal shapes with a low frequency ‘hill’ corresponding to the presence of very-large-scale motions (VLSMs) in the turbulent flow. The high frequency ‘hill’ of the drag force spectra cannot be explained by velocity time series extracted from points around the particle, but instead appears to be induced by the action of the spatial pressure gradients in the overlying flow field. Experiments with lightweight mobile particles confirmed that VLSMs are key contributors to sediment entrainment. Further data are required to clarify the role of particle lift forces, and particle shape in the entrainment process.