Drag Forces on Sediment Particles in Unidirectional Water Flows

The estimation of drag forces acting on aquatic surfaces is of interest in many areas of hydraulic and eco-hydraulic engineering including the assessment of: friction factor; the initiation and rate of sediment transport; appropriate flow regimes for aquatic plants, invertebrates and other biota; and general flow-structure interaction problems. There remains, however, a shortage of data to evaluate mechanisms responsible for drag force generation and to test and refine models coupling velocity and drag force fluctuations. The aim of this study is to address this shortage with comprehensive measurements and analysis of the instantaneous drag forces acting on roughness elements combined with synchronous measurements of the surrounding velocity field.

Experiments were conducted with a roughness Reynolds number of 605 and flow depth (H) to particle diameter (D) ratios varying between 1.9 and 7.5. Stereoscopic particle image velocimetry was used in a transverse-vertical plane passing through the centre of a ‘target’ particle. The protrusion of the target particle, which was equipped with a drag force sensor, was systematically varied between 0 and 0.5D relative to the surrounding hexagonally packed spheres. Long duration measurements (up to 90 minutes) were conducted at each protrusion – flow depth combination in order to provide highly resolved spectra covering the full range of possible scales in the flow.

The results showed that the pre-multiplied drag force spectra have a bimodal shape characterised by a low frequency peak and a high frequency peak. With increasing particle protrusion, the drag force variance becomes increasingly dominated by the contributions from the low frequency process. The low frequency spectral peak is associated with high coherence between the streamwise velocity component and the drag force and likely results from the action of very large scale motions (VLSMs) which extend up to 50H in the flow direction. High frequency drag force fluctuations, previously thought to be related to wake turbulence, however, do not show appreciable coherence with point velocity measurements around the particle. Instead, we propose that the high frequency region of the drag force spectra is dominated by the action of pressure spatial gradients in the overlying turbulent flow. These findings are relevant to sediment transport processes, indicating that the key mechanism of particle entrainment may change from transient pressure events at low protrusion to high velocity events associated with VLSMs at high protrusion. Neither of these mechanisms are accounted for in current sediment transport models.