Threshold for particle entrainment into suspension

Abstract

Laboratory observations regarding the limit conditions for particle entrainment into suspension are presented. A high‐speed video system was used to investigate conditions for the entrainment of sediment particles and glass beads lying over a smooth boundary as well as over a rough bed. The results extend experimental conditions of previous studies towards finer particle sizes. A criterion for the limit of entrainment into suspension is proposed which is a function of the ratio between the flow shear velocity and particle settling velocity. Observations indicate that particles totally immersed within the viscous sublayer can be entrained into suspension by the flow, which contradicts the conclusions of previous researchers. A theoretical analysis of the entrainment process within the viscous sublayer, based on force–balance considerations, is used to show that this phenomenon is related to turbulent flow events of high instantaneous values of the Reynolds stress, in agreement with previous observations. In the case of experiments with a rough bed, a hiding effect was observed, which tends to preclude the entrainment of particles finer than the roughness elements. This implies that, as the ratio between particle and roughness element sizes becomes smaller, progressively higher bed shear stresses are required to entrain particles into suspension. On the other hand, an overexposure effect was also observed, which indicates that a particle moving on a smooth bed is more prone to be entrained than the same particle moving on a bed formed by identical particles.

The influence of turbulent bursting on sediment resuspension under unidirectional currents

“…Overall, in the time series significant variability and intermittency both in Reynolds stress (u , w ) and sediment resuspension (backscatter) was also revealed. Such intermittent nature of u w was expected and observed previously in the laboratory (Grass, 1974;Sumer and Oguz, 1978;Sumer and Deigaard, 1981;Niño et al, 2003;Schmeeckle, 2015) and in the field (Heathershaw and Thorne, 1985;Drake et al, 1988;Soulsby et al, 1994;Kularatne andPattiaratchi, 2008 andYuan et al, 2009). In more detail, the time series of the AMCV run showed 28 major resuspension events (Fig.…”

“…Although Lavelle and Mofjeld (1987) previously reviewed the concept of critical stress for the initial motion of non-cohesive sediment beds under turbulent flow conditions suggesting the non-existence of true threshold in the movement of sediment, their conclusions were based on photographic observations employed in conjunction with current measurements to infer sediment thresholds in the field. Likewise, the work from Niño and Garcia (1996) and Niño et al (2003) identified such instantaneous events from high-speed videos, which limit the number of captured and analysed events. We examined the influence of turbulent coherent structures on sediment resuspension for flows both above and below the measured mean critical resuspension velocity over a flat sandy bed using widely used acoustic instruments.…”

Longitudinal profile and sediment mobility as geomorphic tools to interpret the history of a fluviokarst stream system

“…The d 50 particle represents the median size of the sediment substrate and a lower bracket for estimating sediment mobility. The d 85 particle represents an upper threshold for estimating armor destabilization and wholesale substrate mobility (Parker & Klingeman, 1982;Nino et al, 2003).…”

The morphology and evolution of channels on the Wax Lake Delta, Louisiana, USA

“…Sediment travels as bed load once the critical shear velocity (u * c ), is exceeded and as long as w s =u à sk ≥ 2:5 [Niño et al, 2003], where w s is the particle fall velocity calculated using the methods of Dietrich [1982]. The development of a measurable suspended sediment concentration profile typically occurs when w s =u à sk ≤1 [Bagnold, 1966].…”