Entrainment of coarse particles in turbulent flows: An energy approach

Abstract

[1] The entrainment of coarse sediment particles under the action of fluctuating hydrodynamic forces is investigated from an energy perspective. It is demonstrated that the entrainment of a grain resting on the channel boundary is possible when the instantaneous flow power transferred to it exceeds a critical level. Its complete removal from the bed matrix occurs only if the impinging flow events supply sufficient mechanical energy. The energy-based criterion is formulated theoretically for entrainment of individual spherical particles in both saltation and rolling modes. Out of the wide range of flow events that can perform mechanical work on a coarse grain, only those with sufficient power and duration or equivalent energy density and characteristic length scale may accomplish its complete dislodgement. The instantaneous velocity upstream of a mobile particle is synchronously recorded with its position, enabling the identification of the flow events responsible for grain entrainment by rolling at near incipient motion flow conditions. For each of the entrainment events, the energy transfer coefficient defined as the ratio of the mechanical work performed on the particle to the mean energy of the flow event responsible for its dislodgement obtains values ranging from 0.04 to 0.10. At the examined low-mobility flow conditions, the majority (about 80%) of the energetic structures leading to complete particle entrainment have a characteristic length of about two to four particle diameters.

The Physics of Sediment Transport Initiation, Cessation, and Entrainment Across Aeolian and Fluvial Environments

“…Likewise, moderate force pulses that only barely exceed resisting forces lead to entrainment if their duration is sufficiently long. That the duration of force peaks is as important as their magnitude has also been experimentally confirmed both for particles resting on idealized, fixed beds (Celik et al, , , ; Diplas et al, ; Valyrakis, ; Valyrakis et al, , , ) and natural erodible sediment beds (Salim et al, , ). However, note that, for sediment transport along erodible beds (with the exception of viscous bedload transport), the vast majority of entrainment events are triggered by particle‐bed impacts, except for very weak transport conditions (see sections and ).…”

Taking the river inside: Fundamental advances from laboratory experiments in measuring and understanding bedload transport processes

“…For example, if a large force does not persist long enough to completely move a grain out of its pocket, the grain may wobble, but it will still remain immobile. Impulse, which includes the magnitude and duration of drag forces, may be better correlated to grain movement than drag force magnitude alone (e.g., Diplas et al, 2008) or an energy approach may better represent grain motion (Valyrakis et al, 2013).…”

Dynamics and mechanics of bed-load tracer particles

“…Due in part to the difficulties in measuring tracer particle motion and near-bed stresses during floods, the fluid shear stress is commonly quantified through use of a bulkflow parameter such as the depth-slope product, τ b = ρg h S (Church and Hassan, 1992;Hassan et al, 1991;Ferguson and Wathen, 1998;Haschenburger and Church, 1998;Lenzi, 2004;Haschenburger, 2011), where h is the flow depth (m) and S is channel slope. For coarse-grained streams this simplification is perhaps more reasonable, as particle inertial timescales are large and thus coarse particles are insensitive to a range of turbulent stress fluctuations (Diplas et al, 2008;Celik et al, 2010;Valyrakis et al, 2010Valyrakis et al, , 2013. Although some readers may object to the assumption of steady and uniform flow for a flood, the flow may be considered quasisteady so long as the hydrograph varies slowly compared to the grain inertial timescale (on the order of several seconds), and quasi-uniform so long as water surface slope remains approximately constant.…”