Effects of turbulence on the transport of individual particles as bedload in a gravel‐bed river

Abstract

This study investigates the association between mean and turbulent fl ow variables and the movement of individual particles in a gravel-bed river. The experimental design implemented in the Eaton-North River (Québec, Canada) is based on the simultaneous observations at a high temporal resolution of both particle movements as bedload using an underwater video camera and of the streamwise and vertical fl ow velocity components using a vertical array of three electromagnetic current meters (ECMs). The frequency and distance of displacement of particles larger than 20 mm that were sliding or rolling on the bed were measured from a 10 minutes long fi lm. Mean and turbulent fl ow properties obtained for periods without sediment transport are compared to those when particles were sliding and rolling. When particles are sliding, weak differences are present for the mean streamwise velocity and normal vertical stresses. Instantaneous Reynolds shear stresses are signifi cantly lower for sliding events which was not expected but could be explained by the important dominance of Quadrant 3 events (inward interactions). When particles are rolling, only the vertical normal stresses show a weak difference from those observed in the absence of transport but they tend to occur when Quadrant 2 (ejections) dominate the fl ow fi eld. For both sliding and rolling particles, vertical and/or streamwise fl uid accelerations show high magnitude values when compared to periods without transport. For sliding particles, streamwise acceleration is mostly negative and combines most of the time with a positive vertical acceleration. For rolling particles, streamwise and vertical acceleration are predominantly of opposite sign. These results suggest that fl uid acceleration or deceleration in the streamwise and vertical directions is affecting the pressure fi eld around particles. In this study, fl uid acceleration seems to play a more important role than Reynolds shear stress or normal stresses for bedload movements.

On the evolution and form of coherent flow structures over a gravel bed: Insights from whole flow field visualization and measurement

“…Furthermore, there is a need for such findings to be extended to examine how these turbulent flow structures influence the sediment transport dynamics [e.g., Nikora and Goring , ; Shvidchenko and Pender , ; Maddux et al ., ; Nelson et al ., ; Yager and Schott , ] and influence subsurface hyporheic flows [e.g., Blois et al ., ]. The relation between local turbulence and forces acting on the sediment grains is not understood, and as such there is still little agreement as to which turbulence statistic is the best descriptor to predict sediment entrainment [ Rennie and Millar , ; Schmeeckle et al ., ; Wren et al ., ; Coleman and Nikora , ; Paiement‐Paradis et al ., ]. The mechanism which causes sediment to be entrained is dominated by drag, lift, or a product of both forces [ Vollmer and Kleinhans , ; Schmeekele et al ., ; Yager and Schott , ] although the contribution from each component has yet to be fully quantified.…”

Macroturbulent coherent structures in an ice‐covered river flow using a pulse‐coherent acoustic Doppler profiler

“…Most notably, the formation of an ice cover has immediate and profound impacts on the flow structure of the turbulent boundary layer (TBL). The structure of the turbulent flow in the boundary layer acts on mixing processes, sediment transport (Lapointe, ; Drake et al ., ; Cellino and Lemmin, ; Paiement‐Paradis et al ., ), morphological change (Leeder, ; Best, ) and aquatic habitat (Davis and Barmuta, ; Carling, ). Central to the theme of turbulent flow structure is the occurrence of quasi‐periodic coherent flow motions that promote strong exchanges between the inner and outer layers of the flow.…”

Bridging the gap between turbulence and larger scales of flow motions in rivers

“…These stresses play a critical role in river dynamics as they are associated with both suspended and bedload sediment transport (e.g. Lapointe, ; Nino and Garcia, , Schmeeckle et al , ; Paiement‐Paradis et al , ). While this analysis shows alternating periods of dominance of ejections and sweeps which is to be expected, it also highlights the temporal clustering of events of a certain type giving rise to a dominant velocity signature at larger scales.…”

“…These elongated wedges develop over the entire depth of the flow and their length is two to five times the flow depth (Y ) (Roy et al, 2004). Recent field and laboratory studies have highlighted the role of the macroturbulent flow structures on fish behaviour (Enders et al, 2003;Liao et al, 2003) and on the motion of individual grains as bedload (Paiement-Paradis et al, 2011). These macroturbulent structures have a short duration (in the range of a few seconds) with respect to the scale of the river reach and of the long flow pulsations that have been reported in the literature (Murphy, 1904;Savini and Bodhaine, 1971;Lapointe, 1992;Dinehart, 1999).…”