The influence of turbulent bursting on sediment resuspension under unidirectional currents

Abstract

Abstract. Laboratory experiments were conducted in an open channel flume with a flat sandy bed to examine the role of turbulence on sediment resuspension. An acoustic Doppler velocimeter (ADV) was used to measure the instantaneous three-dimensional velocity components and acoustic backscatter as a proxy to suspended sediment concentration. Estimates of sediment transport assume that there is a mean critical velocity that needs to be exceeded before sediment transport is initiated. This approach does not consider the turbulent flow field that may initiate sediment resuspension through event-based processes such as the "bursting" phenomenon. In this paper, laboratory measurements were used to examine the sediment resuspension processes below and above the mean critical velocity. The results within a range above and below the measured mean critical velocity suggested that (1) the contribution of turbulent bursting events remained identical in both experimental conditions, (2) ejection and sweep events contributed more to the total sediment flux than up-acceleration and down-deceleration events, and (3) wavelet transform revealed a correlation between the momentum and sediment flux in both test conditions. Such similarities in conditions above and below the measured mean critical velocity highlight the need to re-evaluate the accuracy of a single time-averaged mean critical velocity for the initiation of sediment entrainment.

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 ).…”

Modeling Suspended Sediment Transport Under Influence of Turbulence Ejection and Sweep Events

“…Various descriptions of the relationship between ejection/sweep events and the hairpin vortices have been offered. Among others, ejection and sweep events are in different stages of vortex development, implying that ejection and sweep events happen consecutively (Kaftori et al, ; Salim et al, ). Another involves the attached eddy hypothesis that was originally proposed by Townsend (), according to which, vortices that are rooted in the near‐wall region are particularly important.…”

“…In addition to the directions of relative motions, researchers who examined the correlation in flow velocity/pressure time series and the swirling of the flow also observe the existence of the coherent structures particularly the ejections and sweeps in boundary layer flow and wall-bounded turbulence (Chong et al, 1998;Lehew et al, 2013;Sillero et al, 2014). The impact of ejection and sweep events in the near-wall region on sediment transport such as bed load transport, the entrainment/deposition processes, and sediment flux has been shown in recent studies (Cellino & Lemmin, 2004;Dwivedi et al, 2011;Nelson et al, 1995;Noguchi & Nezu, 2009;Radice et al, 2013;Rashidi et al, 1990;Salim et al, 2017;Schmeeckle, 2015). The aforementioned observations imply the existence of temporal and spatial correlations among the movement of sediment particles.…”

“…First, the specific flow structures mentioned above may contribute to the long‐term transport phenomena on the average. Previous studies have demonstrated the influence of the ejection/sweep events on the instantaneous sediment flux in experiments (Noguchi & Nezu, ; Salim et al, ). However, the net contribution of occurrences of ejection/sweep events to the long‐term average sediment flux remains unclear.…”

Geomorphic implications of gravity currents created by changing initial conditions

“…Sediment entrainment is a complex mechanism mainly due to the difficulty in defining the fluctuating nature of turbulent flow (Salim et al, 2017). In Zordan et al (under review, a) the transport of sediment within a gravity current is linked to the bed shear stress, that is here considered a "surrogate" measure of it.…”