###### Abstract
As part of a further investigation into the relationship between turbulence and sediment entrainment, researchers have started to investigate whether there are specific flow events that are associated with the entrainment of sediment. Specifically, the observation of [PERSON] et al. [23] showed that among the random and complex turbulent events, there are elementary organised structures with spatial and temporal coherence. This has generated interest among researchers in relation to further investigating the flow structures that exist in boundary layers, linking the observation of [PERSON] et al. [23] to the sediment entrainment problem. At the time of this work, there has been an extensive research output on the investigation of the coherent flow structures in the boundary layers for rough and smooth wall flows [24; 25; 26]. As for the study of sediment transport, some researchers [12; 14; 27] have reported that although there are chaotic turbulent fluctuations, there are specific events or flow structures that are associated with sediment transport episodes. These events have both temporal and spatial coherence. The method of quadrant analysis has been used widely to report the type of events occurring during sediment entertainment and sediment transport. The method was developed by [PERSON] et al. [28] to quantify the visual observations of [PERSON] and [PERSON] [29] by dividing the turbulent velocity fluctuations based on their signs into \(Q_{1}\) (\(+u^{\prime}\), \(+w^{\prime}\)), \(Q_{2}\) (\(-u^{\prime}\), \(+w^{\prime}\)), \(Q_{3}\) (\(-u^{\prime}\), \(-w^{\prime}\)) and \(Q_{4}\) (\(+u^{\prime}\), \(-w^{\prime}\)). Ejection (\(Q_{2}\)) and sweep (\(Q_{4}\)) events contribute positively to Reynolds shear stresses. These events were named by [PERSON] and [PERSON] [29] based on their experimental observations. Outward (\(Q_{1}\)) and inward interactions (\(Q_{3}\)) contribute negatively to Reynolds shear stresses and were named by [PERSON] et al. [28] for a lack of a better descriptive terminology. [PERSON] and [PERSON] (1985) demonstrated that most of the sediment entrainment has been caused by sweep events (\(Q_{4}\)) and to a lesser extent by outward interactions (\(Q_{1}\)). Multiple researchers [30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40] have suggested that entrainment is associated with high Reynolds shear stress structures, \(Q_{4}\) or sweep, while ejections are responsible for the particles' suspended motion. Additionally, there is a tendency for there to be an existence of a group of increasingly stronger \(Q_{2}\) (ejection) events followed immediately by \(Q_{4}\) (sweep) events during the burst-sweep cycle which have resulted in sediment entrainment. These two quadrants have been reported to contribute to Reynolds stresses and momentum transfer, which is much stronger near the wall region [41; 42]. [PERSON] et al. [43], on the other hand, suggested that the sweep component of the burst-sweep cycle resulted in the majority of the sediment transport due to its common occurrence. The events of outward interactions of the same duration and magnitude can result in the same transportation as sweep events do. In another stream of investigation into sediment transport, some researchers have tried to link sediment entrainment to turbulent flow structures and to fluctuating drag and lift using laser Doppler velocimetry (LDV) and particle image velocimetry (PIV) measurements in addition to pressure measurements [21; 44; 45; 46; 47; 48; 49; 50]. The conclusion of all their studies is that there are \(Q_{4}\) events associated with high positive drag force fluctuations at the time of sediment particle entrainment. As for the lift fluctuations, their role is less clear in sediment entrainment. [PERSON] et al. [46] reported positive lift and drag during sweeps, [PERSON] et al. [51] suggested that sweep may induce negative lift and finally, [PERSON] and [PERSON] [50] have reported there to be advected pressure fluctuations due to passing vortices which lead to a positive lift just before \(Q_{4}\) events.