## 4 Discussion
A possible explanation for this is due to the much higher magnitude of the \(Q_{2}\) and \(Q_{4}\) events compared to the \(Q_{1}\) and \(Q_{3}\) events, eventually resulting at higher stress values ([PERSON] et al., 2017; [PERSON], 1991). This can be clearly seen for example from the joint probability distributions of the velocity component fluctuations (\(u^{\prime}\) and \(v^{\prime}\)), according to [PERSON] (2003); [PERSON] (2003). The joint probability distribution has an elliptical shape which is significantly more elongated in the \(Q_{2}\) and \(Q_{4}\) quadrants. As a consequence, the \(Q_{2}\) and \(Q_{4}\) events are having more high magnitude values while the \(Q_{1}\) and \(Q_{3}\) events will be limited to relatively smaller magnitude values. This observation aligns with the fact that impulses above a critical level, linked to entrainments, should be of a higher value, thus the contribution of the higher magnitude \(Q_{4}\) events will be more pronounced compared to the \(Q_{1}\) events. Additionally, [PERSON] et al. (2017) suggested that sweeps are the more dominant mechanism in sediment entrainment while \(Q_{4}\) and \(Q_{2}\) cancel each other out on an immobile bed, giving rise to the \(Q_{1}\). Other researchers ([PERSON] et al., 2017; [PERSON], 2017) have also reported the predominance of the role of Q\({}_{4}\) and to a lesser extent \(Q_{1}\) in sediment entrainment. According to the rest of the figures in the results section, considering all five runs, for both cases of critical impulse, i.e., \(I_{cr}\) = 0.0445 m\({}^{2}\)/s and \(I_{cr}\) = 0.0063 m\({}^{2}\)/s, for both investigation windows of readings, i.e., 1-5 readings and 1-3 readings and for before and after the occurrences of impulses above the critical impulse, it is clear that most of the transitions between the quadrant readings are \(Q_{4}\) to \(Q_{4}\), \(Q_{1}\) to \(Q_{4}\) and \(Q_{1}\) to \(Q_{1}\). In other words, it is clear from the results of this study that the impulses above the critical impulse, i.e., the impulses that have caused the instrumented particle's entrainment, are relatively short-lived sweep events and to a lesser extent, relatively short-lived outward interactions. This conclusion is supported by the observation that the majority of transitions between quadrant readings occur before and after the first and last quadrant readings of the impulses above the critical impulse being \(Q_{4}\) to \(Q_{4}\) and \(Q_{1}\) to \(Q_{1}\). This is in addition to the transitions between sweeps and outward interactions, i.e., \(Q_{4}\) and \(Q_{1}\). The presence of even a weak but non-negligible secondary flow in a rectangular flume can affect the structure and intensity of the shear flow and subsequently influence the probability of sediment entrainment rates. Secondary flow is significantly affected via the flume's aspect ratio. In this study, the aspect ratio of the rectangular channel for the flows tried, is about 8.2 and it is almost fixed, given that the flow depth is changing less than 5%. [PERSON] et al. (2016), assessed the impact of secondary flow in an open channel for ratios ranging from 3 to 7.5 and found the presence of more secondary flow cells for the higher aspect ratios, which is also closer to the aspect ratio in our experiment. These can affect the flow velocity and flow energy distribution, resulting for example in possible deviations from the mean logarithmic flow velocity profiles, which we have also observed. These are expected to change as the aspect ratio of the flume is altered but given its infinitesimal change for the flow rates trialled herein, it is expected that any contribution of secondary flows to the entrainment rate of the target coarse particle, will remain the same. Finally, do the impulses correspond to coherent flow structures and vice versa? The answer to that is yes and no. An impulse includes the magnitude and duration of the turbulent flow events. However, the impulse criterion is a resistance dependent criterion.