## 4 Discussion
To define the window of investigation of the quadrant readings for the case of \(I_{cr}\) = 0.0063 m\({}^{2}\)/s, the histogram of the durations of the impulses above the critical impulse, Figure 24, was assessed. Close investigation of the histogram of the durations of impulses above 0.0063 m\({}^{2}\)/s indicates that about 90% of the impulses have durations of less than 0.12 s. This duration, 0.12 s, is equivalent to up to three ADV readings. Therefore, it has been decided to investigate the quadrant readings for a window of the first few readings, in the case of \(I_{cr}\) = 0.0063 m\({}^{2}\)/s. The mean time scale of the flow structure that is of comparable size to the instrumented particle, and thought to be relevant to its entrainment [21], has been identified in Equation (3). The authors of [21] have shown that at near threshold flow conditions, the impulses that may lead to coarse particle entrainment have timescales that correspond to about 1-3 times the particle's representative length-scale (diameter). Thus, in our analysis it is considered that the near threshold impulse events are mostly short-lived and as a consequence better represented by a sequence of one or two readings (given the ADV recording frequency and the entrainment inducing impulses). We could also be assessing sequences of three or more readings, but these would primarily correspond to impulses above critical, much less than threshold and near threshold impulses that are relatively more long-lived (e.g., only a small percentage according to [21; 22]). Figure 23: Histogram of the durations of the flow impulses for all five runs of the experiment (for \(I_{cr}\) = 0.0445 m\({}^{2}\)/s). Figure 24: Histogram of the durations of the flow impulses for all five runs of the experiment (for \(I_{cr}\) = 0.0063 m\({}^{2}\)/s). As for the quadrant readings in the flow, it is clear from Figure 4 that there is a slight dominance in the occurrences of \(Q_{4}\) and \(Q_{2}\) quadrant readings in comparison to the occurrences of \(Q_{1}\) and \(Q_{3}\) quadrant readings in the flow. This is similar to some of the observations that have been reported in the literature for a similar type of flow ([PERSON], 1981; [PERSON], 1991b). As for the quadrant readings within the impulses, sweeps have occurred more than outward interactions during the events with impulses above the critical impulse (for both cases \(I_{cr}\) = 0.0445 m\({}^{2}\)/s and \(I_{cr}\) = 0.0063 m\({}^{2}\)/s) as can be noticed from Figures 5 and 16. The difference between the percentages of occurrence of \(Q_{4}\) readings and \(Q_{1}\) readings in the flow (shown in Figure 4) does not justify the difference between the percentages of the occurrences of \(Q_{4}\) and \(Q_{1}\) during the events with impulses above the critical impulse. This conclusion is similar to the findings of [PERSON] et al. (2003), i.e., \(Q_{4}\) moves the majority of the sediment followed by \(Q_{1}\) and to a much lesser extent, \(Q_{2}\) and \(Q_{3}\). However, according to [PERSON] et al. (2003), the main reason that \(Q_{4}\) moves the majority of the sediment compared to \(Q_{1}\) is due to its common occurrence. This is not the case in this study. In other words, and as is clear from Figure 4, for the five runs of the experiment, the percentages of quadrant readings that have occurred in the flow during the periods of ADV recordings at the position of investigation are almost the same with the slight dominance of \(Q_{2}\) and \(Q_{4}\) (i.e., sweep and ejection events). [PERSON] et al. (2017) suggested that a higher probability of occurrence of a high-magnitude force is induced by sweep events. Of importance is to observe that even though the \(Q_{2}\) and \(Q_{4}\) events are somewhat more commonly observed than the \(Q_{1}\) and \(Q_{3}\) events (Figure 4), the majority of the high magnitude impulses are due to \(Q_{4}\) events (according to e.g., Figure 5).