## 4 Discussion  
In this study, the elementary mechanism of the scour onset in waves is studied based on the full view visualization of the sediment behavior under a pipeline. The phenomena in the tests are discussed and analyzed in this section. At the beginning of the test, the sediment motion began adjacent to the downstream mudline (Figure 5b). The sediment incipient motion can be expounded by the distribution pattern of seepage hydraulic gradient under the pipeline. The distribution of the seepage hydraulic gradient under a pipeline is not even. In steady currents, the maximum seepage hydraulic gradient appears at the downstream side below the pipeline, close to the mudline [21; 22]. In regular waves, the near-bottom flow is bidirectional, so the maximum hydraulic gradient can appear near the upstream and downstream mudlines. The waves in this study were non-linear, so the downstream bottom flow and seepage were more powerful than their upstream counterparts. A similar variation mode of the seepage hydraulic gradient can also be seen in [PERSON] et al. [3], where the wave was also nonlinear. As a result, the maximum seepage hydraulic gradient was located near the downstream mudline (Figure 6a). When the test started, the wave height gradually increased to the specified value and the local seepage hydraulic gradient overwhelmed the critical value of the sediment first near the downstream mudline, where the incipient motion of sediment under a pipeline occurred (Figure 6b). Herein, the critical seepage hydraulic gradient indicates the hydraulic gradient in the sediment at the critical point of the seepage failure, which is a function of the buoyant unit weight of soil. After the sediment incipient motion under the pipeline, more sediment began to move, and the oscillating amplitude of the sediment increased (Figure 5d-g). The expansion of the area affected by sediment movement can be attributed to the increase in the rising wave height (Figure 6c). With the wave height gradually increasing to the specified value, the variation amplitude of the seepage hydraulic gradient increased accordingly. Similar variation of hydraulic gradient can be seen in [PERSON] et al. [3]. The area where the local hydraulic gradient exceeded the critical value extended upstream, so the area experiencing sand particle oscillation extended upstream as well. The increasing oscillating amplitude of the sediment can also be explained by rise in the amplitude of the hydraulic gradient variation. The sand oscillation turned to be almost symmetrical around the bottom line of the pipeline after \(t=t_{0}+6.522T\) (Figure 5h-j). This change in the sediment oscillation pattern is analyzed below. At \(t=t_{0}+2.481T\), some sediment particles rushed out from the bottom of the pipeline due to the  
Figure 6: Illustration of the sediment incipient motion under the pipeline (**a**) maximum hydraulic gradient under the pipeline; (**b**) sediment incipient motion near the downstream mudline; (**c**) increasing oscillating amplitude and extending coverage of the sediment oscillation. remarkable rise in sediment oscillation amplitude (Figures 5f and 7a). Thus, the remaining sand fell almost symmetrically around the bottom line of the pipeline due to gravity, and continued moving back and forth between curves A\({}_{1}\)B\({}_{1}\) and C\({}_{1}\)D\({}_{1}\) (Figures 5h and 7b). In this period of time, the wave height may have reached the specified value, but the oscillatory amplitude of the sand particles continued to rise (Figure 5i,j). The rise in the oscillatory amplitude may be attributed to two factors. On one hand, the seepage path under the pipeline was shortened due to the loss of the sand under the pipeline (Figure 7c). Thus, the variation amplitude of the average hydraulic gradient under the pipeline increased with the wave height unchanged. On the other hand, the sand below the pipeline was continuously loosened due to oscillation.