## 5 Conclusions  
Visualization testing was performed to better understand the mechanism of scour onset beneath a pipeline in regular waves. A miniature camera was enclosed in a transparent pipeline to observe the sediment in the onset of scour below a pipeline. At the beginning of the test, the sediment motion began near the downstream mudline, probably due to the distribution pattern of the seepage hydraulic gradient under the pipeline and the asymmetric waves adopted in this study. With the wave height gradually increasing to the specified value, the sand under the pipeline began to oscillate at increasing amplitude and the coverage of oscillation extended upstream. Some sand rushed out from the bottom of the pipeline in this process. After that, the sand oscillation became almost symmetrical around the bottom line of the pipeline. This change was related to the loss of sediment that rushed out. The loss of sand under the pipeline reduced the length of the seepage path, and thus increased the variation amplitude of the hydraulic gradient. The oscillatory amplitude continued to rise, probably because the increase in the variation amplitude of the hydraulic gradient and loosened sediment in the oscillation. Later, more sand rushed out before the onset of scour because of the increasing oscillatory amplitude. The onset of scour occurred when a jet flow with high concentration of sediment load was ejected from the bottom of the pipeline. The remaining sediment was scoured when the bottom flow reversed at the wave trough. The initial scour hole formed and extended rapidly.  
It was found in previous reports that the onset of scour in waves does not occur until the average seepage hydraulic gradient exceeded the critical value several times. This phenomenon was reproduced in the visualization of this study. In this study, the visualization analysis further revealed that the delay in the scour onset under a pipeline involves the increasing amplitude of sediment oscillation due to the increase in the variation amplitude of the hydraulic gradient and the loosened sediment during the continuous sediment oscillation.  
The following are available online at [[http://www.mdpi.com/2076-3417/10/9/2994/s1](http://www.mdpi.com/2076-3417/10/9/2994/s1)]([http://www.mdpi.com/2076-3417/10/9/2994/s1](http://www.mdpi.com/2076-3417/10/9/2994/s1)), Video SI: Video clip of scour onset process beneath a pipeline in regular waves.  
Conceptualization, Y.Z., L.X., and T.-C.S.; methodology, Y.Z., L.X., and T.-C.S.; investigation, Y.Z., L.X., T.-M.W. and T.-C.S.; resources, Y.Z. and L.X.; writing--original draft preparation, Y.Z.; writing--review and editing, Y.Z., L.X., and T.-C.S.; visualization, Y.Z., L.X. and T.-M.W.; supervision, L.X.; funding acquisition, Y.Z. and L.X. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China, grant numbers 11172213 and 51479137, and the China Scholarship Council, grant number 201806260166.  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.