## 2 Experiment Setup  
The experiments were performed in a glass-sided hydraulic flume at the Laboratory of Hydraulic and Harbor Engineering, Tongji University. The flume (Figure 2) is 50 m in length, 0.8 m in width and 1.2 m in depth. A piston-type wave maker capable of producing regular and irregular waves with a period between 0.5 s and 5.0 s was installed at the upstream end of the flume. The maximum wave height was 0.25 m at 0.5 m water depth. The error of the wave maker in the regular waves was lower than 0.1% in wave period and lower than 2% in wave height. A wave absorber was deployed at the downstream end of the flume. A sediment recess, 2.7 m long and 1.0 m deep, was located in the central part of the flume. The side walls and the bottom of the flume were made of impermeable concrete. Medium sand with a medium grain size \(d_{50}=0.283\) mm and geometric standard deviation \(\sigma_{\rm g}=(d_{84}/d_{16})^{1/2}=1.8\) was used as the bed sediment in the study. A transparent model pipeline made of acrylic was used in the experiments (Figure 3). The diameter of the model pipeline was \(D=0.11\) m and the length was 0.80 m. The pipeline was partially buried, with an embedment ratio \(e/D=0.045\). Two jack screws were used to fix the pipeline in the flume, and thus pipeline movement was considered to be negligible. A Logitech C920 camera was installed inside the pipeline to observe the scour process beneath the pipeline. The resolution of the camera was 1920 \(\times\) 1080 pixels and the average frame rate of the video was 23.94 fps with a maximum of 30 fps. A 5 mm-sized mesh was printed on the pipeline surface, covering the majority of the camera view. Two ballast blocks were enclosed in the sealed pipeline to improve stability. The setup of the model pipeline was identical to that previously presented in [PERSON] et al. [9]. Figure 4 is a typical view of the camera, where lines AB and CD indicate the intersection of the bed surface and the pipeline, also called the mudline. The thin yellow dot-dash line in the center of the view illustrates the bottom line of the pipeline; the buried part of the pipeline is symmetrical around this line. A Nortek Vectrino high-resolution acoustic digital velocimeter (ADV) was installed 3.0 m upstream of the sediment recess to monitor the orbital velocity at 1.0\(D\) above the bed (Figure 2). The range of the ADV was \(\pm\)1.0 m/s, and the accuracy was \(\pm\)0.5% of measured value \(\pm\)1 mm/s. The sampling frequency was 100 Hz. A wave gauge was set 1.0 m upstream of the ADV. Figure 2: Sketch of the test flume (not to scale). In this study, the scour onset under a pipeline in waves was recorded using a camera. Before each test case, the sediment underneath the pipeline within the coverage of the mesh was removed and replaced gently. Then the bed surface was carefully leveled. This process ensured that scour onset occurs within the field of view of the camera. A similar arrangement was used in [PERSON] et al. [3]. In some cases where the onset of scour failed to appear inside the camera view, the test case was repeated, and the video recording was abandoned. The test parameters were selected based on [PERSON] et al. [26] as follows: The water depth was \(h_{0}\) = 0.4 m; the wave period was \(T\) = 1.3 s and the wave height was \(H\) = 0.08 m. Thus, the Keulegan-Carpenter number \(KC\) = 2.46, calculated by  
\[KC=\frac{U_{\text{m}}T}{D} \tag{1}\]  
where \(U_{\text{m}}\) is the maximum undisturbed orbital velocity near the bed; \(T\) is the wave period and \(D\) is the pipeline diameter. The Shields number of the wave was \(\theta\) = 0.13, which was calculated by the equations used by [PERSON] et al. [8] derived from [PERSON] [27]. The Shields number was larger than the critical value of the sediment, and thus test condition was in live-bed condition. In this study, the emphasis was qualitative analysis on the elementary mechanism of the scour onset in waves. The potential scale effects on the results are acceptable for a qualitative mechanism study.