## 1 Introduction  
Submarine pipelines are vital channels which transport crude oil, oil products, and water between the oil refineries and offshore platforms. Scour beneath a pipeline is a key concern for pipeline stability and safety. In marine hydrodynamic environments, scour onset can occur due to waves and currents, eventually causing a local scour hole to form under a pipeline. As the scour hole develops along the pipeline, the pipeline may be threatened by vortex induced vibration due to pipeline spanning and vortex shedding, which can eventually cause pipeline failure [1]. These failures can trigger both environmental and ecological disasters.  
Scour under an underwater pipeline has been previously investigated both experimentally and numerically. Laboratory experiment studies have focused on the onset of scour [2, 3], tunnel and lee-wake erosion [4], the mechanism of three-dimensional scour hole development [5, 6], the spanwise scour propagation rate [7, 8, 9], and the equilibrium scour depth [10]. Numerical simulation has also been widely used in the investigations on the aspects above [11; 12; 13; 14]. Other literature has addressed scour protection measures [15; 16; 17; 18; 19].  
The onset of scour is the start of scour hole development under a pipeline, and has been investigated extensively. [PERSON] [2] analyzed the pressure difference on two sides of the pipeline in steady currents, revealing that piping, also known as seepage failure, is the dominant cause of scour onset under a pipeline. [PERSON] et al. [3] investigated the temporal variation of the average hydraulic gradient under the pipeline in an accelerating current and in regular waves. It was pointed out that excessive seepage flow beneath the pipeline encourages scour onset. An equation on the critical conditions of scour onset was also proposed for the steady current condition. [PERSON] et al. [20] studied scour onset with numerical simulation. Systematic analysis was performed on the effects of hydrodynamic parameters on the normalized pressure difference on two sides of the pipeline. There is other literature which reports on the onset of scour [21; 22; 23; 24].  
However, reports on the mechanism of the scour onset are limited, especially in wave scenarios [3]. [PERSON] et al. [3] measured the hydraulic gradient across the pipeline during the scour onset in waves. Some important phenomena were found in the scour onset in waves, including a delay of piping failure after the hydraulic gradient exceeded the critical value of the sediment. The detailed mechanism of the scour onset in waves has not been comprehensively revealed, including the attribution of this onset to the delayed piping. The scour onset of a pipeline is closely associated with the behavior of sediment particles under the pipeline, which was not observed in previous studies. Further investigation is needed to improve understanding of this important phenomenon.  
This study investigates the elementary mechanism of the scour onset in regular waves using visualization flume experiments (Figure 1). A miniature camera enclosed in a transparent model pipeline was used to record full view videos of the sediment movement in the scour onset in waves, which has not been previously observed. The behavior of sediment in the scour onset was analyzed using the videos. The details in the scour onset mechanism were discussed based on the phenomena analysis, including the potential causes of the phenomena in previous studies. Direct observation of the scour onset process provides clues for further mechanism investigation of the scour onset in regular waves. The mechanical analysis in this research can be used in future studies on the critical conditions for the scour onset and countermeasures which may be used in offshore and marine structures, such as the ones studied in [25] and others.  
Figure 1: Sketch for the scour onset under a pipeline in waves.