*3.1. Experiment Setup*

The experiment was carried out in a water tank of 1.2 m in length, 0.8 m in width, and 0.45 m in height. The sand was evenly laid on the ground with a height of 0.1 m. Scour depth was much smaller than the depth of available laid sand. The sand used was the construction sand screened through 0.1 mm to 0.3 mm diameter filters. The diameter of sand in this experiment was 0.2 mm. The distances between propeller tip and sandbed were set to 5.0 mm, 27.5 mm and 55.0 mm according to Hong et al. [10]. The schematic diagram of the experiment is shown in Figure 2a,b.
