*3.2. Measurement Methods*

The measurement system consisted of a laser rangefinder to measure the scour depth and the traverse system to move the laser rangefinder to the designated position for point measurement within the 1 m × 1 m area, as shown in Figure 2. A laser rangefinder was used to measure the scour depth and deposition height. The laser rangefinder used is made by Sndway Co. Ltd. with a measurement range up to 40 m. The laser rangefinder had an aspherical optical focusing mirror, which could enhance reflected light acceptance. The measurement accuracy was within ± 0.2 mm. The lasers had different refractive indices in water and in air. The experimental data needs to be multiplied by 1.33 to eliminate the differences due to the transmission of the light beam through the air and water as consideration of refractive index.

The propellers were not rotating before the measurement and the designed rotational speeds can be adjusted by using the speed control system. Depth measurement was made on a point by point basis through comparison of the undisturbed reference point and the scoured point to obtain the relative scour depth. The laser rangefinder was initially moved to the undisturbed sandbed point and read the measurement point L1, as shown in Figure 3. Then, the laser rangefinder was moved to the sandpit measurement point to read the data L2. The scour depth can be obtained by calculating the εm = (L2 − L1) n, where n is the refractive index of water.

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**Figure 2.** Schematic diagram of the experiment (1.2 m × 0.8 m × 0.45 m). (**a**) Plan view; (**b**) longitudinal cross-section (A-A).

**Figure 3.** Measurement system: (1) Laser rangefinder; (2) optical axis and sliding block; (3) support; (4) experimental tank.

The support structure for the measurement system was built with four columns and four beams surrounding the experimental tank to accommodate the traverse system. The stands were 0.4 m high, and its length and width were the same as 1 m. Two rods were fixed in the direction of the propeller jet (x-direction) on supporting beams with sliders allowing y-direction movement. An additional rod was installed on top of the two rods with sliders on each side allowing the x-direction motion. A fixed clip was used to fix the laser rangefinder the rod. A laser rangefinder had the optical axis perpendicular to the seabed for depth measurement. The single rod allowed the laser rangefinder to move at any axial distance from the flow field. The double rods allowed the laser rangefinder to measure any position in the y-direction. Therefore, all desired points within the x-y measurement grid can be acquired to provide data for scour depth and deposition height.
