**3. VLC Scour Experiment**

An experiment was conducted in a 6 m wide and 30 m long flume, as depicted in Figure 5. Eight VLC sensors including the transmitter (Tx) and the receiver (Rx) were installed in the pier. Sensor 1 was located 5-cm below the bed surface. These sensors were arranged in two rows of eight sensors separated by 5-cm intervals in the vertical direction. The sampling rate was set to 100 Hz to record the time history throughout the scour experiment. Uniform sand with a diameter of 0.88 mm was paved as the bed material in the flume. The approach velocity of the steady current was set to be 0.5 m/s. All dynamic data were monitored through the experimental setup shown in Figure 5. Figure 5 illustrates the scour responses recorded by individual sensors. These responses were further analyzed using the proposed Hilbert–Huang transform (HHT) process.

**Figure 5.** Hydraulic flume test and experimental setup.

Initially, all the sensors are embedded in the soil and the transmit signal cannot be detected by the receiver sensors. At the early stage, the scour depth increases significantly while inflow runs through the pier. As the embedded VLC sensor scoured from the soil due to scour, the significant signal of the waveform can be easily obtained. An obvious example of the sensors 3 and 4 is shown in Figure 6. The variation of waveform magnitude is due to the turbidity and velocity of the turbulent flow that contains time-depended suspension particles. As seen in Figure 6, the scour depth increases gradually after 1 h. Around 2.5 h, a total scour depth of 30 cm is measured in this test, which may be close to the equilibrium state. As experiment finished, the scour hole can be observed as shown in Figure 5. A small camera recorded the scouring process as a comparison to valid the scour depth evolution measured by the VLC. An empirical scour formula with the nonlinear nature of flowing water which responds in exponential form to the light intensity for the VLC monitoring system is also obtained as shown in Figure 6.

**Figure 6.** Monitoring interface and recorded scour responses.

## **4. HHT and Data Analysis**
