*4.5. Analysis Results*

From recorded data, the time history data and HHT diagrams obtained for the first six sensors are plotted in Figure 8. The recorded time history is plotted on the top of each subplot and the corresponding Hilbert amplitude spectrum is shown in the bottom. The time-varying IF demonstrated nonlinear characteristics of the signals. The x-axis represents the time history measured in seconds, and the energy density is illustrated at frequencies between 0 and 5 Hz, which contains the most energy of the vibration.

The HHT diagram for channel 1 shown in Figure 8a reveals a large energy distribution indicated by the significant yellow bar at approximately 90 s. This distribution can be attributed to the rapid jump in the time domain. Similarly, Figure 8b also reveals a yellow bar, indicating a sudden increase in the beginning of the time history. According to the color distribution in Figure 8b, more energy was covered in channel 2 than in channel 1, which fits well with the measured time histories.

The HHT diagrams for channels 3 and 4 shown in Figure 8c,d, respectively, reveal a gradual shift of the yellow bar, which indicates the large variation section of energy in the time domain. Based on the result, the yellow bars in HHT diagrams can be used to estimate the occurrence time of possible scour processes. Figure 8d indicates the scour phenomenon to occur between 150 and 200 s and then it continues till the end of the experiment; this figure also reveals the energy distribution in channel 4 to be greater than that in channel 3.

Finally, as observed from the time history, significant vibration can be reflected from channel 5. Furthermore, the HHT diagram for channel 5 shown in Figure 8e reveals that the yellow energy bar is shifted to 300 s. It can be treated as a proper indicator to evaluate the scour depth, which follows the trend observed from the experiment. Additionally, the HHT diagram for channel 6 shown in Figure 8f reveals two peaks. In contrast to the other five sensors, the first yellow energy bar located at 300 s with the fundamental frequency between 0 and 2 Hz is not considered as scour phenomenon in the HHT diagram. As only slight energy is shown in the diagram, it can be neglected as a small vibration in the time domain. By combining Figures 7 and 8, a possible backbone scour curve can be established to provide a rapid estimation and early warning for scour processes around a bridge.

**Figure 8.** HHT diagrams for the first six sensors.
