**5. Summary**

Offshore wind farms face more severe environmental conditions such as severe storms, typhoons, ocean currents, and waves. Flow induced scour around the foundation of structure is extremely complex. Scour leads to the excavation of sediments, reducing the safe capacity of the structures. The phenomenon of pier scour combines the effects of the vortex system involving time-dependent flow pattern and sediment transport mechanism. A real-time scour monitoring system can improve the safety of structures and afford cost-effective operations by preventing premature or unnecessary maintenance. Numerous studies have explored the mechanisms of hydraulic scour around foundations and have presented several formulas for scour depth estimation around piers. However, scour data from real-time monitoring systems are still inadequate due to the lack of reliable and durable instrumentation techniques.

Normally, water turbidity was measured based on absorption, attenuation, and scattering e ffects by using spectrometers or photometric devices. However, conventional estimation instruments are usually bulky and costly. Experimental results obtained in this study demonstrated the feasibility of the VLC modules for executing both water turbidity and water flow velocity measurements.

According to the results of the underwater turbidity and water flow velocity experiments, the communication light path of the VLC modules can be sensitively a ffected by turbulent movement of particles in water. These notable phenomenon e ffects could be further implemented as an early warning structural health monitoring system by conducting real-time scour measurement and Hilbert transform analysis. In this present study, an on-site scour monitoring system for o ffshore wind turbines has been proposed; specifically, the monitoring system consists of arrays of small VLC modules attached directly to a pile foundation structure and linked to a wireless network to enable remote data acquisition has demonstrated. From the flume experiment results, it revealed that the system was highly sensitive to seabed scour processes. The proposed robust sensory monitoring system has considered for further on-site applications and as an indicator to improve the empirical scour formulas for sustainable maintenance in the life cycle of o ffshore structures.

From the result analysis, the VLC ray is easily a ffected by the suspended particles in the water and the turbidity, especially in the scouring process. Hence, the proposed arrayed-transmission measurement method will be limited by the turbidity e ffect. In the future work, it may be necessary to cooperate with the reflection approach, as a supplement comparison, for simultaneous measuring the back-scattering characters. Combining these two transmission and back-scattering implementations would provide a better real-time approach to measure and discriminate turbidity, flow velocity, and scour depth. These data can be useful to establish a local scouring formula to evaluate structural safety.

**Author Contributions:** Y.-B.L., developed the methodology and took the lead in writing the manuscript. C.-C.C., and T.-K.L., provide data analyzed and results. C.-W.H., and B.-T.C., responsible for image measurement and data collection. J.-S.L., and K.-C.C., revised the manuscript and suggestion to the manuscript.

**Funding:** This study is funded by Ministry of Science and Technology, Taiwan, under Grants MOST: 107-2625-M-492 -003, 106-2625-M-492-011, and 105-2625-M-492-016.

**Acknowledgments:** The authors would like to gratefully acknowledge the experimental sites from the National Center for Research on Earthquake Engineering in Taiwan. In addition, the authors appreciate the Hydrotech Research Institute of National Taiwan University for facilities and support.

**Conflicts of Interest:** The authors declare no conflict of interest.
