Accurate Spatio-Temporal Measurements of Extreme Waves Using LiDAR in the Laboratory

Abstract Measurements of the surface elevation of waves in a laboratory are commonly undertaken with wave gauges with high accuracy for nonbreaking waves. Nonetheless, these sensors are less well suited for steep breaking waves and do not easily nor nonintrusively provide high spatial resolved measurements of the surface elevation; the latter being possible only through large arrays of sensors, where a densely spaced array certainly affect the flow field. In this study, a novel laboratory setup with two LiDARs is proposed and evaluated for different wave conditions. The wave characteristics include wave packets with varying wave heights, periods, and steepness. These are also recorded using wave gauges and collocated high-speed cameras. These tests are performed in order to evaluate the efficiency and accuracy of a system for measuring waves over a large area with high resolution in space and time in an original and non-intrusive way. It has also enabled measurements of the steep waves, and even the overturning and splashing processes of the breaking waves is captured with high accuracy when comparing to the signal of video recordings. While the potential of high-resolution spatio-temporal measurements using LiDARs is clear, there are limiting factors. The most important relates to the availability of reflective surfaces resulting in return signals. In most offshore laboratory physical models, the water is too transparent to provide a consistent return signal. To this end, we explore a range of methods, which have not previously been reported in the field to increase the roughness of the free surface and improve the reflective properties. The accuracy of the LiDAR within the different configurations is quantified using robust error metrics against the more traditional measuring techniques. The ability of the LiDAR system to measure the shape of the steepest waves is compared to side-view video recordings. In accessing these methods for conditions of the tank, we provide clear and practical recommendations about the deployment of LiDARs for laboratory wave measurements. In using the most efficient techniques, we provide comparisons between LiDAR and camera measurements showing clear benefits of the proposed approach.