Research into the breaking of ocean surface waves, their kinematics and their statistical properties has been ongoing for decades. An important component of this research involves the capability of mapping the breaking point within a wave. Contemporary methods such as visual inspection, acoustic noise or water conductivity measurements have their limitations. Similarly, RADAR, which is used for both normal waves and breaking waves, is unable to consistently and accurately detect the point of breaking. However, recent works have shown positive results using LiDAR instruments. These have been used primarily in coastal areas, but some promising results have also been obtained from measurements in the open sea. This paper describes the experimental campaign within a laboratory using a LiDAR instrument (LSLiDAR CH128x1), along with several wave gauges. The experiment consists of tests with focused waves for both instruments. The experiments show that there is agreement between the data of the LiDAR instruments and the wave gauges, especially when detecting the surface elevation when breaking is initiated. However, in the absence of wave breaking, the data containing measured surface elevations is sparse. This is because the surface is nearly non-detectable, due to a failure of backscattering. Finally, results are more promising in the time domain when assessing the performance in the vicinity of the LiDAR.
The increasing number of offshore activities, such as the application of wave energy devices, and fixed or floating wind turbines, has raised the demand for accurate and detailed knowledge of ocean wave behavior. Ongoing research on wave measurements is focused on wave shape, wave kinematics, and statistics. One of the challenges encountered in this area of research involves describing the wave shape and kinematics at the point at which a wave breaks. Waves have been measured for decades both in the laboratory, as well as in coastal and open sea environments. In wave tanks and flumes, the surface elevation is typically measured at a single point using one capacitance or resistance-type wave gauge, or more than one point, using an array of such wave gauges (Shepherd, 1997). The wave buoy is a commonly used 1D-measurement instrument and is applied both in the open sea (Hasselmann et al. 1973), as well as in coastal areas (O’Reilly et al., 1996). Alternatively, a RADAR system, either real or synthetic, is also used (Marom et al., 1990) and (Dankert et al., 2003). Measurements from a buoy and an Acoustic Doppler Current Profiler (ADCP) are compared in (Kabel et al., 2024). This study shows good agreement between the two full-scale wave measurement devices. The aforementioned devices are used in both in the laboratory, as well as in making full-scale measurements. However, they only measure the waves at a single point. Moreover, it is challenging to capture the overturning crest of a plunging breaker using these instruments. This information is required to identify the breaking of a wave (Babanin, 2011). It is therefore highly unlikely that these single-point measurement devices can capture the overall maximum of the crest height over a larger area (for example, the area spanned by an oilrig). This is partially caused by the fact that all the three – dimensional characteristics of waves are disregarded in making single-point measurements. Wave gauges also have the tendency to underestimate the crest of breaking waves, since the upper part of the crest contains entrapped air, in the form of active and residual foam (Wang and Lee, 1986). Moreover, steep waves are underestimated by the wave buoy, since the buoy is incapable of staying on the wave crests. Different two - or three-dimensional measuring techniques have been developed to overcome these drawbacks. Laboratory studies (Govender et al., 2002), (She and Cannings, 2007) and (Blenkinsopp and Chaplin, 2008) have investigated wave geometry and kinematics (wave phase speed) along a limited length in a wave flume with the help of visible imaging using a camera placed at Still Water Level (SWL), that looks through a glass wall of the flume. However, the images are affected by side-wall boundary effects, and reflections in the glass etc., which introduce uncertainties. Waves can also be mapped using the technique of Particle Image Velocimetry (Reul et al., 2008). In this approach, two pictures are taken in quick succession, following which image correlation is used to track the surface elevation and estimate the velocities of the waves. However, the aforementioned remote sensing techniques are slow and demanding, due to the process of setting up and calibrating the equipment, and post-processing the obtained data. The application of remote sensing techniques, such as synthetic RADAR systems (Marom et al., 1990) and (Dankert et al., 2003), IR imaging (Jessup et al., 1997) and LiDAR systems (Kabel et al., 2019) in the open seas have been investigated. The LiDAR systems have shown promising results both in laboratory (Blenkinsopp et al., 2012), (Allis et al., 2011), as well as full-scale conditions (Martins et al., 2016) and (Carini et al., 2021). The following sections contain a description of the test setup comprising a LiDAR system, wave gauges, and cameras, and used at Imperial College, London. The post-processing of the LiDAR data is described, and spatio-temporal maps of waves breaking at different distances relative to the LiDAR equipment are shown. The focused wave with the breaking point relative to the LiDAR resulting in the largest densities of measurements on the wave surface is shown in a spatial domain mapping the wave profile, and in the time domain as well. Finally, the paper contains a discussion on the estimation of wave phase speed.
