Field Measurement of Wave Overtopping Frequency and Intensity at Sea Dike Using Shore-Based Video Images

A tentative flied measurement of wave overtopping over a rubble mound sea dike using video images was carried out during two storms in the summer of 2018 at Rizhao Coast, Shandong Province, China. A shore-based video monitoring system was mounted at the top of a lighthouse behind the sea dike to collect coastal images with a sample frequency of 1 Hz in the beginning 10 min of each hour during daylight. Image georectification, describing the transformation from the pixel coordinates to world coordinates, was calibrated following the two-step calibration methodology. The number, location, width and duration of overtopping events were extracted in the rectified time-stack images and the process of extraction was performed automatically using 150 virtual pixel sensors distributed along the sea dike crest. Finally, a total of 7120 individual overtopping events were detected. Temporal and spatial variations of overtopping intensity, defined as the product of width and duration to estimate the dimension of an individual overtopping event, were analyzed. Using the EurOtop 2018 and JTS 145-2015 formulas, overtopping volume was also calculated based on the measured offshore wave parameters and tidal level at the nearby Rizhao ocean station. The correlation coefficient between the calculated results and the observation was 0.66 and 0.64, respectively. During storm AMPIL, the alongshore variation of overtopping intensity was revealed to be dominated by the alongshore variation of surf zone width. While during the moderate storm JONGDARI, it was affected by the surrounding coastal structures. The present study suggests the feasibility of shore-based video monitoring technique to capture the main features of wave overtopping at coastal dikes, providing new possibilities to monitor wave overtopping in the field and to improve our calculation method and tools.