Study on Appropriate Time Interval of Image Averaging for Rip Current Detection

There are from 2000 to 3000 rescues every summer at bathing beaches in Japan. A total of 54% of drowning accidents were caused by rip currents; therefore, rip currents are the main cause of drowning accidents in Japan. To analyze the rip current surveys, a method for visualizing rip currents with average images can be used. It is a method for visualizing currents by averaging consecutive images of the coast into a single image. It was investigated the appropriate time interval for image averaging for the Onjuku Beach in Japan. However, the research had only investigated on a single beach and under specific wave conditions. In this study, the appropriate time interval of image averaging was investigated by comparing accuracy for time intervals set as 1, 2, 3, 4, 5, 10, 15, 20, 25, 30, 40, 50, 60, 70, 80, 90, and 100 min in various wave heights. As a result, the accuracy was more than 95% in the validation dataset at all intervals, and 100% at 10 min or more intervals especially. The same trend was observed in the test dataset, but the accuracy ranged from 53 to 70%. When the wave height was 0.5 m or higher, the accuracy ranged from 74 to 88%. According to those results, it was found that this method can be used when the wave height was 0.5 m or higher, and 10 min or more intervals were appropriate for image averaging on the beaches with constant wave direction.