Reef rip current generated by tide and wave during summer season: Field observation conducted in Yoshiwara Coast, Ishigakijima, Okinawa, Japan

• Main Authors: De Leon, Mario P., Nishi, Ryuichiro, Kumasaka, F., Takaesu, Tsuyoshi, Kitamura, Ryosuke, Otani, A.
• Format: text
• Published: Animo Repository 2008
• Subjects: Rip currents—Japan; Drowning—Japan; Waves; Coral reefs and islands—Japan; Civil and Environmental Engineering
• Online Access: https://animorepository.dlsu.edu.ph/faculty_research/6202
• Institution: De La Salle University

In 2004 and 2005, five drowning accidents in Ishigakijima, Okinawa were documented by Japan Coast Guard and were found to be caused by a strong offshore current commonly known as “reef current”. This type of current has been identified to be responsible in generating the circulation of water and transportation of sediment in coral reefs. In this paper, however, high-speed offshore current in coral reefs is specifically referred to as “reef rip current”. Considering that the generation mechanism of reef rip current is not revealed yet, the researchers conducted a hydrodynamic study in Yoshiwara Coast, Ishigakijima, Okinawa in two phases (normal and storm conditions) during summer season to determine the location and time of occurrence of the strong offshore current that will significantly address safe utilization of coral reef and carbonate beach thereby reducing drowning accidents. The nearshore hydrodynamic project that was undertaken included field measurement of wave, current and tide during normal and storm conditions with the implementation of bathymetry survey by laser method. The results revealed the maximum 20-minute average offshore velocity at 1.2m/s. In addition, it was observed during low tide condition that most of the water inside the lagoon is only discharged through the reef gap.