Miyu Kojima

Investigation of the Conditions for Drowning Accidents at Major Swimming Beaches on the Shonan Coast

Academic Advisor: Naoyuki Inukai

This study aims to clarify the conditions under which water accidents occur at Yuigahama Beach and Katase Higashihama Beach on the Shonan coast, and to obtain knowledge useful for accident prevention. Both beaches are located near major tourist areas and attract large numbers of visitors each year. However, several recent accidents have involved swimmers being swept offshore. To examine the causes, this study analyzed past accident records, meteorological and oceanographic conditions at the time of the incidents, and conducted field investigations including bathymetric surveys and current observations.

Accident data from the Japan Coast Guard were reviewed to identify the dates, wave heights, and wind directions associated with past incidents. The results showed that accidents did not occur only during high-wave conditions; some occurred even when wave heights were around 0.5 m. This suggests that rip currents play a significant role, and that wave direction and offshore meteorological disturbances are important factors.

Field surveys revealed differences in beach topography between the two sites. Yuigahama Beach has a relatively gentle average slope (about 1/55), with an estimated rip current spacing of approximately 244 m. In contrast, Katase Higashihama Beach has a steeper slope (about 1/44) and shorter rip current spacing (about 169 m), indicating a denser distribution of rip currents. Dye-tracing observations confirmed offshore-directed flows at both beaches, demonstrating that even moderate current velocities can carry swimmers away from shore.

Analysis of offshore wind conditions further indicated that accident risk increases not only during onshore winds but also when offshore winds coexist with typhoons or low-pressure systems, which generate long-period swell. These findings suggest that accident occurrence depends on both local coastal topography and broader oceanographic conditions. Future work should focus on detailed wave analysis and the development of real-time safety assessment systems.