Abstract
Koga beach is located in the north of Kyushu. This beach is a pocket beach and this has the groins at the both ends of the beach, furthermore there is a submerged breakwater at the offshore of the beach. In this beach, an accident of 4 people drowned occurred in August 2017. When the accident occurred, 2 children played at the water’s edge of the beach, however the big waves suddenly reached the beach and the children carried away to the offshore. In this research, we tried to grasp the reason why the accident occurred. Firstly, we conducted the field survey one month after the accident, and we got the geographic information of the beach and the geographic feature of the domain. After the survey, we made the geographical grid data for the numerical simulation. Secondary, Koga beach is located at the base of a peninsula, therefore, when the prevalent wave from NNE direction incidents to this domain, the wave in this domain diffracts and attenuates. In contrast, when the wave from NNW direction incidents to this domain, the wave does not attenuate. When the accident occurred, the wave always incident from NNE direction, however the wave direction changed NNW direction in an instant. Therefore, we simulated the wave height profiles in this domain. In this case, we simulated two cases, that is, NNE and NNW wave direction cases, and compared the wave height between at the offshore and at the beach. As the results, when the wave direction was NNE, the wave attenuated around Koga beach. Finally, we simulated the wave dynamics in the pocket beach, and we grasped how the wave moved in the pocket beach. As the result, the wave velocity became over 3 m/s around the accident occurred point.
