Field Study on the Physical Properties of Beach Gravels on the Kotogahama Beach, Kochi Prefecture

A study on the formation process of the beach was conducted at Kotogahama Beach, Kochi Prefecture, Japan. The 5 km beach was formed by sand and gravels supplied from the Wajiki River and the Akano River. A field measurement was conducted in July 2021 by using a UAV. Orthographic mosaic images with spatial resolution of 1 cm were obtained by UAV. Alongshore distribution of maximum gravel size was found to decrease on the west side of the Wajiki River. Further measurements were conducted in August and September 2022, focusing on the shape and the imbrication structure of the gravels. More than 3000 gravels were sampled, and the imbrication structure was recorded by a smartphone camera equipped with GPS and compass. Flatness of gravels increased westward from the river mouth, reconfirming that the long-term longshore transport of the beach gravels is westward. On the other hand, the imbrication structure of the gravels was strongly influenced by the direction of the incoming wave caused by the precedent typhoon, representing the short-term migration characteristics of beach gravels. These results demonstrate that the long-term sediment movement could be revealed from size and flatness of gravels while the short-term sediment movement could be represented by the imbrication structure.