Impact of the Japanese traditional river training structure 'Seigyu (Crib spur dike)' on river morphology and its geometrical changes

The traditional Japanese river training structure, known as “Seigyu”, designed to divert flow, reduce flow velocity, and promote geomorphological changes, is expected to contribute to the management of river morphology. Seigyu is highly impervious at the bottom giving it a unique function. Twelve Seigyus were installed in the Kizu River in Japan to assess their ability to serve as a sediment management structure, which is necessary when considering sediment replenishment from upstream dams constructed upstream. Images and videos were recorded by a UAV to assess the flow and morphology around Seigyu via image-based analysis. Additionally, the shift in its function caused by the transformation of Seigyu itself, such as tilting and clogging, was investigated. The detected geomorphological changes indicated that Seigyu contributed to the formation of complex morphological patterns and the continuous shift in the sediment budget from erosional to depositional. Even after tilting approximately 25°, Seigyus’ effects were sustained. High-velocity flow, which occurred between two Seigyus in the early stages after implementation, did not appear or at least decreased in velocity after debris was trapped between the Seigyus. These results suggest that the effects of Seigyu persist even after its transformation, and future simulations and implementations should consider these changes.