Field surveys of September 2018 landslide-generated waves in the Apporo dam reservoir, Japan: combined hazard from the concurrent occurrences of a typhoon and an earthquake

We report and analyze a case study of landslide-generated waves that occurred in the Apporo dam reservoir (Hokkaido, Japan) culminating from the rare incident of hazard combination from the September 2018 Typhoon Jebi and Hokkaido earthquake (Mw 6.6 on 5 September 2018). The typhoon and earthquake were concurrent and produced thousands of landslides in the area by the combined effects of soil saturation and ground acceleration. Here, we report the results of our field surveys of the landslides that occurred around the Apporo dam and generated damaging waves in the reservoir. We identified six landslides at a close distance to the dam body; the largest one has a length of 330 m, a maximum width of 140 m and a volume of 71,400 m3. We measured wave runup at a single point with height of 5.3 m for the landslide-generated wave in the reservoir and recorded the damage made to the revetments at the reservoir banks. By considering the locations of the landslides and their potential propagation paths, we speculate that possibly three of the six surveyed landslides contributed to the measured wave runup. The surveyed runup was reproduced by inputting landslide parameters into two independent empirical equations; however, other independent empirical relationships failed to reproduce the observed runup. Our field data from the Apporo dam can be used to improve the quality of predictions made by empirical equations and to encourage further research on this topic. In addition, our field data serves as a call for strengthening dams’ safety to landslide-generated waves in reservoirs.