Time series analysis of slope displacements using UAV photogrammetry and its relationship with rainfall intensity

In this study, UAV (Unmanned Aerial Vehicle) photogrammetry was used to monitor the ground displacement on the slope below a coal waste-rock stockyard and to investigate the role of rainfall on its displacement. The study area is a mountainous region in Korea, where coal mining is active, and coal waste-rock is stored on the slopes of the mountain. In this area, material removal work was undertaken to reduce the steepness of the slope and driving forces in order to prevent disasters, but the strategy requires continuous monitoring to confirm the stabilization of the slope. For slope monitoring, a total of six UAV photogrammetry campaigns were conducted between April 2019 and August 2020. As a result of data processing, an orthoimage and DSM (Digital Surface Model) were generated. The ground displacement was estimated through four steps: slope extraction, displacement area evaluation, horizontal displacement, and vertical displacement analysis. During the study period, the maximum vertical displacement was 3.3 m and the maximum horizontal displacement was 3.5 m. The horizontal displacement was effectively evaluated through the measurement of the movement of the drainage system using orthoimages captured with a periodic survey. The effects of rainfall on ground displacement were also investigated. A very clear linear relationship between rainfall intensity higher than 20 mm/day and ground displacement was identified. The accumulated amount of rainfall also showed a good correlation with slope movement, but the frequency of rainfall intensity below 20 mm/day showed a relatively low correlation with ground displacements.