As the novel techniques including air-borne and ground- LiDAR, InSAR and UAV grow popular, investigation of rock slope is facing a transformation. When acquiring comprehensive and complete information of rock slope in a faster way becomes easier than ever in the support of novel techniques, research on integration of massive data is now thriving. To deliver an in-depth and detailed assessment on the failure mechanism of a coastal cliff, unmanned aerial vehicle and close-range photogrammetry are used in this research to produce high-resolution digital surface model. The case cliff is located on an abandoned old route of Suhua Highway in eastern Taiwan, where the old highway passes through a mountain ridge with single-line one-way old Heping tunnel. In replace of a former coastal highway that had collapsed under the Pacific Ocean, the old highway itself was abandoned also because of continual collapse. Successive landslides identified from 1974 to 2017 manifest a repeat cycle of collapse, rock blocks piled on the seashore, and rock blocks disappeared due to marine erosion. The cliff is high, steep and undulating with wave-cut notches. Unmanned aerial vehicles allow one to take pictures of a cliff from every possible angle of view, particularly the perpendicular and looking up views that traditional aerial photos fail to provide. The digital surface model generated from UAV images provides a convenient and detailed measure to investigate and survey the case cliff. Under the threat of plane sliding, wedge failure, flexural toppling and wave erosion, the continual collapse of the case cliff in the future is predictable.
First built in 1870, Suhua Highway in eastern Taiwan had went through expansion, improvement and multiple disaster-related route modification. Suhua Highway started with a trail built by Qing government of China, Japanese government then constructed a single-lane one-way coastal highway in 1916–1932 (Yang and Cheng, 2000; Li, 2003). Taiwan government initiated a route improvement that lasted for nearly half a century since 1949. Until 1990, the expansion into two-way highway was completed. Suhua Highway passes through unfavorable geography with many cliffs, where movement of Pacific Plate and Philippine Sea Plate generates densely distributed folds and faults, and complicated engineering geological conditions. Located on the brink of the Pacific Ocean, highway slope retreat with time because of wave scouring, release joint that is parallel with the slope, head ward erosion that is perpendicular with the slope, and downward erosion. Various forms of collapse obscured the highway at times (Chen and Wang, 2012).