Simultaneous monitoring of a collapsing landslide with video cameras
Abstract. Effective countermeasures and risk management to reduce landslide hazards require a full understanding of the processes of collapsing landslides. While the processes are generally estimated from the features of debris deposits after collapse, simultaneous monitoring during collapse provides more insights into the processes. Such monitoring, however, is usually very difficult, because it is rarely possible to predict when a collapse will occur. This study introduces a rare case in which a collapsing landslide (150 m in width and 135 m in height) was filmed with three video cameras in Higashi-Yokoyama, Gifu Prefecture, Japan. The cameras were set up in the front and on the right and left sides of the slide in May 2006, one month after a series of small slope failures in the toe and the formation of cracks on the head indicated that a collapse was imminent.
The filmed images showed that the landslide collapse started from rock falls and slope failures occurring mainly around the margin, that is, the head, sides and toe. These rock falls and slope failures, which were individually counted on the screen, increased with time. Analyzing the images, five of the failures were estimated to have each produced more than 1000 m3 of debris, and the landslide collapsed with several surface failures accompanied by a toppling movement. The manner of the collapse suggested that the slip surface initially remained on the upper slope, and then extended down the slope as the excessive internal stress shifted downwards. Image analysis, together with field measurements using a ground-based laser scanner after the collapse, indicated that the landslide produced a total of 50 000 m3 of debris.
As described above, simultaneous monitoring provides valuable information about landslide processes. Further development of monitoring techniques will help clarify landslide processes qualitatively as well as quantitatively.
