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. |