Landslide dams are less frequent than other landslides, and their very existence is often very-much short-lived, because the temporary dam tends to collapse rapidly. Because of the resulting lack of evidences, there has been less research done on this topic, although the potential catastrophe they can be at the origin of needs assessment. For this purpose, the present contribution aims at differentiating landslides that trigger dams against those that do not inside a group of valleys of the Kii peninsula in Japan, where landslides occurred after the typhoon Talas in 2011. Using topographic map before the event and LiDAR data in its aftermath, the authors have calculated the factor of safety (FS) of different landslides in the same valleys, comparing the data of landslides that created dams against those that did not. The results show that landslides that triggered dams seemed to have a higher FS than those that did not. The authors suggest that it is because larger landslides are needed and thus appear more stable, but also because at the location where the slopes are stable, the riverbed can incise further instead of growing horizontally, and thus the sediments damming the channel have more chances to block it (as it is narrow) and stop the river from flowing.
Since its last eruption from 1990-1995, Unzen Volcano (Shimabara Peninsula, Japan) has been quiescent. At its summit, a complex Dacitic dome spreads towards the East, in the direction of the Mizunashigawa-valley. In precarious equilibrium, sliding over previously erupted material, the dome has been generating rockfalls, and as the surrounding gullies have been eroding headwards, the stability of the dome is further reduced. Even if the volcano is in a dormant stage, its monitoring is therefore essential for disaster risk management. Therefore, the present contribution aims to (1) quantify the minute dome movement as a whole and (2) divided by lobe, in order to understand deformation; and (3) calculate what is the link between rainfalls and the dome movement. The method relies on the Unzen GBSAR system (Ground Based radar interferometry system) and on hourly rainfalls from raingages stations at Unzen Volcano. As a result, the authors have identified that (1) there is a time-delay between rainfall events and dome movements, and that peak rainfall alone is not sufficient to trigger dome movement; (2) the lower part of the dome rises and falls more rapidly than the upper part of the dome when rainfall is less than 100 mm/48 hours, and (2) the upper and lower parts of the dome move up and down at the same level when rainfall exceeds 100 mm/48 hours. In turn, when rainfall exceeds 250 mm/48 hours, then the upper part of the dome also displays further downward movement, so that the entire dome might be moving down like an accordion.
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