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Update:November 20, 2019

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Locating Shallow Landslides Induced by Heavy Rainfall


Article title

Geophysical features of shallow landslides induced by the 2015 Kanto-Tohoku heavy rain in Kanuma city, Tochigi prefecture, Japan

Author (affiliation)

Yasuhiko Okada (a), Chisato Konishi (b)

(a) Department of Disaster Prevention, Meteorology and Hydrology, FFPRI, Tsukuba, Ibaraki, Japan.

(b) OYO corporation, Saitama, Japan.

Publication Journal

Landslides, Springer・International Consortium on Landslides DOI:10.1007/s10346-019-01252-1( External link )

Content introduction

Significant damage was caused during the heavy rainfall in Kanto-Tohoku in September 2015 because of shallow landslides and debris flows. The importance of disaster prevention and mitigation efforts was recognized once again. In the city of Kanuma in Tochigi Prefecture, slopes with inclinations of approximately 30°, which are not considered very steep, failed on a mountain hill behind private homes. Debris and muddy water were discharged, causing damage downstream. The depth of the failure was approximately 1.5 m, and although classified as shallow landslides, it did not occur in a concave topography as observed in zero-order basins where most shallow landslides occur. It is to be noted that the landslide occurred at a location prone to high risks of failure due to the unavailability of evidentiary preliminary assessments.

Consequently, survey lines were set in the vicinity of the location where the slide occurred and debris was discharged, and geophysical surveys were conducted to examine the structure and stiffness of the soil layers of the slopes. The results showed that near the locations where failure occurred, sandy topsoil layers with many voids in comparison with the surrounding soil were thickly accumulated, with a depth of nearly two meters at its maximum. It is believed that these values were approaching the maximum limit value of failure initiation.

Therefore, the implementation of some countermeasures with respect to slopes with increasing risk of failure, where thick topsoil layers are formed owing to the influence of geological and crack structures underground, although not identified when observing the surface, is necessary. Many aspects are still unexplained with respect to the failure mechanism. However, prediction accuracy may significantly improve with the geophysical surveys currently being conducted. In recent years, equipment for geophysical surveys has progressively become lighter and less expensive, and is therefore expected to be used in the future. In the midst of predictions of high frequency of local torrential rains due to climate change, the results obtained herein will be useful for improving landslide disaster prevention in advance.


Figure: Overview of the shallow landslides 

Figure: Overview of the shallow landslides that occurred in Kanuma City, Tochigi Prefecture.
Created by processing aerial photographs (taken in 2015) from the Geospatial Information Authority of Japan

Figure: Velocity distribution of S-waves

Figure: Velocity distribution of S-waves (transverse seismic waves) measured along the longitudinal line set at the right side of the failure when facing upward from downslope. “Warmer” colors represent a low S-wave velocity, indicating that soil layers with many voids are being accumulated. In the vicinity of locations where failure occurred, while the ground surface was essentially level, a structure characterized by a thicker soil layer structure with numerous voids as compared to the surroundings areas was observed. (The white line represents the position of the bottom of the topsoil layer, estimated from simplified penetration tests)