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[HGM03-02] Shear deformation progression induced by change in soil moisture at the bottom of slope soil layer on the upper slope of forest and its role in slope materials transfer
Keywords:forest slope, shear deformation. , soil moisture change, slope material transfer, climatic warming
On the upper slopes of forests, the control mechanism for the slow movements of mass (i.e., soil creep) differs between times of torrential rain (saturated soil moisture conditions) and unsaturated conditions. Shear deformation of the soil layer driven by hydro-forces during torrential rain (i.e., saturated conditions) (Sonoda 1998) and that under specific unsaturated soil moisture conditions play a key role in slope material transfer. To clarify the mechanism of the latter, following Sonoda and Kurashige (2017), Sonoda et al. (under referees of journal) enlarged the measurement range to the deeper part of the soil layer and investigated the relationship between soil layer behavior and soil moisture change under unsaturated conditions. The strain in the soil layer was measured using strain probes, while the pressure head of the soil water was measured using tensiometers. The results showed that shear deformation progressed at the bottom of the soil layer on the upper slope during the transitional period from soil drying to soil wetting and sometimes developed into shear destruction. A weak soil strength zone was produced at the bottom of the slope soil layer, which was presumed to affect the sliding of the slope soil layer during torrential rains. In other words, under the unsaturated conditions during the transitional period from soil drying to soil wetting, the shear deformations at the bottom of the slope soil layer are highly likely to play an important role in the total slope material transfer. The data underlying this mechanism has been introduced.
Furthermore, under existing global climatic warming, thermal waves and torrential rains are striking in the middle-latitude districts, and notable changes in climate from dry to wet have been observed. In Japan, where 70% of the country’s land area is covered with vegetation in a temperate humid climate, the phenomenon of soil drying is fairly muted. However, the range of soil moisture is predicted to increase (i.e., from dry to wet) from now on. Accordingly, on the upper slope, the progression of shear deformation at the bottom of the slope soil layer induced by gaps between dry and wet climates has intensified. Combined with the shear deformation of the soil layer and material transfer driven by hydro-forces, the amount of slope materials transferred from the upper slope to the middle slope has increased. On the middle slope, multiple collapses of the soil layer accelerated the slope material transfer and have increased the amount of slope materials being supplied to the foot of the slope (i.e., talus). Overall, under the influence of the effects of climate warming, the amount of material transferred from the upper slope to the foot slope is expected to increase in the future.
References
Sonoda, M. 1998, A numerical simulation of displacement of weathered granite on a forest slope. Trans. Japan. Geomorph. Union 19(3), 135-154.
Sonoda, M. and Kurashige, Y. 2017, Characteristics of surface soil creep on a forest slope in Japan. Geomorphology 288, 1-11.
Sonoda, M., Kurashige, Y., Yamada, S. and Nonaka, M. 2024, Soil creep induced by moisture changes at the inner slope soil layer in a forest of weathered granite, Japan.
Furthermore, under existing global climatic warming, thermal waves and torrential rains are striking in the middle-latitude districts, and notable changes in climate from dry to wet have been observed. In Japan, where 70% of the country’s land area is covered with vegetation in a temperate humid climate, the phenomenon of soil drying is fairly muted. However, the range of soil moisture is predicted to increase (i.e., from dry to wet) from now on. Accordingly, on the upper slope, the progression of shear deformation at the bottom of the slope soil layer induced by gaps between dry and wet climates has intensified. Combined with the shear deformation of the soil layer and material transfer driven by hydro-forces, the amount of slope materials transferred from the upper slope to the middle slope has increased. On the middle slope, multiple collapses of the soil layer accelerated the slope material transfer and have increased the amount of slope materials being supplied to the foot of the slope (i.e., talus). Overall, under the influence of the effects of climate warming, the amount of material transferred from the upper slope to the foot slope is expected to increase in the future.
References
Sonoda, M. 1998, A numerical simulation of displacement of weathered granite on a forest slope. Trans. Japan. Geomorph. Union 19(3), 135-154.
Sonoda, M. and Kurashige, Y. 2017, Characteristics of surface soil creep on a forest slope in Japan. Geomorphology 288, 1-11.
Sonoda, M., Kurashige, Y., Yamada, S. and Nonaka, M. 2024, Soil creep induced by moisture changes at the inner slope soil layer in a forest of weathered granite, Japan.