11:30 AM - 11:45 AM
[AHW28-10] Hillslope hydrological processes and mass movement mechanisms constrained by shallow subsurface structures reflecting dynamic soil geneses
★Invited Papers
Keywords:Granitoids, Soil structure, Hillslope hydrological processes, Pore connectivity parameter, Shallow landslide
Hillslope hydrological processes leading to shallow landslides were explored with special focus on differing soil evolution history in adjacent mountainous terrains underlain by granite and granodiorite. We targeted northern Abukuma Mountains, northeastern Japan, where heavy rainfall due to typhoon Hagibis caused severe shallow landsliding in 2019. Ground-based surveys, geotechnical tests, and hydrological monitoring were conducted in hillslopes with typical landslide scars in each geological condition to reveal soil coverage, hydraulic properties of regolith, and shallow subsurface pore-pressure fluctuations in response to rainwater infiltration. Numerical simulation was also carried out to reproduce characteristics in the observed pore-pressure responses, and then to reconstruct hydrological processes derived by the typhoon event that eventually triggered shallow landslides.
The soil layers possess characteristic structures and vertical profiles of physical properties corresponding to each geological condition. On the investigated granite slope, a coarse-grained, highly permeable soil layer, generally thinner than 1 m, covers the hard, impervious weathered bedrock (saprock). Saturated hydraulic conductivity abruptly decreases about two orders of magnitude at the soil–saprock interface. The granodiorite slope, on the other hand, is composed of thicker soil cover (>1 m in thickness), which can be divided into two layers: a loose, humus-rich upper part and a dense, stiff lower soil over the soft, heavily weathered bedrock (saprolite). The transitional boundary between these soil layers is a gradual decrease in saturated hydraulic conductivity about one order of magnitude from the upper to the lower parts. These characteristic soil structures seemed to have developed via production and transport of soil particles by bedrock weathering and soil creep. The uniform colluvial soil layer on the granite hillslopes is generated most probably through overall mixing during its discrete accumulation, whereas the superimposed soil layers on the granodiorite hillslopes would have evolved via colluvium overlaying onto the deforming residual materials with incomplete intermixing.
These inherent subsurface structures resulted in marked differences in the hydrological processes inside. Pore pressure in the soil layer on the granite hillslope increases quickly even by a small rainfall event, leading to frequent saturation at the soil–saprock interface. In the granodiorite hillslope, in contrast, pore pressures respond sequentially from shallower to deeper layers with a large lag-time only under a certain amount of rainwater supply. Numerical simulation using Hydrus-2D program with conditions that mimic actual subsurface structures successfully reproduced the characteristic responses of the observed pore pressures. The pore connectivity parameter differs significantly between the two geological conditions, suggesting a wide variety in the contribution of matrix flow and preferential flows in the soil water behavior. Simulations for the disaster-induced heavy rainfall demonstrated the formation of an extensive saturated zone in the granite hillslope on the saprock at downslope, with which the water level reaches the ground surface. With the same rainfall input onto the granodiorite hillslope settings, a perched groundwater table was formed near the boundary between the upper and lower soil layers.
The hillslope hydrological processes governed by the distinct soil structures should affect also the occurrence of rainfall-induced soil mass movements. On the granite hillslopes, formation of saturated zones triggers shallow landsliding along the soil–saprock interface as well as gully expansion. Such erosion is likely to be triggered by a short intense rainfall owing to the high drainage capability of the hillslope offered by the preferential flows. On the granodiorite hillslopes, formation of the perched groundwater table would induce the shear failure within the soil layer. Efficient storage of the infiltrated water in the pores during the predominant matrix flow demands an intensive rainwater input with a large amount to trigger a shallow landslide.
The soil layers possess characteristic structures and vertical profiles of physical properties corresponding to each geological condition. On the investigated granite slope, a coarse-grained, highly permeable soil layer, generally thinner than 1 m, covers the hard, impervious weathered bedrock (saprock). Saturated hydraulic conductivity abruptly decreases about two orders of magnitude at the soil–saprock interface. The granodiorite slope, on the other hand, is composed of thicker soil cover (>1 m in thickness), which can be divided into two layers: a loose, humus-rich upper part and a dense, stiff lower soil over the soft, heavily weathered bedrock (saprolite). The transitional boundary between these soil layers is a gradual decrease in saturated hydraulic conductivity about one order of magnitude from the upper to the lower parts. These characteristic soil structures seemed to have developed via production and transport of soil particles by bedrock weathering and soil creep. The uniform colluvial soil layer on the granite hillslopes is generated most probably through overall mixing during its discrete accumulation, whereas the superimposed soil layers on the granodiorite hillslopes would have evolved via colluvium overlaying onto the deforming residual materials with incomplete intermixing.
These inherent subsurface structures resulted in marked differences in the hydrological processes inside. Pore pressure in the soil layer on the granite hillslope increases quickly even by a small rainfall event, leading to frequent saturation at the soil–saprock interface. In the granodiorite hillslope, in contrast, pore pressures respond sequentially from shallower to deeper layers with a large lag-time only under a certain amount of rainwater supply. Numerical simulation using Hydrus-2D program with conditions that mimic actual subsurface structures successfully reproduced the characteristic responses of the observed pore pressures. The pore connectivity parameter differs significantly between the two geological conditions, suggesting a wide variety in the contribution of matrix flow and preferential flows in the soil water behavior. Simulations for the disaster-induced heavy rainfall demonstrated the formation of an extensive saturated zone in the granite hillslope on the saprock at downslope, with which the water level reaches the ground surface. With the same rainfall input onto the granodiorite hillslope settings, a perched groundwater table was formed near the boundary between the upper and lower soil layers.
The hillslope hydrological processes governed by the distinct soil structures should affect also the occurrence of rainfall-induced soil mass movements. On the granite hillslopes, formation of saturated zones triggers shallow landsliding along the soil–saprock interface as well as gully expansion. Such erosion is likely to be triggered by a short intense rainfall owing to the high drainage capability of the hillslope offered by the preferential flows. On the granodiorite hillslopes, formation of the perched groundwater table would induce the shear failure within the soil layer. Efficient storage of the infiltrated water in the pores during the predominant matrix flow demands an intensive rainwater input with a large amount to trigger a shallow landslide.