The role of subsurface water flow paths on hillslope hydrological processes, landslides and landform development in steep mountains of Japan

Abstract

Abstract:Hydrological monitoring was conducted in high-relief watersheds in the Japan Alps to investigate the relationship between hillslope hydrological processes and landform evolution in steep granite and shale mountains. In the Koshibu watershed, underlain by Mesozoic shale, the drainage density and frequency was significantly lower than in the Yotagiri watershed underlain by granite. Drainage micro-morphology analysis showed that hillslopes in the watersheds K1 and K6 (Koshibu basin) are mostly combinations of talus and bedrock exposures. In contrast, watershed Y1 (Yotagiri basin) is composed of several zero-order streams with hollows. Infinite slope stability analysis indicates that the regolith shear strength in the K6 watershed (Koshibu basin) is lower than that of the Y1 hillslope, but groundwater levels were higher in the Y1 hillslope than in the K6 hillslope during storm events. These data suggest that, although the shear strength of the soil is stronger in the Yotagiri watershed, the slopes are unstable because of the groundwater conditions, whereas deep-seated landslides may occur episodically in the Koshibu watershed associated with extreme storms and very high antecedent soil moisture. These differences would strongly contribute to the different observed hillslope processes and drainage characteristics.

Runoff generation mechanisms in high-relief mountainous watersheds with different underlying geology

“…The scar of an old rockfall deposit was detected in the upper section of the Y2 watershed; several shallow landslide scars were also detected (Onda et al, 2004). The channel slope in the Y1 watershed is extremely steep (approximately 30°), and the bedrock is exposed in most sections of the main channel; it is covered only by a thin veneer of boulders and gravel.…”

Effects of hillslope topography on runoff response in a small catchment in the Fudoji Experimental Watershed, central Japan

“…First‐order streams were defined according to the method of Takayama (1972), in which every topographic convergence at the valley‐head on the map is regarded as the first‐order stream. This method is useful for identifying small valley‐heads, and many studies have used this approach (Onda, 1994; Komatsu, 1999; Onda et al , 2004). We defined the convergent type of hillslope (valley‐head) as the area that exists above the head of the first‐order stream.…”

Hydrogeomorphology: overview of an emerging science

“…A field study in the Ashio Mountains of central Japan examined detailed sources of both runoff and sediments in small catchments of differing lithologies (Hattanji and Onda, 2004). Similarly, INTRODUCTION 599 Onda et al (2004) investigated the interactions of stormflow generation, lithology, and slope stability in two catchments in the Japanese Alps underlain by shale and granite. Asano et al (2004) employed geochemical signatures in precipitation, soil water, groundwater, and stream water to elucidate water sources and pathways in two Japanese catchments.…”