Influence of specific flowpath of bedrock groundwater causing shallow landslide at weathered granitic mountain

Understanding the groundwater dynamics in the soil layer is one of the key issues for accurate landslide prediction in steep mountains. Recent studies have pointed that the water movement from bedrock layer to soil layer may trigger the landslide occurrence. In order to have deeper and precise understanding, we need to get the results from direct observations of bedrock groundwater. Our research group has focused on the dynamics of bedrock groundwater using densely nested bedrock wells in a steep catchment. In our catchment, we recognized an existence of shallow landslide (2×5m) which recently occurred at a ridge on the surface topography in this investigation and also found the springs from the landslide scar. This study could aim to elucidate the bedrock groundwater movement and its influence to the landslide based on the hydrological and hydrochemical observations of bedrock spring and groundwater. The results have shown that the bedrock groundwater was distributed in three different aquifers: upper, middle and lower aquifers. The discharge of bedrock spring had rapid response for rainfall, similar to the response of lower bedrock aquifer. The chemistries of bedrock spring were close to that of bedrock groundwater located in the upper point rather than other groundwater of the lower aquifer. The groundwater in the middle aquifer moved to the lower aquifer across the topographic catchment boundary located on the upper areas of landslide scar. These results indicated that the bedrock groundwater flowed from neighboring catchment and exfiltrated as bedrock spring at the scar of landslide. Our research demonstrated that the bedrock groundwater flowpath across the surface divide that was not able to detect from surface topography is important for predicting the shallow landslide in steep catchments.