THE PROPOSAL OF THE RISK ASSESSMENT METHOD FOCUSING ON THE GEOLOGICAL STRUCTURE AND THE HYDROLOGICAL CHARACTERISTICS ON THE SLOPE WHERE THE GEOLOGICAL BOUNDARY EXISTS

Abstract

In the Nachi River basin, the shallow landslides and the debris flows occurred by the heavy rainfall from Typhoon Talas in 2011. This basin is characterized by the existence of geological boundaries, which is considered to be one of main causes of the landslides. In this study, we conducted geophysical and hydrological surveys to make it clear. First, we conducted airborne electromagnetic survey. Second, the groundwater level was observed using the boreholes. The surveys were conducted on landslide occurred and not occurred slopes and the results were compared. Third, we conducted hydrological observation of the surface soil layer near the geological boundary using the tensiometers and the soil moisture meters. As a result, we have found that both the drainage characteristics and the distribution of resistivity were different on the two slopes. We have also found that geological boundaries affect hydrological behavior.