Geomorphological characteristics of the collapsed slopes due to Typhoon No. 14, 2005 at the upstream of the Monobe river, Kochi, Japan

Katsuo SASAHARA, Hiromasa HIURA and Shoji DOSHIDA

2 large landslides and many surface failures occurred due to the Typhoon No. 14, 2005 at the upstream of the Monobe River in Kochi prefecture. The first large landslides was the Nakao landslide which occurred on the Butsuzo tectonic line between the Shimanto belt and Chichibu composite belt and the second was the Nakanishidani landslide which located at the chichibu composite belt. The fractured sandstone layer slipped on the weathered mudstone layer in both landslide areas. These landslides occurred due to the rainfall which had a large total rainfall amount of 1203 mm and relative small maximum hourly rainfall intensity of 75mm/h. It should be emphasized that the rainfall of more than 30mm/h continued during 10hours. The geomorphological characteristics of the slope on which 2landslides occurred were examined based on the LiDAR data with1m mesh. The gentle slope with the knick line at the boundary of the slope was recognized above the slope of both landslides. The linear depression zone and the stairs]like slope with the combination of steep slope to gentle slope were found in the gentle slope on the ridge of the Nakao landslide. These geomorphological features were the sign of old deformation of the ridge and slopes. Active landslides were found near the ridge above the Nakanishidani landslide. The continuous knick line or the gentle slope band traversed at the middle of the side slope of the ridge of both landslides.