Coupling Fluvial Processes and Landslide Distribution Toward Geomorphological Hazard Assessment: Cases in Tectonically Active Mountain Ranges in Taiwan and Japan

Thursday, 17 December 2015: 14:55
309 (Moscone South)
Ching-Ying Tsou1, Masahiro Chigira2, Yuki Matsushi3, Noriyuki Arai2, Su Chin Chen4 and Zheng-yi Feng4, (1)Hirosaki University, Department of Agricultural and Environmental Engineering, Aomori, Japan, (2)Disaster Prevention Research Institute, Kyoto University, Kyoto, Japan, (3)Kyoto University, Kyoto, Japan, (4)National Chung Hsing University, Department of Soil and Water Conservation, Taichung, Taiwan
Large-scale destabilization of mountain slopes, which are affected by long-term river incision, give rise to the risk of catastrophic failures in tectonically active ranges. We found deep-seated gravitational slope deformations (DGSDs) induced by these processes in the Chishan River and Dahan River in the Central Range in Taiwan and the Kumano River in the Kii Mts. in Japan. These areas comprise paleosurface remnants with moderate relief at higher elevations and incised V-shaped inner gorges below them, which were made by the recession of knickpoints. Our studies include field surveys, mapping of DGSD and landslide scars, and cosmogenic nuclide exposure dating of several landform surfaces.

In the Dahan River catchment, rims of paleosurfaces, which have a minimum age of ca. 150 kyr, are distributed up to 600 m above the present river bed, acting as a proxy of fluvial dissection associated with phases of river incision since the middle to late Pleistocene. The relationships between slope movements and the topography modified by the river incision show that about 53% of all DGSDs, or all large DGSDs (>106 m2) and catastrophic landslides occurred on slopes along the rims of paleosurfaces, suggesting they could be fundamentally controlled by long-term river incision. Catastrophic landslides observed along or below the rims of paleosurfaces were preceded by buckling of alternating beds of sandstone and mudstone on parallel or underdip cataclinal slopes dipped at 50° to 58°. This suggests that the peripheral zones of the paleosurfaces may be most susceptible to future catastrophic landslides, particularly on parallel or underdip cataclinal slopes comprising alternating beds of sandstone and mudstone dipping at 50° to 60°.

The 2009 Typhoon Morakot-induced Shiaolin landslide along the Chishan River and the 2011 Typhoon Talas-induced catastrophic landslides along the Kumano River also occurred on the gravitationally deformed slopes along the rims of paleosurfaces.