Late Quaternary landscape evolution and genesis of the 2009 catastrophic landslide in the Hsiao-lin area, southwestern Taiwan

Abstract With cumulative rainfall of 1700 mm, Typhoon Morakot triggered a catastrophic landslide at Hsiao-lin (9 August, 2009), which dammed the river, buried the village and killed > 400 people. We conducted a geomorphic study to understand the origin of this landslide in the context of the long-term evolution of the hillslope. The landslide originated from a broad, poorly drained slope (21°–23°) and ran down along two first-order channels. The erosion, up to 80 m thick, has created a concave-shape slope and exposed mostly loose debris, which implies that the landslide was mainly the reworking of ancient colluviums. The genesis of the concave-shape slope where the source sediments of the landslide resided reflects the weakness of the mudstone/shale bedrock underlying this slope, in contrast to the sandstone-based transport area. The two channels in the transport area also follow old faults or joints. Given that the source area has been subject to erosion during large landslides, the subsequent deposition here was likely achieved by minor colluvial processes. This deposition continued until the catastrophic failure in 2009, which could be controlled by (1) the amount of sediments deposited, which determined the slope angle, and (2) the headward expansion of the channels from the transport area, which enhanced groundwater convergence and later provided routes for the landslide materials to run downslope. Extensive mass-wasting motions had occurred in the area, as shown by the prevalence of > 20 m thick landslide/debris-flow sequences on the lower part of the hillslope, which are dated 21, 14.9, 13.7 and 12.0 ka by the radiocarbon method. These mass-wasting events had hindered the long-term incision of the trunk river in response to the rapid tectonic uplift. At least the 12.0 ka event, which caused extensive mud-flow deposition, had dammed the river. Overall, this study shows how bedrock lithology and structures controlled the shape of a hillslope and caused extremely unsteady mass-wasting transport in the active mountains of Taiwan.