Characteristics of rainfall-induced landslides in Miocene formations: A case study of the Shenmu watershed, Central Taiwan

Abstract

Fractured and high-permeability Miocene formations, exhibiting shallow soil layers and dense faults, and an increasing number of heavy rainfall events have caused severe and frequent landslides in the Shenmu watershed of Central Taiwan. The Shenmu watershed exhibited a landslide ratio that was typically greater than 1.0% during heavy rainfall events between 1996 and 2009, and can be considered one of the most landslide-prone areas in Taiwan. The landslide ratio in the lower hillslopes is approximately 2.8 to 43.3 times greater compared with that in the upper hillslopes. Analyzing based on a self-organized criticality (SOC) perspective indicated that the β value of the Shenmu watershed (1.24–1.26) was the lowest among landslide-prone areas, implying that small landslides dominated the distribution of landslides documented in available records. The six large landslides that occurred include four shallow landsides and two deep-seated landslides (with a landslide depth greater than 10m). Approximately 82.3% of the small landslides constituted bank-erosion landslides in the downstream watershed, whereas 96.2% of the large landslides occurred in upstream areas with dense faults. Large landslides occurred during only heavy rainfall events in which the accumulated rainfall was greater than 1000mm in fractured and high-permeability strata with dense faults. The large landslides deposited considerable amounts of sediment into streams, inducing significant bank-erosion landslides in the downstream watershed. Bank erosion in the downstream watershed caused additional small landslides along the river. Large landslides were typically caused by a fault density of 693.6m/km2, fractured strata with a permeability of 10−3 to 10−4m/s, and high levels of accumulated rainfall exceeding 1000mm. The apparent increase in bank erosion, which increased from 103% to 136% between 1996 and 2009, resulted from an increasing number of heavy rainfall events and the amount of sediment deposition caused by major landslides. Frequent landslides result from the increasing number of heavy rainfall events, fractured rock exhibiting dense faults, and steep and shallow-soil morphology in the Shenmu watershed.