Disaster Investigation and Failure Analysis of Debris Flow from Morakot Typhoon at Liugui Town, Kaohsiung, Taiwan on August 8, 2009

摘要

The techniques of in-situ investigation, close-range photogrammetry, laboratory tests, stereographic projection and slope stability analysis were used to investigate debris flow at Hsia-Hsinkai Tribe in Liugui town, Kaohsiung on August 8, 2009. Because test results show that 50- 80% of the soil aggrades are larger than 2 mm (#10 size), the study area can be considered as gravel debris according to the Taiwan specification. The internal friction angle of the aggrading soil materials ranges from 33.1° to 40.9° using direct shear tests. The outcrops found on site are analyzed with a stereographic projection method. Finally, an analysis of slope stability on the upslope reveals that the deposited geomaterials and rock slope on both sides of the valley are unstable when they are gradually saturated. The heavy precipitation is the primary factor that triggered the occurrence of the debris flow. Topographical variations before and after the 2009 Morakot Typhoon disaster were provided by a three-dimensional digital topography model created by a close-range photogrammetry technique. The elevation variation after the disaster is calculated using a digital elevation model (DEM) and can validate the relation between the source area and the depositional area. In Taiwan, the principal areas of debris flow can usually be divided into three sub-areas: a source area, a flow area and a depositional area. To investigate deposit length and mass flow volume, some independent zones in the study area are divided to verify zones locations after a disaster. It was found that the length of the depositional area is shorter than expected. Hence, the excess pore water pressure between the debris flow and the bedrock could be a significant factor to cause the debris flow. In this paper, we present an explanation of the debris flow based on Newtonian slide theory.