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 analysiswere used to investigate debris flow at Hsia-Hsinkai Tribe in Liuguitown, 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 areacan be considered as gravel debris according to the Taiwanspecification. The internal friction angle of the aggrading soil materialsranges from 33.1° to 40.9° using direct shear tests. The outcrops foundon site are analyzed with a stereographic projection method. Finally, ananalysis of slope stability on the upslope reveals that the depositedgeomaterials and rock slope on both sides of the valley are unstablewhen they are gradually saturated. The heavy precipitation is theprimary factor that triggered the occurrence of the debris flow.Topographical variations before and after the 2009 Morakot Typhoondisaster were provided by a three-dimensional digital topography modelcreated by a close-range photogrammetry technique. The elevationvariation after the disaster is calculated using a digital elevation model(DEM) and can validate the relation between the source area and thedepositional area. In Taiwan, the principal areas of debris flow canusually be divided into three sub-areas: a source area, a flow area and adepositional area. To investigate deposit length and mass flow volume,some independent zones in the study area are divided to verify zoneslocations after a disaster. It was found that the length of thedepositional area is shorter than expected. Hence, the excess pore waterpressure between the debris flow and the bedrock could be a significantfactor to cause the debris flow. In this paper, we present an explanationof the debris flow based on Newtonian slide theory.