Investigation of the Large Landslide Evolution Using Slope Unit and Discrete Element Method
Date Issued
2014
Date
2014
Author(s)
Lin, Cheng-Han
Abstract
During the Typhoon Morakot, which happened on August 7 to 9 in 2009 in Taiwan, the heavy rainfall caused many more than 10,000 m^2 area of large landslides. One of them is the Butangbunasi river large landslide in Kaohsiung City. According to the DEM analysis, the Butangbunasi river large landslide source area is about 3.5 million m^2, and the total landslide material is more than 80 million m^3 through DEM analysis. The source materials of the Butangbunasi river landslide is four times of the Hsiaolin landslide, but the severity of damage did not cause the same. The previous research proposed the Hsiaolin landslide is catastrophic landslide, but this study consider that the Butangbunasi river large landslide may be caused by several small-scale landslide in sequence. The earlier deposit materials would produce the energy dissipation to the subsequent landslide block, and reduce the final disaster region of the large landslide.
In this study, we generated the slope unit by the object-based image analysis to recognize the mechanic of the different small-scale landslide events of the large landslide evolution. The ideal size of the slope unit should separate the direction of movement of different landslide events and the material volume has an influence. This study used the aspect and basin layers in the segmentation process, and carried the concept of local variance (LV) for defining meaningful segmentation scale parameters. Segmentation results had been evaluated visually, based on field work, expert knowledge and landslide inventory. Compared to previous researches have been heavily dependent upon trial-and-error exploration, the LV method can speed up the process and have quantified standard in the selection of appropriate scale parameters.
According to the slope unit aspect, we can divide the Butangbunasi river large landslide into five landslide events, and number the events as A, B, C, D and E from east to west. With the slope unit as a framework, the software PFC3D, which is based on the discrete element method, was simulated the complex behavior in the kinematic process and the geometry of deposition. Base on numerical model, the friction coefficient for each slip face should decrease into 0.06 that the landslide block will start sliding and the friction coefficient of each particle as 0.09 can obtain the best deposition regions which were closely matched with satellite photos after the Typhoon Morakot. The bonding strength as 60MPa will cause the disintegrative rockslide to develop into fragmentation and debris avalanches, which demonstrated by the field work. Those best fitting microcosmic parameters will be used to simulate the kinematic process of the Butangbunasi river large landslide.
Based on simulation results, the large landslide started in 11 to 30 sec and the rock block become the debris avalanche. After failure initiation, the average velocity was about 40 m/s which permits the debris to move in Laonung river. At 452 s after the event, the debris avalanche came to rest, forming a landslide dam in the middle of the Butangbunasi river. In the 3-D model most of the energy will dissipate because of the collision interaction of landslide blocks and deposition during the movement. The result helps with reconstructing historical large landslide events, which is due to the sequence of small-scale landslide events, and also predicting the affect region of the disaster.
Subjects
布唐布那斯溪
大型崩塌
能量消減
斜坡單元
分離元素法
SDGs
Type
thesis
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