Using Rainfall and Groundwater Quality Data to Monitor and Analyze the Occurrence of Debris Flows
Date Issued
2009
Date
2009
Author(s)
Liu, Che-Hsin
Abstract
The contents of this study are mainly divided into two parts. One is to evaluate the critical rainfall threshold of debris-flow occurrence. The other one is to monitor and analyze the groyndwater quality during the slope failures which were triggered by groundwater level rise and consequently induced the occurrence of debris flow.n the first parts of this study, twenty-eight first order streams in the Nan-Tou area of Taiwan were chosen as the samples. After analyses, five factors of the stream samples were found to be higher related to the occurrence of debris flow. Through the analyses of three dimensional Fisher’s linear discriminant function, the predicting equations for the critical rainfall threshold of debris flow occurrence in this area were obtained. To reflect the effect of the great 1999 Chi-Chi earthquake on the occurrence of debris flow, some modifications were made for the aforementioned predicting equations. The modifications included the peak ground acceleration (PGA) during earthquake and the effect of the time increase after the Chi-Chi earthquake on the thixotropy. Aside from these, in this study, the rainfall data of some typhoons occurred after the great earthquake were used for analyses and tests. From the results obtained in this study, it might be concluded that the critical rainfall threshold of debris flow occurrence dropped noticeably right after the Chi-Chi earthquake, the critical rainfall threshold was elevated gradually. In addition, the critical rainfall threshold of debris flow occurrence and the effective rainfall path proposed in this study might be used for predicting the occurrence of debris flow and its occurring time. The predictions were found to be very close to the facts.s for the second parts, soil samples were used in this study for seepage tank tests in the laboratory were collected from the sides of two streams with high debris flow potential at Shenmu and Fengchiou village in Nantou County, Taiwan. While the tests were being conducted, observations were made to investigate the relationships among displacement of the slope, quality of the seepage water and occurrence of mass movement. The results showed that according to the change rate, displacement could be divided into two stages, namely, the initial failure displacement stage and primary failure displacement stage. While the displacement of the slope was in primary failure displacement stages, the probability of slope failure became much higher. Before general slope failure, electrical conductivity (EC) and sulfate ion (SO42-) concentration of the seepage water increased significantly. The time when EC of the seepage water started to increase rapidly was much earlier than that when displacement of the slope started to increase significantly. Therefore, from the hazard mitigation view, there will be a longer time for response if EC of the seepage water was monitored.
Subjects
debris flow
critical rainfall threshold
effective rainfall path
mass movement
electrical conductivity
sulfate ion
Type
thesis
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