利用降雨及地下水水質資料進行土石流發生之監測及分析

Abstract

本研究主要分為兩個部分，其一為推估土石流發生的臨界降雨量基準，另外即針對因地下水位上升導致邊坡崩塌所引發的土石流，進行地下水的監測分析。 在土石流發生臨界降雨基準部分，以1999年集集大地震前台灣南投地區28條一級溪流為樣本，分析得到與土石流發生較為相關的五個影響因子，經由三維費雪區別函數分析而得此地區之土石流發生臨界降雨基準預測公式。並以此預測公式為基礎，提出機率化階段式臨界降雨基準的方法。為反應集集大地震對土石流發生的影響，乃對上述臨界降雨基準預測公式做若干的修正。修正的因素包括地震時當地之地表最大加速度和地震後隨時間增加而恢復的影響。此外，本研究亦將地震後之颱風雨量資料加以分析和驗證，以瞭解臨界降雨基準修正後之適用性。從整個研究結果，略可推論研究區域內之土石流發生臨界降雨基準已因集集大地震之發生而大幅下降，且隨著大地震後時間之增加而逐漸提升；另外，本研究所提出之土石流發生臨界降雨基準和有效雨量路徑可作為對土石流發生與否及發生時間的預測，且其預測結果與事實相近。 另外研究中以台灣南投地區神木村及豐丘村土石流潛勢溪流的土壤樣本進行室內滲流箱試驗。模擬邊坡崩塌時，觀測邊坡位移、滲流水水質與邊坡崩塌的關係。試驗的結果顯示，邊坡發生崩塌前，依位移量大小明顯分成初始破壞位移區和主要破壞位移區兩個階段，若位移量進入到主要破壞位移區時，則邊坡發生崩塌破壞的機率將大增。且崩塌前滲流水中的電導度值及硫酸根離子有明顯的上升。而滲流水電導度開始明顯上升的時間，較邊坡有明顯位移的時間來得早。因此若以防災應變的觀點來看，有其較長的應變時間。

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. In 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. As 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.