Effects of Physiographic Factors on the Occurrence Probability of Debris Flows
Date Issued
2015
Date
2015
Author(s)
Huang, Hsiao-Yu
Abstract
This study focuses on the establishment of a model for evaluating the occurrence probability of debris flows using physiographic factors and rainfall, and the effects of physiographic factors on the occurrence probability of debris flows. Firstly, the watershed of Chenyulan Stream was selected as a study site. In this site, after analysis, four physiographic factors were adopted for their significance to the occurrence of debris flows, including landslide ratio (the ratio of landslide area over watershed area), average steepness of the streambed, effective watershed area, and form factor. Among the four factors, landslide ratio was found the most significant. Then, the method of logistic regression analysis, accompanied with the physiographic factors and rainfall parameter were used to establish the model for evaluating the occurrence probability of debris flows. To obtain better results for this model, the effects of the division methods of rainfall event, the calculation methods of effective accumulated rainfall, and whether the physiographic factors were quantified using membership function, on the debris flow warning model. Therefore, two methods for dividing rainfall event and calculating effective accumulated rainfall (namely, RET1 and RET2) and two methods for evaluating physiographic factor(namely, the genuine values, PF1, and the values converted using membership function, PF2), i.e. four combinations in total were used to analyze and establish the model for evaluating the occurrence probability of debris flows. The results shows the model established using the method of rainfall event division by Fan et al.(2003) and the physiographic factors converted using membership function is the best, and because the physiographic factors are considered, its prediction accuracies reach as high as 80%, which is higher than that predicted using the debris flow warning rainfall by the government. The model developed in this study is consistent with the mechanism of debris flow occurrence. The occurrence probability of debris flow increases with landslide ratio (DN), average steepness of the streambed (SN), effective watershed area (AN), and form factor (FN). If the occurrence probability of debris flow remains constant, when landslide ratio (DN), average steepness of the streambed (SN), effective watershed area (AN), and form factor (FN) increase, the rainfall for triggering debris flow decreases. Based on the model for evaluating the occurrence probability of debris flows established as above, in this study, the models were also built for the basin of Kaoping stream and the combined area of the watershed of Chenyulan stream and the basin of Kaoping stream. For the two models, four physiographic factors: landslide ratio, average steepness of streambed, effective watershed area, and form factor, were adopted for their significance to the occurrence of debris flow. Among them, landslide ratio was also found the most significant. The prediction accuracies in the basin of Kaoping stream and the combined area of the watershed of Chenyulan stream are approximately 70% and 60%, respectively. In general, the model established for the watershed of Chenyulan stream is the most accurate. It is suggested the model developed in this study be built for future. When the physiographic factors significantly change, the model could be used to adjust the warning rainfall of debris flow.
Subjects
landslide ratio
physiographic factor
rainfall
debris flow
probability
logistic regression
Type
thesis
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