Effects of Watershed Runoff Factors on Debris Flow Occurrence–Using the Watershed of Chenyoulan Stream as An Example
Date Issued
2015
Date
2015
Author(s)
Chang, Ching-Fu
Abstract
The mechanical relationship between debris flow and surface runoff has been studied and described through channel experiments and numerical modeling. However, research on using surface runoff to predict the occurrence of debris flow at watershed scales is relatively rare. The objective of this study is thus to investigate the effects of surface runoff in watersheds on debris flow occurrence. With the watershed of Chenyoulan stream as an example, this study uses six different hydrologic factors, including one rainfall factor and five runoff factors, to establish debris flow warning models, whose performances and accuracy are then compared and analyzed. To start with, based on the results from statistical tests, effective watershed area(EWA), maximum streambed slop steepness(MSS), and landslide ratio(LR) are selected for model establishment, because of their mutual independence and their significant relation to debris flow occurrence. These physiographic factors are then quantified by two methods, namely their genuine values and their degree of membership(DOM). Secondly, through synthetic unit hydrograph and validation by observed data, Chenyoulan stream’s runoff is simulated, from which five runoff factors are obtained. The effective accumulated rainfall is also obtained for the purpose of comparison. Finally, twelve debris flow warning models are established using logistic regression; each of them comprises one hydrologic factor and two or three physiographic factors quantified by either their genuine values or their DOMs. The results show that using DOM to represent physiographic factors makes the performances of models better. Furthermore, there are five hydrologic factors that yield significantly better model accuracy, which is higher than 81%. These factors include effective accumulated rainfall(EAR), unit area one-hour peak direct runoff(q1), unit area three-hour peak direct runoff(q3), unit area five-hour peak direct runoff(q5), and estimated Shields stress parameter(SS). The findings of this study and the consequential debris flow occurrence contour maps can be applied for disaster prevention and site rehabilitation purposes. Based on acceptable debris flow occurrence probabilities and updated landslide ratios, the authorities can establish threshold values of hydrologic factors for issuance of debris flow warning. On the other hand, with the help of designed storms and acceptable debris flow occurrence probabilities, the authorities can also set up site rehabilitation goals of landslide area reduction for debris flow prevention.
Subjects
debris flow
logistic regression
landslide ratio
rainfall runoff routing
Type
thesis
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