Applying Logistic Regression Method to Analyze the Effect of Landslide on Rainfall Threshold Value for Debris Flow Warning –Using the Site of Shenmu, Nantou as An Example

本研究主要為探討崩塌及其他地文因子，對土石流降雨警戒基準值（rainfall threshold value for debris flow warning）之影響，並進而推估之。土石流降雨警戒基準值為現今臺灣地區發布土石流警戒參考之依據，以供為預警指標，並於每年或遇重大土砂災害事件後檢討修訂。 本研究之研究區域位於南投縣信義鄉神木村，蒐集研究區域內土石流發生之事件，利用羅吉斯迴歸法分析。並統計前人研究中最常被使用作為探討土石流事件發生與否之地文因子，利用溪床平均坡度、崩塌地面積及有效集水區面積等地文因子推估土石流降雨警戒基準值。根據分析結果得知，資料比例以發生土石流：未發生土石流為1：1時，有較高之敏感度，高達87%。顯示當土石流發生事件與不發生之土石流事件資料比例較相近時，有較佳之預測結果。 另外本研究中利用羅吉斯迴歸法分析推估得到土石流降雨基準之公式。推估公式所用之參數為溪床平均坡度、崩塌地面積、有效集水區面積及有效累積雨量。並以賀伯颱風之事件作驗證，研究顯示當地文條件類似賀伯颱風時，有效累積雨量只要達到713.5mm即有可能會發生土石流。The main purpose of this study is to investigate the effects of landslides and other physiographic factors on the rainfall threshold value for debris flow warning and then to evaluate it. In Taiwan, the rainfall threshold value for debris flow warning has been used for years as a basis to announce debris flow warning and it is amended annually or after the event of great mass movement disasters. The site of this study was selected at Shen-mu, Nan-tou. The debris flow events occurred in the site were collected and the method of logistic regression was used for analysis. Statistical analysis was made on the physiographic factors which were mostly commonly used in the previous studies to investigate the occurrence of debris flow. In this study, average riverbed slope steepness, landslide area and effective watershed area were used to estimate the rainfall threshold value for debris flow warning. From the results, it was found that when the ratio of the data in proportion to the occurrence of debris flow and non occurrence of debris flow was 1 to 1,the sensitivity was higher and up to 87%. This indicated that when the debris flow events of occurrence and non occurrence were similar, there would be a better prediction. In addition, the equation for evaluating rainfall threshold value for debris flow using the logistic regression analysis was obtained. The parameters used for evaluation were average riverbed slope steepness, landslide area, effective watershed area and effective cumulative rainfall. The event occurred during Herb typhoon was used as an example for validation. If the physiographic factors are similar to that during Herb typhoon, debris flow may occur while the effective cumulative rainfall reach 713.5mm