The Discussion on Occurrence Factors of Debris Flow In Disaster Area of Typhoon Morakot
|關鍵字:||土石流;發生因子;統計分析;數值高程地形;debris flow;occurrence factors;statistical analysis;DEM|
3. 採隨機森林演算法(random forest, RF)針對多變量因子間關連性進行研究與探討，發現莫拉克颱風時，土石流顯著發生因子分別是「有效累積雨量」影響最大，「時雨量」次之，之後依序「各潛勢溪流線50公尺環域範圍內之崩塌率」、「10度以上有效集水區高度平均值」。
Typhoon Morakot lashed out on Taiwan during Aug. 7 to 9, 2009. Morakot dumped heavy rainfall in southern Taiwan, especially around the Central Mountain Range in Pingtung, Chia-Yi, and Kaohsiung County. Comprehensive field investigation was carried out by government and private organizations after Typhoon Morakot, and valuable information of debris flow was gathered. In addition, after Typhoon Morakot, the occurrence factors of debris flow become more challenging in Taiwan, and many aspects need to be considered. The scope of this project covered mainly the discussion on the occurrence factors of debris flow in seriously damaged areas which include Nantou, Chia-Yi, Tainan, Kaohsiung, Pingtung, Taitung. A total 218 torrents were included. Field investigation data and disaster records of Typhoon Morakot were utilized to analyze the occurrence factors of debris flow in three aspects: terrain, rainfall, and sediment source. The project’s results were summarized as following. 1. After “principal component analysis” and “Pearson product-moment correlation analysis”, 101 factors were reduced to the most important 18. The highest AUC(Area Under Curve) value under SRC(Success Rate Curve) is 0.5670, which means that single factor could not predict the occurrence of debris flow. 2. Bubble chart and 3D scatter diagram were selected to describe the relationship of average slope of watershed, landslide rate along the stream within the 50m buffer zone, as well as the rainfall during Typhoon Morakot. The above charts were used to summarize the range of factor values which tends to occur in the debris flow in Typhoon Morakot. 3. Furthermore, RF(Random Forest) Theory was utilized to research the relationship with multi-variables. Significant factors which affect the occurrence of debris flow were effective accumulated rainfall, hourly rainfall, landslide rate along the stream within the 50m buffer zone, and average elevation value of effective watershed higher than 10 degrees sequentially. 4. According to the results, the most significant factor is clearly the rainfall factor during Typhoon Morakot. 5. Also, the Shenmu area of Nantou County was selected for demonstration, and rainfall data and historical disaster events were used to find the lower boundary line for debris flow events. Satellite images before and after Typhoon Fanapi were purchased for the interpretation of landslide area. Four historical DEMs were produced to explain the terrain change from the downstream to upstream in DF199 of Nantou County.
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