Links between Landslides, Rainfall and Sediment Discharge around Langang and Chishan catchment

本研究在旗山溪集水區流域選用了四個颱風事件，包括2008年卡玫基颱風、2009年莫拉克颱風、2010年凡那比颱風以及2011年南瑪都颱風，在蘭陽溪集水區流域選用了三個颱風事件，包括2005年海棠颱風、2008年辛樂克颱風以及2009年芭瑪颱風，探討其崩塌地分布狀況。由實測輸砂量及輸砂濃度，探討輸砂量在乾濕季的差異、異重流事件以及颱風和地震前後之變化。最後利用台灣寬頻地震網於莫拉克颱風期間之資料，來進行崩塌地動訊號分析及定位，來找出其邊坡崩塌時間點與降雨之關係。 研究結果顯示，旗山溪集水區流域四個颱風事件之崩塌率介於1.7 %至7.27 %，2009年莫拉克颱風過後，崩塌率上升了4.64 %，但從凡那比颱風崩塌率1.34%及南瑪都颱風崩塌率0.19%，顯示大部分的坡體在莫拉克颱風過後，土地復育的現象已逐漸出現。在蘭陽溪集水區流域三個颱風事件之崩塌率介於0.62 %至1.07 %，在該流域下游之崩塌率介於0.29%至0.6 %，在上游介於2.12 %至2.52 %，崩塌有向上游集中之趨勢存在。且兩流域發生的崩塌多以靠近坡腳為主。在輸砂量部分，旗山溪集水區流域單位面積之年輸砂量為1,797 t/km2/yr，在莫拉克颱風前平均輸砂濃度為533 ppm，莫拉克颱風過後平均輸砂濃度上升2.52倍達到了1,362 ppm，在甲仙地震過後，其平均輸砂濃度上升5.7倍達到了7,883 ppm，蘭陽溪集水區流域，單位面積之年輸砂量為10,395 t/km2/yr，且在1980至2010年的實測輸砂濃度紀錄中，共有4次異重流事件，當日雨量達到360 mm時便有可能誘發異重流事件，其異重流再現周期為7年。 在莫拉克颱風期間，共偵測到了12個崩塌產生之地動訊號，訊號來源分布於旗山溪、荖濃溪、太麻里溪以及林邊溪四個區域，地動持續時間介於25 s至100 s，對應崩塌面積介於0.3 km2至2.7 km2，崩塌發生當下雨量介於712 mm至1967 mm，所有偵測到的訊號皆位於颱風期間最大降雨強度的後面2小時至39小時，此時間差受控於雨水滲入至地表面下深處之破壞面，當孔隙水壓上升使安全係數小於1.0時便會產生破壞。運用台灣寬頻地震網來分析颱風期間之崩塌，能夠讓我們清楚且快速地得知崩塌發生之位置，此位置將提供後續災害處理之評估，以達颱風期間災害應變處理之功效。This research focus on landslides distribution, sediment discharge, rainfall energy and seismometers signal. During the period 2005-2011, we investigated rainfall-induced landslides along Chishan catchment of southern Taiwan and Lanyang catchmen of Northern Taiwan from different typhoon events. The investigated result landslide ratios was 1.7% in Typhoon Kamegi, 7.27% in Typhoon Morakot, 5.61% in Typhoon Fanapi and 4.5% in Typhoon Nanmadol. After Typhoon Morakot, the decrease of landslide ratio reveals the slope reclamation little by little. The landslide ratios ranged from 1.07% to 1.32% in Lanyang catchments. In downstream area the landslide ratios ranged from 0.29 % to 0.6 %. In upstream area the landslide ratios ranged from 2.12 % to 2.52 %. Landslides distribution gathered in upstream area. On average the sediment discharge of Chishan catchment was 1,797 t/km2/yr and of Lanyang catchment was 10,395 t/km2/yr. Sediment concentration was 2.52 times after Typhoon Morakot and 5.7 times after Jiaxian earthquake in Chishan catchment. Discharge suspended sediment to the ocean at hyperpycnal concentrations (>40 kg/m3). During the period 1980–2010, 4 hyperpycnal flow events detected in Lanyang catchment When the accumilated rainfall reached 360 mm can induce it probably. The recurrence interval for a hyperpycnal flow was about 7 years. During typhoon Morakot 12 landslides were detected in Southen Taiwan. Landslide areas from 0.3 km2 to 2.7 km2 were detected by using signal analysis from seismometers. Our findings point out that the accumulates precipitation ranges from 712 mm to 1967 mm if rainfall-triggers a landslide area (more than 0.3 km2). It implied that using broadband stations to analysis rainfall conditions for big landslide would be a potential tool