河道彎道水力侵蝕崩塌預測暨連結度之研究

Abstract

崩塌的發育與河流作用息息相關，河流水力侵蝕邊坡坡腳而導致崩塌發生，而崩塌土砂進入河道會改變河流形貌。但目前研究對於河流水力侵蝕和邊坡的交互作用研究仍然相當缺乏，特別在山區岩床河道，許多邊坡穩定模式往往未考慮河流水力侵蝕效應而無法反映崩塌空間分布，本研究透過地理資訊系統的方法萃取河流水力侵蝕作用的空間分布，並且將邊坡單元與河流連接，統計歸納崩塌與河流水力侵蝕因子關係，並依據崩塌與河流水力侵蝕因子的量化分析成果，採決策樹建立水力侵蝕崩塌預測模式，並以石門水庫集水區巴陵壩上游五公里至下游義興壩範圍為研究區。研究結果顯示，從主流縱剖面和邊坡單元面向，研究區河道中流量、集水面積、河道蜿蜒度、水深、河寬與崩塌密度具有顯著相關，若考慮河流距離後，則是河道蜿蜒度和河道邊界剪應力最能解釋崩塌地之分布。此外，河岸邊坡類型會影響崩塌多寡，依研究區域崩塌密度排序基蝕坡最高，源頭坡次之，居末為滑走坡，基蝕坡和滑走坡在下游高蜿蜒度河段水力侵蝕崩塌差異愈趨明顯，且採決策樹整合河流水力侵蝕崩塌預測模式明顯比水文型無限邊坡模式為佳，整體正確率由54.7%提升至83%，顯見水力侵蝕作用對於河岸侵蝕和誘發崩塌所演的角色更為重要。 水力侵蝕崩塌連結度結果可知水力侵蝕崩塌顯著分布，當(1)標準化河流距離小於等於 0.298時；(2)邊坡安全係數介於 0.5 至 1.5之間；(3)河川級序6級河且為基蝕坡；(4)單位河川水力指標大於19,848.3 W/m^2；(5)河道蜿蜒度介於1.246 ~1.486區間；(6)河道邊界剪應力達1,921.1 Pa以上等情況為易產生水力侵蝕崩塌之區位。 此外，蒐集1968~2015年共2,001筆現地觀測資料，以穩健迴歸分析 (robust regression)率定崩塌體積-面積關係式，計算艾利颱風河岸邊坡崩塌體積、崩塌侵蝕率及崩塌規模尺度 (landslide magnitude scale, mL)，結果顯示在不同河岸邊坡類型 (基蝕坡、滑走坡和源頭邊坡)侵蝕速率具顯著差異，基蝕坡於6級河達最大約為60 mm/year、滑走坡於1級河達最大約為32 mm/year及源頭坡於1級河達最大約為16 mm/year，崩塌體積集中在主河道近岸高蜿蜒河段基蝕坡區域，相較於中上游增加3倍，且三種河岸邊坡類型崩塌規模尺度皆達10年以上。 綜合各項分析可知，不同河岸邊坡類型相互呈現顯著性差異，再者，河川級序愈高或河道蜿蜒度愈高時，基蝕坡和滑走坡的崩塌侵蝕速率的差異愈大，顯示基蝕坡受到較強的流速和二次流效應，侵蝕作用較為強烈，而滑走坡則水流速度較慢，泥砂沉積物堆積在坡腳保護，不容易被水力侵蝕，因此，水力侵蝕作用在下游或河道彎道河段扮演更重要的角色。此外，藉由比較無限邊坡模式和整合水力侵蝕因子的模式，整合水力侵蝕因子確實能增加模式的準確性，特別是在下游的河段，故本研究突顯河道彎道水力侵蝕對於山區崩塌和地形演育的重要性。

Mountain channels are strongly coupled with adjacent hillslopes. Hydraulic erosion cuts channel bank resulting in hillslope instability. Conversely, aggregated landslide sediment changes the channel morphology. However, the knowledge of stream-hillslope coupling is still lacking, especially in the bedrock channel and mountainous regions, and the effects of hydraulic erosion on landslide have not been considered in landslide susceptibility models. This study used a geographical information system to extract the spatial distribution of hydraulic erosion processes and linked it to the spatial units of channel-hillslope. Then, this study quantified the relationship between “hydraulic erosion on landsliding” and “integrated hydraulic erosion and riverbank landslide” to develop an integrated landslide probability model of hydraulic erosion (ILAPHE). The study area covered a 5 km meandering stream between Balin Dam and the Yixing dam within the Shihmen reservoir watershed. The results of this study on hydraulic erosion and landslides can be summarized in two parts. First, an analysis based on the longitudinal profile of the main stream and the slope units shows that river discharge, drainage area, sinuosity, water depth, and width were significantly correlated with landslide density. Normalized distance to stream, river sinuosity and boundary shear stress were the best predictors of landslides. On the other hand, hillslope type affected landslide density the most in headward slope, followed by undercut slope and slip-off slope. And the difference between undercut and slip-off slopes in landslide density increased with stream order and sinuosity. Moreover, with inclusion of hydraulic erosion, ILAPHE significantly improved the accuracy and reliability of predicting riverbank landslide, with a modified accuracy of 83%. Second, hydraulic erosion had most apparent effects on landsliding along the meandering river when the normalized distance to stream was less than or equal to 0.298, the safety factor of hillsope was between 0.5 and 1.5, sixth-order streams existed in undercut slope, the unit stream power index was more than or equal to 19,848.3 W/m2, the sinuosity was between 1.246 and 1.486, and the boundary shear stress was more than 1,921.1 Pa.To quantify landslide magnitude, 2,001 landslides from historical typhoon events in 1968~2015 were mapped, and landslide volume, erosion rate, and landslide magnitude scale of Typhoon Aere were estimated by using a landslide volume-area relation. The results show that the maximum landslide erosion rates on the undercut, slip-off, and headward hillslopes were 60 mm/year (sixth-stream order), 32 mm/year (first-stream order), and 16 mm/year (first-stream order), respectively, and were significantly different among them. Landslide volume peaked along the fifth or sixth-order stream with high sinuosity and undercut hillslopes, and the landslide erosion rate of a six-order stream was three times higher than the middle and upper-streams. Moreover, the landslide magnitude scale reached above 10 years for all three types of hillslopes. The results also show that landslide erosion rates increased with sinuosity, boundary shear stress or stream order on the undercut slope, but decreased on the slip-off slope. This suggests that the effects of hydraulic erosion play a more important role on the meandering or downstream river than on the straight or upstream river by eroding the materials on the undercut slopes and depositing sediment on the slip-off slopes. Furthermore, comparing the infinite slope stability analysis with and without using hydraulic erosion factors, it shows that the usage of the hydraulic erosion factors can improve the model performance, especially for the downstream area. This study highlights the need to understand more about the fluvial effects on landslides of hydraulic erosion and topography evolution in mountainous areas along meandering river.