中小型埋積河谷地形特徵與地形反應之研究：以新店溪支流為例

Abstract

臺灣因地狹人稠，山區中小型溪流沿線的狹窄平緩谷床，也常被開發利用。這類谷床常是沈積物淤埋而成，暗示該集水區內較易因崩塌或土石流作用而供給大量沈積物，如不當開發很可能造成災害。若能了解此類埋積河谷的谷床地形單元分布和集水區特徵，並了解極端事件中流域發生的地形作用對谷床產生的影響，應有助於自然災害的減災。本研究以新店溪中游的平廣溪、小坑溪與磺窟溪三條支流為例，嘗試以地表幾何形態自動分類和數值航測系統判釋谷床地形單元分布，透過數值地形的分析了解各項地形特徵和潛在地形作用，並以2015年8月蘇迪勒颱風之影響來檢視極端事件造成這些中小型流域的地形再作用情形。 研究結果顯示，本研究區三條溪流之埋積谷床的寬度從8到200公尺不等，谷床地形單元以高約2-3公尺高的小階為主，其中面積較大之平廣溪的支流土石流扇發育較佳，佔據主流谷床的中游段。蘇迪勒颱風誘發研究區內的地形反應，以平廣溪主流谷床最顯著，多處被淤埋或溢淹，其他兩溪的源流和多條支流溪溝可判釋出沖蝕特徵。該些溪溝大多有歷史崩塌紀錄或埋積、土石流扇特徵，受強降雨事件驅動，發生土石流再作用。 參考周憲德(2016)推估溪流主導營力之流域險峻值(Melton’s ratio)和主流長度的界檻值，可計算得知本研究區各子集水區和小坑溪主流的主導營力均為土石流，平廣溪與磺窟溪往下游則轉為高含砂水流和洪水作用。蘇迪勒颱風之降雨強度雖可達200年一遇之程度，但降雨延時短、總降雨量有限，研究區的整體流域崩塌率並不高。平廣溪流域相對降雨最多，以致原就好發土石流之源流段和支流溪溝河道上儲存的土砂被驅動，加以主流谷床之上游與下游段邊坡發生崩塌，故使其上游段谷床被土石流掩埋、下游段受到洪水溢淹。值得提出的是，此次土石流作用的河段還包含部份理論推估的高含砂水流河段，這應該和歷次降雨事件類型與土砂供應來源具有空間偶然性有關。 本研究以位於西部麓山帶砂頁岩地層為主的三個中小型流域為例，發現蘇迪勒颱風這種短延時高強度的降雨事件，雖然沒有造成邊坡普遍性崩壞，但足以驅動支流河道的土砂下移與主流局部河段邊坡崩壞，使主流埋積谷床發生地形再作用。在臺灣山區有非常多類似形態的埋積谷床，應仍屬現生地形，若其支流溪溝或源流段儲存相當多沈積物，則表示具有較高的地形動態性，值得進一步檢視其地景特徵所暗示的災害風險。本研究也發現，由於中小型溪流的谷床多窄小，若採用數公尺精度的DEM，透過地表幾何形態自動分類工具(Geomorphon)判釋谷底的平地形態(flat)，可以快速判別谷床的有無與大致分布位置，但若要進一步區劃細部的地形單元範圍，則需要逐一測試並組合形態，相當費時，故建議採數值航測人工判釋圈繪為宜。Taiwan is a highly populated mountainous island. Under the extremely high demand for land, even narrow waste-filled valley bottoms have been developed. Waste-filled valleys imply landslides and debris flow happen easily and supply deposit in that watershed, improper development may cause hazard. This research attempts to develop a (semi-)automatic procedure to identify the waste-filled valleys, of which the valley beds are characterized by debris flow fans and filled deposits. Three tributaries of the Xindian River in the northern Taiwan, are chosen as the study area, including the Ping Guang, the Xiao Keng and the Huang Ku Stream. The landform element classification is firstly applied (Geomorphon) to automatically identify the landform element of the waste-filled valleys and then verified by the result of the digital aerial photogrammetry. Melton’s ratio (MR) is adopted to infer the major geomorphic processes alone the streams. On-site investigation report immediately after the major event - Typhoon Soudelor in August, 2015 is also consulted.. The results show that the width of the valley bottoms is about 8 to 200 meters and the main landform element of the valley bottom is fill terraces with scarp 2 - 3 m. Tributary debris flow fans are relatively well-developed in the Ping Guang Stream and occupy the mid-stream valley bottom. Triggered by Typhoon Soudelor, with extremely high intensity but short duration rainfall pattern, the valley bottom of the Ping Guang Stream experienced the most significant geomorphic change and the sediment transport occurred in quite few tributary channels, which are characterized by valley fill, debris flow fan or historical landslides. MR infers that the dominant geomorphic process in the upper streams and sub-catchments in the study area is debris flow, while it turns into hyper-concentration flow and sediment-laden flow in the downstream of the Ping Guang and Huang Ku stream. Short duration of Typhoon Soudelor only induced low landslides ratio in the study area. Sediments contributing into main valley, however, were probably stored in the tributary channels during the previous events. It should also be noted that debris flow extended into the hyper-concentration flow segment inferred by MR during Typhoon Soudelor along upper Ping Guang Stream. It is mainly due to the coupling of sediment supply both from the source area and landslides on the immediate valley sides. This research demonstrates the dynamic response on the valley bottoms of the lower-order drainage basins, which is not just controlled by the existing geological and geomorphic settings but also the pattern of the geomorphic-forming events.