埋積谷床地形特徵、作用與流域地形計測之探討：以新店平廣溪為例

The Study on Landform Feature, Process and Geomorphometric Analysis of Waste-filled Valleys: A Case Study of Ping Guang Stream, Xindian

臺灣山區中小型河谷常見由沈積物淤埋而成的較平緩河段，當強颱豪雨驅動流域內沈積物搬運時，很可能造成谷床上建物、農地的災害，但對其地形變動的探討有限。本研究擬探討中小型埋積谷床地形特徵、地形作用和流域地形計測參數的關連，主要研究方法為數值航測地形判釋、流域地形計測，並結合前人現場調查和流域險峻值（Melton's ration）的分析結果，研究樣區為新店溪中游支流平廣溪，2015年8月蘇迪勒颱風時曾誘發較顯著的地形變化與造成災害。數值航測判釋結果顯示，平廣溪主流谷床以比高約2－3m的埋積階地為主要地形單元，中段並有多個支流扇狀地分布，顯示該溪谷主要受到土石流與洪水溢淹作用而建成，流域險峻值（MR值）估算結果亦大致符合。不過蘇迪勒颱風幾處特定地點之谷壁邊坡崩塌所供給的大量土砂，導致此次平廣溪土石流作用的河段比MR值推估的略向下游延伸，而且該溪流最下游的洪水溢淹與谷壁崩塌、新店溪水位高漲都有關係。由此可知，雖然每次降雨事件誘發的地形反應具有空間上的偶然性，但當代單一大規模事件（例如，蘇迪勒颱風）通常只是在其既有的地形單元上「再作用」，因為谷床地形特徵反映的是更長期無數次大規模事件作用的結果。臺灣山區類似平廣溪谷床地形的河段很多，結合判釋既有地形特徵、流域地形計測與現場實察，進行綜合評估，將有助於理解埋積谷床的災害風險評估。

In Taiwan, waste-filled valleys of small mountainous catchments imply that the valley floors are subject to geomorphic hazards triggered by debris flow, bank erosion and flooding. This study intends to demonstrate the spatial distribution of landform features and of geomorphic change by the major rainfall event in a waste-filled valley. The methods applied include digital aerial photogrammetry, geomorphometric analyse (Melton's ration, MR) and on-site investigation report analysis. The study area is the Ping Guang Stream (PS Stream), a tributary of the Xindian River in the northern Taiwan, which experienced the most significant geomorphic change during Typhoon Soudelo in August, 2015. The main landform unit of the valley bottom along the Ping Guang Stream is fill terraces with scarp 2-3 m and a few tributary fans locate in the mid-stream. During Typhoon Soudelo, the dominant geomorphic process in the upper streams was debris flow and turning into flooding in the downstream, which is in line with the result of MR. It is regarded as the reworking of geomorphic processes within the existent geomorphic framework of the Ping Guang Stream valley although the location of landslides and raising water level of the Xindian River, i.e., the spatial and temporal contingency, did increase the area of debris flow and flooding. There are numerous waste-filled valleys as the Ping Guang Stream in the mountainous Taiwan, a comprehensive investigation as demonstrated in this research will greatly benefit the future natural hazard assessment.