二仁溪中上游泥岩區河道演育之研究

Abstract

二仁溪流經臺灣西南部泥岩分布區，呈現惡地地形及曲流地形。本研究欲找出二仁溪中上游河道之變遷位置及類型，與河岸邊坡後退特徵；另一方面觀察一人工平台上之小型河道（紋溝）的地形地貌特徵，並觀察其在水力作用下的侵蝕或堆積現象，以歸納出其發育模式，藉以瞭解泥岩的紋溝與河道變遷特徵。 套疊分析第一、三、四版1/5000的像片基本圖，歸納出研究區河道變化類型包括：頸切斷、流槽切斷、向外擴張、河彎向下游移動與轉動，以及特殊的向內縮減現象。其中以曲流切斷現象最能顯現26年間的河道變化。頸切斷最大變化規模可達335 m，但研究區中多數頸切斷發生的原因極有可能是道路工程所致，或魚池開挖所影響。以坡腳侵蝕方式導致河岸邊坡逐漸後退之河道向外側擴張，其量最大可達30 m（26年期間），選取其中91個地點之平均值達18 m（26年），約等於平均每年向外側擴張0.7 m。曲流切斷之分布因曲流河段多分布在古亭坑層，尤其在古亭坑背斜的西翼支流崧子腳溪上游較密集分布，應與地層之高傾角有關。此外崧子腳溪上游之河道向外擴張方向與地層傾向一致，其他地方則關連性不大。 對於小型河道的觀測上，以現地觀察、野外降雨模擬試驗、沖水試驗，及沖蝕針剖面觀測進行。歸納出小型河道的發育模式可分為四階段：1.初始階段，主要受到降雨及漫地流之侵蝕，溝內物質由下游往上游呈團塊崩落，形成向源侵蝕的加長狀態。2.侵蝕營力以流水為主的小型河道。3.側蝕現象明顯的曲流河段。4.下游水量匯聚更多，面臨平台階地崖，高度急遽下落，坡度遽增，在下游處產生V形深谷。其中曲流河段的側蝕以坡腳侵蝕為主，與二仁溪河道現象上能互相印證。

This study focuses on the understating of morphological characteristics and evolutionary model in the mud rock area, southern Taiwan. According to the archive map analysis, comparison of channel evolution is mapped. Rill erosion and deposition is also observed during field experiments. The channel evolution and development of rill of mud rock are studied. The main research area is at the upstream area of River Erhjen. The results show that it is useful to identify channel evolution by using historical map. According to the photo maps of 1976, 1988 and 2002, there are several types of morphological changing, including neck cutoffs, chute cutoffs, migration and rotation. The biggest neck cutoff is about 335 m in length. However there are several neck cutoffs in the study area are caused by land use change by human activities. From the field survey, field rainfall simulations and monitoring of erosion pins, this study demonstrates the way of channel erosion. The biggest of slope retreatment is about 30 m within 26 years studying periods.