日月潭的地形地貌及淤積形態研究

Abstract

日月潭是台灣最重要的觀光湖泊及重要的電力調節設施，但由其東側引水口而來之溪水每天不斷將含泥的濁水溪渾水引入，造成從引水隧道口往外形成淤泥淤積，在水庫內堆積成沖積扇，除降低水庫有效之庫容空間，也嚴重導致潭水在淤泥處的生態受衝擊使得環境改變。此外日月潭是個70年前因抽續發電目的而改變形貌的盆地，由於水力的開發迫使邵族原來在日月潭畔的舊部落，現今被淹在水中。本研究利用日月潭水庫為實驗場所，利用多音束測深系統、側掃聲納及震測系統取得水下資料，觀察此區域的水下地形及地貌，討論和水下考古有關的水下特徵，並說明潭底沖積扇在過去十年來生長的狀態，間接預測水庫內沉積變化的趨勢，或許可提供為水庫維護的重要參考依據。 結果顯示日月潭西側月潭多年來幾乎無沉積物的產生，拉魯島之水下形貌幾乎一致；而西側日潭則會受東側引水口所帶來之泥砂淤積所影響地形高程。地形資料顯由潭深處漸往淺水域(721-748公尺)，高程位置的變化趨勢分為三個段落，斜率最大的為728m~739m這段，為2006-2014年來淤積的主要區域位置，也是沖積扇主要生長的範圍。新武界隧道在2006年啟用後，可以由2006-2012年變異圖上看到兩個淤積中心，顯示了新舊武界出水口所送出的淤泥推積在不同的位置上，且新武界送出的淤泥量比較多，並往正西方向推移，有向依達邵侵襲的趨勢；側掃聲納影像資料對比，可以找到相對應之地貌特徵如梯田構造、道路、古河道等，表示這117餘年來水下樣貌幾乎沒有改變，且在水下發現和古地圖上相似之古建物樣貌，但其真正結果下一步需要經由水下攝影機來佐證；震測資料可以發現地層間有壓密現象的發生，且日潭東側大於西側。此壓密現象是連續性的，沉積物會因為壓密作用於層間產生氣體。並且可以在震測剖面上看到基盤有下陷的情形，此現象或許和構造有關，並不完全是由壓密現象所引起。

Sun Moon Lake is a famous attraction and an important electricity generation facility in Taiwan. But the underground tunnel’s outlets in the east side of the lake which bring water to the lake will bring water mixed with a large number of muds continuously and accumulate near tunnel outlets, resulting in an alluvial delta in the reservoir. The accumulation will not only reduce the water capacity but also make serious ecological impact to the lake. In addition, the Lake is a basin which changed its appearance to reservoir 70 years ago due to conducting pumping storage project. The development of the project caused the village of Ita Thao near the lake been submerged in the water In this study, we use multi-beam echo sounders, side scan sonar and sub-bottom profiler to collect bathymetric data to observe the underwater terrain and topography of this area, in order to discuss the archeology features under the water and descript the growth of the alluvial over the past decade. We expect to predict the changes of accumulation within the reservoir, perhaps it can provide an important reference for reservoir maintenance. The results show that the west side of Sun Moon Lake has almost no deposit over the years, the Lalu Island which is in the middle of the lake appears almost the same underwater topography like 117 years ago ; But the west side of the lake is controlled by the sand and mud brought from the tunnel outlet.Bathymetry data shows the topographic elevation change is been dividing into three parts, the part which has the the maximum slope (728m ~ 739m) is the main area that the alluvial grew the most since 2006. The new Wu Jie tunnel was enabled to use after 2006, we can clearly see two deposition centers between 2006 to 2012 in deviation topographic map, showing that new Wu Jie tunnel sent the silt out in a different direction from old Wu Jie tunnel. Also, the new Wu Jie sent more silt than the old one, and there is a tendency that the alluvial extends west, and has the tendency to invade toward Ida village; Compared the side scan sonar image to the maps which was 117 years ago, we can find the corresponding structure features underwater such as terraces, roads, rivers and other ancient building remains, indicating that the underwater appearance has very little changed in this 117 years. We also found ancient buildings which the shapes are very similar to the ones on ancient map, but it needs to be confirm via underwater cameras, and that’s our next step. Chirp sonar data indicates the phenomenon of compaction can be found between the stratum layers, and the east side is more notable than the west side. This phenomenon is continuous, and the sediment in the stratum layer will generate gas. We can also see the ancient base plate sunk, perhaps this phenomenon was related to geological structure, not entirely caused by the compaction phenomenon.