霧社水庫內沉積物的沖積與生長

Growth of the deposit wedge in the Wushe Reservoir

陳又嘉(Yu-Chia Chen)

指導教授 ： 宋國士

國立臺灣大學/理學院/海洋研究所/碩士(2012年)

https://doi.org/10.6342/NTU.2012.03061

摘要

水庫是一種人工興築的湖泊。台灣地區山高水急，每當降雨發生時，由集水區所匯集的雨水便挾帶大量泥砂經由河流運送，進入水庫區，並淤積於水庫底部，占去水庫蓄水的空間，降低水庫的蓄水功能。本研究以山谷型水庫—南投縣霧社水庫為研究案例，觀察霧社水庫內1998年至2010年來的沈積情況，描繪出沉積物堆積範圍、淤積型態及堆積量，了解沉積物的移動速率、搬運方式和造成淤積量多寡的原因，並利用反射震測了解庫底沉積物物理特性、壓密狀況及沉積層次，最後針對此研究結果進行定性說明，依其特色做相關分類，給未來水庫運作及水文研究參考。
本研究將霧社水庫由北至南分為個兩個部分各別討論，分別是沖積扇區和沉滓區。水庫中上游向下游移動的砂土，其搬移過程有兩種主要型態:第一類砂石搬運的方式為庫底滾動。第二類為以細泥物質為主的懸浮性搬運，和水一起流動，被搬運到較遠的地方。當水流攜帶沉積物進入湖泊或水庫等靜止水體時，所帶動的沉積物會在水庫的上游向下游方向將粗粒淤積，形成所謂的沖積扇，當沖積扇不斷沉積至靜水體中時，河流中挾帶的物質在沖積扇面上覆蓋稱為頂層(Topset Slope)，較粗的物質先沉積，沿著河床安息角坡度構成向外傾斜的層次，稱為沖積扇的前坡(Foreset Slope)，輸入的沉積物中較細的物質會被搬運到較遠、接近壩體的地方沉積，稱為底層(Bottomset Slope)。而本研究依此又將霧社水庫沖積扇細分成幾個部分，分別是頂層、前坡和扇底來討論。
根據1998年至2007年的實測庫底地形資料，發現頂層區有逐年增厚的趨勢，但各處增加的厚度不均勻，扇底區為每年等量增厚；沖積扇前坡因為高水位輸入效應，前坡位置有北移、高程增高的情形。此外，降雨量的多寡影響了輸入水庫的水量，進而影響到沉積物的搬運距離，水量較小時，沉積物在扇頂區的淤積位置和水道的走向有相對應的關係，沉積物依水道彎曲而左右堆置；水量較大時，也就是在2002年後，受到大的降雨量事件的影響，使得往後幾年的扇頂淤積位置和水道的走向沒有明顯的關係。從每年的各高程的淤積量變化得知，水庫從北到南有三至四個淤積(侵蝕)的中心，各代表不同的沉積(侵蝕)型態，推測可能是因為不同的水流事件，加上庫底水道彎曲影響所造成。
而在霧社水庫沉滓區(南區)，主要以懸浮的細粒泥質沉積為主，這些細粒泥質沉積物是在雨季時被水攜入水庫到達沉滓區，會於水流平靜時均勻散佈下沉於庫底，如長期未再受底流影響，會慢慢排水壓密成薄泥層。
根據歷年的實測庫底高程值，可觀察到幾乎每年的高程值都會增加，有少數年高程值降低，比對水庫上游雨量站資料，發現沉積物增加量的多寡與每年的單日大降雨量(大於100公厘)事件數呈正相關；再將每年實測高程對照震測圖，可以發現庫底沉積物有壓密及下沉的情況，且可分成上下兩層來探討；在2004年至2010年，壓密的情況不明顯，但有下沉的情形，而在1998年至2004年，壓密的情形較2004年至2010年顯著，在十二年內總下沉量為3.3公尺，最大壓密量發生在2003年至2004年間。另外我們也發現，在震測圖上有一不屬於沉積物沉積，而是氣體游離所造成的層面，位於2003年至2004年層面間，推測是由於2004年敏督利颱風，大量泥沙忽然被帶進水庫，覆蓋在底層上，沉積物中的細菌分解有機物質所釋放出的氣體無法逸散所致。

關鍵字

霧社水庫 ； 水庫 ； 沖積扇 ； 降雨量 ； 水深測量 ； 淺層震測

並列摘要

The sedimentary problem of mountain reservoirs in Taiwan is getting serious year by year. These sediments mainly are carried by currents and cause of erosion of land surface. When eroded sediments enter downstream areas, sediment transport capacity becomes lower, and leads to deposit in reservoirs (or lakes). This phenomenon brings about some problems for small mountain reservoirs: First, the efficiency capacity of reservoirs will decrease as the range of sedimentation increasing. Second, the function of generating power will be damaged. Thus understanding the interaction between deposit wedges and mountain reservoirs is a dominant issue. In this study, we selected Wushe reservoir, a mountain reservoir, which is situated in Nantou city (in central Taiwan), for a case study. The goal of the study is to investigate the bathymetries every year, evaluate the moving routes and carried ways of sediments, clarify the composition of sedimentation and explain the sub-bottom depositional sequences which are compared with daily rainfall data and annual rainfall data.
We used some reconnaissance techniques to survey this reservoir. Field measurements were performed to investigate the bathymetry in Wushe reservoir, we collected data of underwater landform and sub-bottom bedding information by using high resolution Multibeam Survey System (MBS) and seismic-reflection system (Chirp Sonar). Now, we already had the bathymetric data for more than ten years, moreover, in 2010, we used Chirp Sonar to analyze the sedimentary bedding situation in this area.
In this study, Wushe reservoir is divided into two parts individually to discuss: the alluvial fan area and the suspension deposit area. There are two main moving process types in the reservoir: bed-load and suspension. In this thesis, we divided alluvial fan to three parts: the topset beds, the foreset beds and the bottomset beds, where sediments are finer than upstream and close to the dam.
In 1998 to 2007, the elevation of topset beds was increasing, and the bottomset beds became thicker averagely. The position of foreset beds moved northward, and its elevation became higher. In addition, the amount of rainfall affects the sediment transport distance. If the input water is small, the positions of sediments deposit correspond to the thalweg in reservoir. If the input water is large, the relationship becomes unobvious. In 2002, due to the large rainfall event, there was no obvious relationship between the thalweg and deposit sediment locations. In these years, there are three to four deposition (erosion) peaks in the reservoir, and it represents of the different types of deposition (erosion).
According to the measured bathymetry data for twelve years, we can observe that the elevation value was increase year by year. The volume of increasing sediment is associated to the rainfall, especially greater than 100 mm. After comparing the annually measured elevation and sub-bottom seismic bedding data, we can find that there are sediment compaction and sinking in the reservoir. In 2004 to 2010, the compaction was not obvious, but it was sinking, while in 1998 to 2004, the compaction significantly serious than 2004-2010. In twelve years. The total sink is 3.3 meters, and the maximum amount of compaction occurred in 2003 to 2004. In addition, we also found that it has free gas layer, which does not belong to sediment deposition, in the sub-bottom seismic bedding data. The level is in 2003 to 2004. It is estimated that due to the 2004 typhoon Mindulle, it caused the large amount of sediment transported into the reservoir, covering at the bottom, and the bacterial decomposition of organic substances can not be released from sediments.

並列關鍵字

Wushe reservoir ； reservoir ； alluvial fan ； rainfall ； water depth investigate ； sub-bottom seismic reflection

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