| Summary: | 碩士 === 國立臺灣大學 === 海洋研究所 === 100 === 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.
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