圖書典藏及影音數位平台

民國98年8月莫拉克颱風為曾文水庫帶來9,108萬立方公尺淤積量，壩前淤積高程由EL.165公尺提升至EL.179公尺，遠高於發電及永久河道放水道(PRO)進水口高程，且上游集水區主河道尚有大量土砂未流進水庫。莫拉克颱風災害後，政府通過「曾文南化烏山頭水庫治理及穩定南部地區供水計畫」，但此特別條例期程有限，無法完全解決曾文水庫淤積問題。曾文水庫原先未定義有效庫容的容積，惟有鑑於水資源開發困難度與日俱增，且曾文水庫目前的庫容已不敷嘉南地區農業、工業及生活用水的需求，而欲恢復莫拉克颱風前庫容並不容易，依據100年度本所辦理「曾文水庫防洪防淤整體綱要計畫」的結論，以曾文水庫目標年民國117年預估庫容4.35億立方公尺為曾文水庫有效庫容。 參據「曾文水庫防洪防淤整體綱要計畫」，曾文水庫年入庫泥砂量為560萬立方公尺，繞庫排砂及庫區防淤隧道等水力排砂設施可排除約60%上游來砂，惟40%泥砂需靠上游集水區減砂入庫、陸挖清淤及機械抽泥等方式排除，然影響水力防砂設施之排砂效益因素繁多且複雜，包含入庫砂量、水文量、操作時機、持續操作時間，排砂流量及進水口高程等因子，針對水庫淤泥力學特性對於防淤效益之影響尚未有較深入之研究。因此本計畫擬參考國內外水力排砂經驗，研析在曾文水庫維持高水位操作且不影響正常供水前提下，於非颱洪期間評估以低耗能及低耗水之水壓吸引排砂工法排除庫底既有淤泥之可行性。本計畫同時進行曾文水庫現地淤泥力學試驗及淤泥輸送能力試驗以建立水力排砂數值模擬分析所需之參數，並進一步評估水壓吸引排砂工法之防淤效益。

1. Preface Tseng-Wen Reservoir is the largest reservoir in Taiwan located between Chiayi country and Tainan city. It is the most important water resource facility for Chianan plain. Unfortunately, the Tseng-Wen Reservoir was suffered from the sediment disaster during 2009 typhoon Morakot. It cause the 9,108-cubic-meter capacity decrease. Therefore, the project is aimed to develop a new desilting scheme, namely hydrosution sediment-removal scheme, to recovery the capcity of Tseng Wen Reservoir. 2. Collecting catchment condition in Tseng Wen Reservoir The area of the catchment of Tseng Wen Reservoir is 481.1km2, and its slope is steep. The geology of the catchment is composed of sandrock, mudrock, and shale. Owing to the fact that the mudrock and shale is erosible, the sediment-yield is large. The capacity of Tseng Wen Reservoir is 479,550,000 m3, and the catchment yields 5,600,000-cubic-meter sediment per year into the Tseng-Wen Reservoir with insufficient low-level discharge facilities. Frome the point of views of capacity recovery, there are several sediment-removal alternatives have been reviewed, the proioity sediment-removal alternatives to bulit sediment-removal tunnel (995cms) and to improve PRO (177cms). These two alternatives release 1,460,000 cubic meter sediment per year. However, 1,460,000-cubic-meter sediment discharge is still less than sediment load from the catchment, the dredgeing and hydrosuction sediment-removal may take into account. 3. Analyzing feasibility of hydrosuction sediment-removal scheme Hydrosuction sediment-removal scheme has been applied in many reservoirs locate in America, China, Japan, France, India …etc. The hydrosuction sediment-removal measure may adapt under the high water level condition and the material of deposition composes of fine particle such as fine sand, silt, and clay. From the field investigation, the material of sediment in Tseng-Wen Reservoir is mainly silt and clay. Due to cohesion of clay, the water jet technique may be place in the reservoir. The water level in Tseng-Wen Reservoir changes violently in whole year. For high water level and water utilization concern, the hydrosuction sediment-removal measure may work from July to October. Owing to the faot that the low-level discharge facility is insufficient at when pipeline of hydrosuction across over the crest of the dam, the hump of pipe bend will exceed 10m limition. Thus, the low- level sediment-removal tunnel-to-build has the position advantage if the pipeline across through the turned is permitted. 4. Drilling investigation and analyzing sediment mechanical property This work is to investigate on six different borehole within the range of 600 meters wide. A laboratory test results are list as follows: (1) 90% of silt in Tseng-Wen Reservoir is fine content, which has similar physical quality with low plastic clay and is undercosolidated soil with more water (around 40% to 50%) in content; (2) Tseng-Wen Reservoir depsition has the hyfraulic conductivity around 1x10-8 cm/sec and shear strength is only 0.5~8 kPa, which will easily to collapse or slide; (3) sediment deposition silt in Tseng-Wen Reservoir is mainly inorganic matter of nearly more than 92% without any heavy metal content and thus can become reclaimed material; (4) the silt of Tseng-Wen Reservoir has no chloride but has very high content of phosphorous (450~650 mg/kg), which could be caused by the drainage due to agricultural activities at the river front and inner bank. 5. Executing hydrosuction experiment Hydrosuction experiment could be divided into two parts, sediment transporting experiment and sediment layer driving experiment. Muddy water transporting experiment used three categories of sediment concentrations, three categories of water level, and two categories of diameter of pipe. The results show that the energy loss is larger when the higher sediment concentration is releasing. Mud deposit startup experiment is also divided into two parts, 1g and 30g. The results of mud deposit startup experiment show that the mud deposit can be easierly drove and massly removed when the jetting power is larger, the diameter of pipe is larger, and the water level is higher. However, only if some special dencity (over 1.42g/cm3), mud deposit startup experiment could drainage high-concentration mud water.(over 400,000 ppm) 6. Analyzing hydraulic numerical simutation From field data, experiment results, and international researches, simulating and analyzing the efficiency of hydrosuciton. Flow-3D is applied for the work item. Simulating mud deliver by real diameter of pipes, the results(friction coefficient) are close to the experiential formular. And the results of mud deposit startup simulation are similar to experiments. The consolidation level of mud deposit is the key factor for hydrosuction efficiency. 7. Preliminarily study hydrosuction scheme in Tseng-Wen Reservoir According to hydraulic condition in Tseng Wen Reservoir, labortary experiment, water supply demand, and international reaserches for hydrosution, setting important restricts of hydrosuction in Tseng-Wen Reservoir, such as transport velocity, water resource restrict, friction corfficient, and planning five feasible preliminarily hydrosuction alternatives: (1) Case 1A: The lowest workable level is EL. 209.7m. It can release 1.5 million-cubic-meter sediment every year. The unit cost of releasing sediment is 38.8 NT. dollars. (2) Case 1B: The lowest workable level is EL. 205.5m. It can release 2.1 million-cubic-meter sediment every year. The unit cost of releasing sediment is 27.6 NT. dollars. (3) Case 2A: The lowest workable level is EL. 213.6m. It can release 1.5 million-cubic-meter sediment every year. The unit cost of releasing sediment is 44.4 NT. dollars. (4) Case 2B: The lowest workable level is EL. 211.3m. It can release 1.4 million-cubic-meter sediment every year. The unit cost of releasing sediment is 64.4 NT. dollars. (5) Case 2C: The lowest workable level is EL. 201.7m. It can release 0.6 million-cubic-meter sediment every year. The unit cost of releasing sediment is 89.5 NT. dollars. 8. Planning universal desilting in Tseng-Wen Reservoir Actually, hydrosuction sediment removal method belongs to mechanical removal, and mechanical removal usually is used to dredge or dig local sediment. Hydraulic desilting is major used to carry out sediment from reservoir. Thus, hydrosuction sediment removal method is set to release sediment depositing entrances of the tunnel and PRO before typhoon and release sediment after typhoon. Owing to water resources demand, hydrosuction sediment removal method cannot be used when the water level in the reservoir is not high enough. The water level must higher than the level that the reservoir will restrict to supply water. 9. Value of the project According to five index items, technique, construction, operation, and economical benefit, case 1B is a top priority. However, case 1B need to co-ordinate with the sediment-releasing tunnel. According to results in the project, hydrosution sediment-removal scheme can certainly achieve the purposes of low-waste water resource, low-waste energy, and low cost . It is worth popularizing to every reservoirs.