| Shimen Dam | |
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Spillway and powerhouse, with forebay of dam visible in the center. |
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| Country | Taiwan |
| Location | Taoyuan County |
| Coordinates | |
| Status | In use |
| Construction began | 1956 |
| Opening date | 1964 |
| Construction cost | NT$ 4.85 billion |
| Owner(s) | North Water Resources Bureau |
| Dam and spillways | |
| Type of dam | Embankment, earth/rock-fill |
| Height | 133 m (436 ft) |
| Length | 330 m (1,080 ft) |
| Impounds | Dahan River |
| Type of spillway | Six-gated overflow |
| Reservoir | |
| Creates | Shimen Reservoir |
| Capacity | 218,000,000 m3 (177,000 acre·ft) |
| Catchment area | 763 km2 (295 sq mi) |
| Surface area | 1,977 acres (8.00 km2)[1] |
| Power station | |
| Installed capacity | 400 MW |
| Annual generation | 200 million KWh |
Shimen Dam (Chinese: 石門水壩; literally "Stonegate Dam"; anglicized as Shihmen or Shihman) is an embankment dam crossing the Dahan River in Taoyuan County, Taiwan. Serving mainly for municipal water supply and flood control, the dam creates Shimen Reservoir in the mountains south of Longtan. The construction plan was created in 1938 under Japanese rule, but was not implemented immediately because of the start of World War II. The dam was the largest in Taiwan when construction ended in 1964.[2]
Each day, Shimen supplies 1.4 million tonnes of water to residences and industry and 1.8 million tonnes of water to agriculture in Taoyuan County, Taiwan|Taoyuan County and New Taipei City. It is integral to the water supply/regulation system of northern Taiwan.[3] The dam cost NT $4.85 billion to construct.[4]
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The dam site lies in a steep canyon of the Dahan (大漢) River near the aboriginal town of Fusing, at the head of a 763 km2 (295 sq mi) catchment area.[1] The canyon, with walls up to 500 metres (1,600 ft) high, was formerly home to the summer villa of Chiang K'ai-shek. The historic arched Amuping Stone Bridge and a nearby Earth God shrine, among other landmarks, were also covered by the Shimen Reservoir as it filled.[5] [6] Before the dam was built in the 1950s, the flow of the Dahan River below the dam was copious enough in the rainy season to make it navigable from the coast to points in Taipei and Tamsui.[7]
In August 2004, Typhoon Aere hit northern Taiwan, dumping 973 millimetres (38.3 in) of rain on the region. The resultant erosion caused by rainfall caused landslides to carry up to 20 million tons of debris into the Shimen Reservoir. Although turbidity levels in the reservoir usually measure roughly 40 NTU.[8] and the water filtration plant can handle up to 3,000 NTU, it was clogged by the sediment in the water which reached a peak of 70,000-120,000 NTU. As a result, homes, businesses, and farms that relied on Shimen Reservoir had their water supply cut off. For several days, these customers had to subsist on trucked-in water.[2]
Also in August 2004, the entry fee to the dam, NT$80/person, was lifted, allowing free public access to the dam facilities.[9]
In 1994, the Amuping bridge and shrine became visible again because of record low waters in Shimen Reservoir. Since then, the shrine has been covered by steady siltation of reservoir waters, though the bridge remains intact and is visible during low water.[5]
In early 2008, the Taiwanese EPA announced a plan to spend NT$200 million ($6.16 million USD) to control nutrient levels in Shimen and three other large reservoirs in the region. High nutrient levels heavily affect water quality, causing eutrophication. The four reservoirs provide water for nearly ten million people, which is half of the country's population.[10]
Funding for the dam totaled NT$4.85 billion.[4] $21 million USD (NT$694 million) was loaned to the construction project by the United States.[11]
In July 1956, work on the dam commenced.[4] The upstream cofferdam for the Shimen Dam was then constructed, reaching a height of 70 metres (230 ft). One of the largest cofferdams built for its time, it also pioneered the use of roller-compacted concrete in gravity dam building. The roller-compacted concrete was used to build the core of the cofferdam, while the rest of the dam was completed by an embankment of coarse, pit-run aggregate. [12] With the upstream cofferdam completed, construction was able to proceed; the dam structure was completed in June 1964[4] and reservoir filling was completed later.
The Shimen Dam is a curved concrete and earthfill gravity dam 330 metres (1,080 ft) long and 130 metres (430 ft) high.[13] The hydroelectric power plant of the dam is located on its west side, while a six-bay spillway section is located shortly to the east. The spillways are fed by water through a series of tainter gates; they end in a hydraulic jump that dissipates water energy. [14] Intakes for the water supply system are located on the upstream face, and consist of five outlets[3] leading to a water treatment plant. [2] All water released into the river from the dam flows into its forebay, formed by a combination concrete gravity and earth-fill dam directly downstream from Shimen.[15] (A forebay is a dam located downstream from a larger dam in order to regulate irregular water releases. [16])
Due to irregular and dangerous sediment patterns in the Shimen Reservoir, with sediment travelling in invisible underwater plumes, a solution clearly had to be found. As a result, a large sediment monitoring system is in place. The first of the systems was installed before 2005 at two points in the lake. The system, called "Vertical Profiling System" or "YSI 6600EDS",[8] measures a series of water quality factors by depth, with intervals of 5 metres (16 ft). The turbidity, chlorophyll content, pH level, and dissolved oxygen levels are sent to the operator of the dam every three hours. A future plan for the system is to help record algae growth, preventing hypoxic events in the water and benefiting fish and other wildlife.[2]
The hydroelectric power plant at the base of the Shimen Dam uses water drawn from the Shimen Reservoir to power generators. The plant has a capacity of 400 megawatts,[17] generating roughly 200 million kilowatt-hours of energy annually. A key factor in the electricity grid of northern Taiwan, it runs as a peaking power plant. [4]
Water drawn from the reservoir feeds to twenty-eight districts of northern Taiwan, supplying municipal water to about 3.4 million people. The reservoir also contributes water to many agricultural and industrial users.[4] An old-growth forest, the 49,420-acre (200.0 km2) "Chi-Lan Formosan Cedar Forest", lies upstream of Shimen Reservoir. Runoff from the forest area feeds the reservoir, and the two sources of water supply are responsible for supplying nearly half of the population throughout five northern counties.[18]
Annually, the reservoir produces 80,000 kilograms (180,000 lb) of commercial fish. Fish production generates an annual revenue of NT$29.33 million.[1]
The steep, mountainous upper reaches of the Dahan River have long been a flooding hazard. When first built, the Shimen Dam was capable of protecting against large floods, but sedimentation has since reduced this capacity.[1][2]
The dam and reservoir attract roughly 160,000 visitors per year, especially during high water releases, and overall are "a direct contribution to the rapid expansion of the island's economy".[5]
The Formosan cedar forest upstream of and surrounding the Shimen Reservoir, said to have qualities and values of a World Heritage Site,[18] is being extensively logged. Although environmentalists and others have attempted to set limits on logging, results were not successful and logging continues. As logging occurs, large swathes of bare ground are exposed to more rapid erosion, leading to landslides and mudflows that may cloud the Shimen Reservoir. As after Typhoon Aere, a high sediment content in the lake potentially can clog the water filtration works, cutting off domestic water supply.[18] The sediment monitoring systems positioned in the lake are only used for monitoring, and as a result, have no means to control sediment flow.[8]