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Zengwen Dam, also spelled Tsengwen Dam, () is a major earthen dam in
Dapu Township Dapu may refer to: China * Dapu, Hengdong (大浦镇), a town of Hengdong County, Hunan. * Dapu, Liucheng County (), town in Guangxi Province * Dapu, Yixing (), town in Yixing city, Jiangsu Province * Dapu, Yongchun County (), town in Fujian Pro ...
,
Chiayi County Chiayi is a County (Taiwan), county in Taiwan. Located in Regions of Taiwan, southwestern Taiwan surrounding but not including Chiayi City, it is the sixth largest county in the island of Taiwan. Its major tourist destination is Alishan Natio ...
,
Taiwan Taiwan, officially the Republic of China (ROC), is a country in East Asia. The main geography of Taiwan, island of Taiwan, also known as ''Formosa'', lies between the East China Sea, East and South China Seas in the northwestern Pacific Ocea ...
on the
Zengwen River The Zengwen River is the fourth longest river in Taiwan after the Zhuoshui River, Gaoping, and Tamsui, with a total length of about . It flows through Tainan and Chiayi County.It is located in the southwestern part of the island. Names '' ...
. It is the third tallest dam in Taiwan, and forms Zengwen
Reservoir A reservoir (; ) is an enlarged lake behind a dam, usually built to water storage, store fresh water, often doubling for hydroelectric power generation. Reservoirs are created by controlling a watercourse that drains an existing body of wa ...
(曾文水庫), the biggest reservoir in Taiwan by volume. The dam stores water for irrigation of the
Chianan Plain Chianan, Chia-nan, or Jianan may refer to: * Chiayi–Tainan Plain, also known as the Chianan or Jianan Plain (, ''Jiānán Píngyuán''), a large plain on Taiwan Island * Chianan Irrigation (, ''Jianán Dàzùn''), also known as the Kanan Irrigat ...
, Taiwan's most productive agricultural region, and provides flood control along the Zengwen River which flows through
Tainan City Tainan (), officially Tainan City, is a special municipality in southern Taiwan, facing the Taiwan Strait on its western coast. Tainan is the oldest city on the island and commonly called the " prefectural capital" for its over 260-year histo ...
. The dam supports a 50
megawatt The watt (symbol: W) is the unit of Power (physics), power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantification (science), quantify the rate of Work ...
hydroelectric power station Hydroelectricity, or hydroelectric power, is electricity generated from hydropower (water power). Hydropower supplies 15% of the world's electricity, almost 4,210 TWh in 2023, which is more than all other renewable sources combined and also mo ...
.


History


Chianan irrigation project

The first proposals for the dam were made as early as the 1930s during the
Japanese occupation of Taiwan The island of Taiwan, together with the Penghu Islands, became an annexed territory of the Empire of Japan in 1895, when the Qing dynasty ceded Fujian-Taiwan Province in the Treaty of Shimonoseki after the Japanese victory in the First Sino-J ...
. The Japanese built the
Chianan Irrigation Chianan Irrigation (), also known as the Kanan Irrigation System, was built to support agricultural production in the Chianan Plain of Taiwan. The name "chia-nan" was derived from two place names among its surrounding area called Chiayi and Ta ...
system to increase Taiwan's agricultural output, which was supplied primarily by the Zengwen River. Because the Zengwen River does not cross the area of the
Chianan Plain Chianan, Chia-nan, or Jianan may refer to: * Chiayi–Tainan Plain, also known as the Chianan or Jianan Plain (, ''Jiānán Píngyuán''), a large plain on Taiwan Island * Chianan Irrigation (, ''Jianán Dàzùn''), also known as the Kanan Irrigat ...
to be irrigated, a
tunnel A tunnel is an underground or undersea passageway. It is dug through surrounding soil, earth or rock, or laid under water, and is usually completely enclosed except for the two portals common at each end, though there may be access and ve ...
was punched through mountains to divert Zengwen water into the
Guantian River Guantian District () is a rural district (Republic of China), district in central Tainan, Taiwan. It is the hometown of former President of the Republic of China, Taiwanese President Chen Shui-bian. It hosts a population of 20,866 residents. ...
. Between 1920 and 1930, the
Wushantou Dam Wushantou Dam () is an embankment dam in Guantian District, Tainan, Taiwan. The dam was designed by Yoichi Hatta and built between 1920 and 1930 during Japanese rule to provide irrigation water for the Chianan Plain as part of the Chianan Irrigat ...
was built, forming an impoundment of the Guantian River to store of water for seasonal use. At the time of completion, it was the largest dam in Taiwan. Also built was the Tungkou Weir on the Zengwen River, a long structure designed to divert the water into the Guantian tunnel.Power Plant
rasb.gov.tw
The system irrigates about of land which otherwise does not receive enough consistent rainfall to grow crops such as rice.


Initial proposal

Due to Taiwan's highly seasonal rainfall, the Zengwen River contains little water except during the wet season from May to October, during which massive floods occur that exceed the capacity of the diversion tunnel.
rasb.gov.tw
Out of the average annual flow of the Zengwen River, only 25 percent of the water could be effectively used.
rasb.gov.tw
Furthermore, the Zengwen river has a high silt content, which was causing the Wushantou reservoir to quickly lose its capacity. In 1939, Japanese engineer Hatta Yoichi proposed the construction of a concrete gravity dam at a place called Lioutengtan, to control the floods and block silt, but these plans were dropped due to the intervention of
World War II World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...
.


River diversion

Construction finally began at the Lioutengtan site on October 31, 1967. Two tunnels were excavated through the rock underneath the south abutment of the future dam site. The outer diversion tunnel was long and the inner diversion tunnel was . A rolled-earth embankment
cofferdam A cofferdam is an enclosure built within a body of water to allow the enclosed area to be pumped out or drained. This pumping creates a dry working environment so that the work can be carried out safely. Cofferdams are commonly used for constru ...
was constructed to divert the Zengwen River and protect the dam site from flooding, up to an elevation of .


Dam and spillways

After the river had been diverted, the foundation could be excavated down to bedrock and drained. Construction of the dam could not begin until several faults in the bedrock had been stabilized and treated with concrete and
grout Grout is a dense substance that flows like a liquid yet hardens upon application, often used to fill gaps or to function as reinforcement in existing structures. Grout is generally a mixture of water, cement, and sand, and is frequently employe ...
. Construction of the main embankment using the
hydraulic fill Hydraulic fill is a means of selectively emplacing soil or other materials using a stream of water. It is also a term used to describe the materials thus emplaced. Gravity, coupled with velocity control, is used to effect the selected depositio ...
method began on 31 October 1970 and continued until 21 April 1973.Tseng wen
rasb.gov.tw
The Zengwen Dam is high, long and thick at the base, and contains of material. The dam was constructed using material from the spillway and foundation excavations, as well as the river bed upstream where the reservoir would be formed. The concrete spillway was constructed to the north (right bank) of the dam and consists of a multi-level open channel controlled by three
radial gate The Tainter gate is a type of radial arm floodgate used in dams and canal locks to control water flow. It is named for its inventor, the Wisconsin structural engineer Jeremiah Burnham Tainter. Tainter, an employee of the lumber firm Knapp, ...
s. Due to a large volume of unstable sandstone, clay and shale that comprised the slope where the spillway was to be built, about of material had to be removed before concrete placement could begin. Construction of the spillway was carried out simultaneously with the main dam. The spillway can discharge up to when all the gates are open. The dam is also equipped with two
sluice gate A sluice ( ) is a water channel containing a sluice gate, a type of lock to manage the water flow and water level. There are various types of sluice gates, including flap sluice gates and fan gates. Different depths are calculated when design s ...
s for de-silting operations, and two flood outlet tunnels with capacity. The entire project consisting of dam, spillway, and power station was formally completed and dedicated on October 31, 1973, exactly six years after the beginning of construction. The total cost of construction was US$97,373,000.


Power station

The Zengwen hydroelectric power station was constructed underground in the south (left bank) of the dam and required the excavation of of earth and rock, and of concrete treatment. The facilities were constructed using the
cut and cover A tunnel is an underground or undersea passageway. It is dug through surrounding soil, earth or rock, or laid under water, and is usually completely enclosed except for the two portals common at each end, though there may be access and ve ...
method. The Tongda tunnel was built to allow workers and equipment to access the plant after it had been completed. The power station's single 50,000
kilowatt The watt (symbol: W) is the unit of Power (physics), power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantification (science), quantify the rate of Work ...
generator first came online in October 1973. The average annual power generation is 184,900,320
kilowatt hour A kilowatt-hour ( unit symbol: kW⋅h or kW h; commonly written as kWh) is a non-SI unit of energy equal to 3.6 megajoules (MJ) in SI units, which is the energy delivered by one kilowatt of power for one hour. Kilowatt-hours are a commo ...
s. Because dam releases are determined by irrigation and domestic water supply needs, power generation is secondary (incidental) and does not always correspond to demand on the grid. Due to the large elevation difference between Zengwen Dam and the head of Wushantou reservoir, further development of run-of-the-river hydropower plants below the dam may be feasible. An 11,500 kilowatt facility is proposed, which would generate up to 49 million kilowatt hours annually.


Reservoir

On March 2, 1973 with construction on the dam 96 percent complete, the first diversion tunnel was plugged; on April 28, 1973, the second tunnel was blocked, allowing the reservoir to begin storing water.reservoir
rasb.gov.tw
The filling was approximately five months ahead of schedule, enabling water to be stored for the 1973 irrigation season rather than beginning in 1974 as originally planned. The Zengwen reservoir is the largest lake in Taiwan, with an initial design capacity of , and a surface area of when full, more than twice that of
Sun Moon Lake Sun Moon Lake ( zh, t=, p=Rìyuè tán, poj= ; Thao language, Thao: ''Zintun'') is a lake in Yuchi, Nantou, Yuchi Township, Nantou County, Taiwan. It is the largest body of water in Taiwan. The area around the lake is home to the Thao people, o ...
. The full water level is above sea level. Although the main purpose of the Zengwen reservoir is irrigation, it is also operated for flood control during the typhoon season. If the
Central Weather Bureau The Central Weather Administration (CWA; ) is the government meteorological research and forecasting institution of Taiwan (the Republic of China). In addition to meteorology, the Central Weather Administration also makes astronomical observation ...
issues a typhoon warning, and the reservoir level is higher than the flood-control pool, the gates may be opened to drain the extra water ahead of time, to reduce the risk of a sudden uncontrolled overflow that might endanger lives and property downstream. The reservoir has allowed an increase of of water per year for irrigation and industry, and another of drinking water. This makes possible the irrigation of an additional of land, while improving the reliability of supply for already existing farmland. It also allows for boats to access remote villages in Dapu township, which prior to the dam's construction could only be reached by winding mountain roads. The Zengwen Reservoir Management Bureau was established on January 1, 1974 to coordinate operations of the dam and reservoir. On January 23, 1998 the bureau was combined with other reservoir administrations in southern Taiwan, to form the Southern Region Water Resources Office.


Watershed and environmental issues

The Zengwen reservoir receives its water from an extremely steep and rugged catchment basin of in the
Alishan Range The Alishan Range ( zh, c=阿里山山脈, p=Ālǐ Shān Shānmài) is a mountain range in the central-southern region of Taiwan. It is separated by the Qishan River from the Yushan Range, the tallest range in Taiwan, to the east of the Alish ...
of south-central Taiwan. The elevation of the land ranges from on the shore of the reservoir to over in the
Alishan National Scenic Area The Alishan National Scenic Area is a mountain resort and nature reserve located in Alishan, Chiayi, Alishan township, Chiayi County, Taiwan. Geography Alishan is in area. Notable characteristics include mountain wilderness, four villages, ...
near the summit of the range. About 77 percent of the watershed is forest, 13 percent is agriculture, 9 percent other uses and less than 1 percent is urbanized. The reservoir has suffered severe siltation (sedimentation) from the watershed, which has caused a large reduction in its capacity. Siltation has reduced the volume of water available for irrigation and hydropower, as well as reducing the effectiveness of the Zengwen reservoir to control flooding. The siltation rate has greatly increased in recent years, in part due to a sharp increase in the intensity of typhoon rains, as well as a rising frequency of drought. In August 2009,
Typhoon Morakot Typhoon Morakot, known in the Philippines as Typhoon Kiko, was the wettest and deadliest typhoon to impact Taiwan in recorded history. The eighth named storm and fourth typhoon of the 2009 Pacific typhoon season, Morakot wrought catastrophic d ...
caused massive flooding and mudflows which carried of sediment into the reservoir, almost instantly wiping out 13 percent of its capacity. In June 2013, the Zengwen Reservoir capacity was estimated at , about 69 percent of its original volume. Due to lack of funding,
dredging Dredging is the excavation of material from a water environment. Possible reasons for dredging include improving existing water features; reshaping land and water features to alter drainage, navigability, and commercial use; constructing d ...
operations in the Zengwen reservoir have been limited, and the effective storage capacity has continued to decrease. However, after Typhoon Morakot the Taiwanese government approved a NT$54 billion project to remove silt and conduct watershed restoration work at Zengwen, Wushantou and Nanhua reservoirs. In January 2018, a long sluicing tunnel began operation at the base of the dam, allowing sediment from the bottom of the reservoir to be moved downstream. The project was constructed from March 2013 to November 2017 and cost NT$4.1 billion. It is expected to remove 1.04 billion tonnes of sediment each year.


Recreation

The Zengwen dam and reservoir are part of the Zengwen Reservoir Scenic Area. The reservoir was first opened to the public in July 1974.Tseng wen
rasb.gov.tw


See also

*
List of dams and reservoirs in Taiwan This is a partial listing of dams and reservoirs in Taiwan (Republic of China). List of dams and reservoirs See also * List of power stations in Taiwan References * Reservoirs, dams and weirs of Taiwan
(Taiwan Water Resources Ag ...
*
List of power stations in Taiwan This page is a list of power stations in the Republic of China (Taiwan) that are publicly or privately owned. Non-renewable power stations are those that run on coal, fuel oils, nuclear power, and natural gas, while renewable power stations run ...
*
Nanhua Dam Nanhua Dam (), originally named Houjie Dam (), is a dam across the Houku River, a tributary of the Zengwen River in southern Taiwan. Located at Nanhua District, Tainan, the dam was built to provide municipal water to Tainan and Kaohsiung City ...
*
Wushantou Dam Wushantou Dam () is an embankment dam in Guantian District, Tainan, Taiwan. The dam was designed by Yoichi Hatta and built between 1920 and 1930 during Japanese rule to provide irrigation water for the Chianan Plain as part of the Chianan Irrigat ...
*
Electricity sector in Taiwan The electricity sector in Taiwan ranges from generation, transmission, distribution and sales of electricity, covering Taiwan island and its offshore islands. Regulator Electricity sector in Taiwan is regulated by its state-owned electric ...


References


External links

{{commons category
Current water levels at Taiwan reservoirs
(Chinese) 1973 establishments in Taiwan Dams completed in 1973 Dams in Chiayi County Earth-filled dams Hydroelectric power stations in Taiwan