Three gorges dam is the tallest dam in the world constructed in China across Yangtze River. It is a gravity dam and is the largest hydroelectric project in the world, which was approved in 1992. Estimated cost is $26 billion. Structural work finished by May 2006. The water flows from top left to bottom right. The dam body divided into left side, the spillway and the right side, when facing downstream side .The underground power plant is in the south west part of the dam. For the construction of this dam, 27, 20,000 m3 of concrete and 4, 63,000 kg of steel was used; enough for 63 Eiffel towers. Its length is 2.335km, height is 185m, base and top width 115m and 40m respectively. It has a capacity of 39.3 km3, catchment area of 1,000,000km2 and surface area of 1,045km2. Other features of this dam are that it is not an obstruction to navigation, resistant to erosion sedimentation and flooding. At present, there are 32 working generators. Its installed capacity is 22,500 mw and annual generation is 100,000gwh. Although, there are economic benefits from flood control and hydroelectric power, there are also concerns about the future of over a million people who will be displaced by the rising waters, the loss of many valuable archaeological and cultural sites, as well as the effects on the environment.
2. FACTS AND FIGURES
· The Three Gorges dam will consist of a 610-foot high wall running 1.3 miles from bank to bank.
· The reservoir created by the backflow of the dam will extend 360 miles up river to Chongqing ("Chong-ching"), a distance equal to nearly half the length of California.
· Once operational, the dam will produce the energy of 15 nuclear power plants.
· The project is estimated to be completed in 2009 at a cost of over $30 billion.
· In the past 2,000 years, the Yangtze River has experienced 215 catastrophic floods.
· In 1998 flooding in the area expected to be controlled by the dam resulted in 4,000 dead, 14 million left homeless and $24 billion in economic loss.
· When the dam is completed, 13 cities, 140 towns and over 1,300 villages will be submerged by the Three Gorges Reservoir.
· To make way for the Three Gorges Dam, 1.5 million people will have to abandon their homes. More than 160,000 citizens have already been relocated.
· Upon the dam's completion, 1,300 known archeological sites will be lost forever under water.
· Over 360 million people live within the watershed of the Yangtze River. If the one in one thousand chance of a dam collapse occurred, the millions of people who live downstream would be endangered.
Construction formally began in November 1997. The Yangtze River was closed by dropping loads of huge rocks using a 32 tonne dump truck. The river was divided into 3.7 km long canal, using massive 580m long, 140m high coffer dam so that three gorges dam could be built on the river bed. In 2003, China blocked the Yangtze river and began building a 600 km long reservoir in the water. Live television broadcast showed water in the sluice being cut off slowly. Water rows at a rate of about a half cm every hour until mid June. When the water reached an interim level of 135 m above sea level ; 100 m above what it was before , commercial ships began passing through the locks. By August, two 700MW generators started functioning. The exploration of the coffer dam done with 191 tons of dynamite sent water shooting 30m into the air. Before the exploration, the water was zapped with electricity to keep the fish out of harm’s way.
4. BENEFITS OF THREE GORGES DAM
4.1 Power Generation
Electricity production in China was scarce comparatively. The fully completed three gorges dam will contribute about 100 TWH of generation per year . The Three Gorges dam is the world’s largest hydro electric power station by total capacity eventually reaching 22500 MW. It will have 34 generators: 32 main generator each with a capacity of 700 MW and two plant power generators each with capacity of 50 MW. Among these 32 main generators, 14 are installed in the north side of the dam, 12 m in the south side of dam and remaining six in the underground power plant in the mountain south of dam.
The main generator worth about 6000 tone each and designed to produce more than 700MW of power. The designed head of generator is 80.6m. The flow rate varies between 600-950m3 depending on the head available. The greater the head, the lesser water needed to reach full power . This dam uses Francis turbine having diameter 10.4 m and rotation speed of 75rpm.
The Yangtze river is one of the busiest waterways in the world . At present the shipping traffic can only pass the dam by means of 2 lane 5 chamber lock chain. The installation of ship locks is intended to increase river shipping from 10 million to 100 million tons annually with transportation cost cut by 30 to 37 percent . Shipping will become safer since the gorges are really dangerous to navigate.
Each of ship locks is made of five stages with transit time at around four hours. Maximum vessel size permitted is 10,000 tones. The locks are 280 m long , 35 m wide and 5 m deep. That is 30 m larger than those on the St. Lawrence seaway but only half as deep. Before the dam was constructed, the maximum freight capacity at the three gorges dam site was 18 million tones per year . From 2004 to 2007, a total of 198 million tones of freight passed through the locks. The freight capacity of the river increased 6 times and the cost of shipping was reduced by 25%. The total capacity is expected to reach 100 million tones. The time taken for this system is more than 3 hours.
4.4 Ship Lift
In addition to canal lock, a ship lift is a kind of elevator for vessels under construction .The ship lift is designed to be capable of upto 3000 tons. The original plan specified a lift having capacity of 11500 tons. The vertical distance travelled is 113m and the size of the ship basin is 120X18X3.5m and has a moving mass of 34000 tons. A conflicting factor is that the water level can rise very dramatically. The ship lift must work even if water level varies by 12m on the lower side and 30m on the upper side. Construction of ship lift started in October 2007 and is anticipated to be completed by 2014. It will be mainly used for passenger transport.
The main component of this structure ate 4 169m high reinforced concrete tower each measuring 40X6m on plan. The four towers are built on a continuous foundation slab measuring 119X57.8m directly on granite rock. Between the towers the steel ship chamber which is 132m long is suspended from 256 ropes that are connected with counter weights . 128 double room pulleys at the top of the tower. Each pair of towers on the long side of ship chamber is flanked by shear walls. The walls and towers are connected by coupling beams distributed evenly over the height. Two bridges between the towers are located above the ship chamber, 1 for the central control room and one for the visitors’ platform. The guided counterweights made of high density concrete run in shafts inside the tower. The ropes are deflected by the rope pulleys at the top of the structure which are supported by reinforced concrete girder mounted on the shear walls and the towers. Rope pulleys are protected by shear walls. Two steel structures on the top of the building with main runway. The cranes also serve the machine room on top of the ship chamber.
· The moving mass of approx 34000 tons ( water+ ship chamber+ counterweights + ropes) and usable space of 120X18X8m area are larger than any similar structure completed to date.
· As part of a dam complex with power station flood protection and two chains of locks , the ship lift is subjected to short term operational water level fluctuations upto 50cm per hour on the downstream side.
· Hydrological water level fluctuations of 30m on the upstream side and 11.8m on the downstream side require special gate equipment at the lower and upper buoys.
At the end of ship chamber an anti collision device positioned at a distance of 4.5m from the backplate of the chamber gate is provided to prevent damage by ships that do not stop in time. This device take the form of a rope 50cm above the water line. The rope is installed below a rope barrier which also serves as a walk way. The maximum impact energy for the design was 1600kNm.
5. DESIGN CALCULATION
· Incorrect operation of the drive
· Sunken ship
· Ship collision
· Rope breaking
· Differing water level
· Water pressure when one gate is open
· Chamber completely full/empty
6. WATER LEVEL CONTROL ON CHAMBER
To ensure control during operation the chamber is motor driven during both ascent and descent. On the ascent, the chamber is moved with slightly higher water level than the normal value of 3.5m. On the descent , the chamber is slightly less full so that the motor must always be in operation. The construction is however designed in such a way that the lift can also be operated in generating motor driven loads in alternating stresses on the pinions which were taken into consideration in the design process. After the ship has entered into the chamber , the chamber gate is closed and the water level is adjusted to the normal level within 5 minutes by pumps at a rate 250m3/sec. Once the outer gates are closed , the water in the gap between the gates is evacuated within 90 seconds and the temporarily stored in a pipe system below the chamber. While it is moving the water is pumped steadily back into it, which takes a minute. This keeps the flow speed of chamber low and avoids additional forces on ship hawsers.
The three gorges ship lift is a structure that is not only unusual in its purpose but also in its components. With its enormous dimensions and complex engineering solutions, the ship chamber effectively represents a movable building.
7. FLOOD CONTROL
The most significant function of a dam is to control flooding which is a major problem for the seasonal river, Yangtze. Millions of people live on the downstream side of the dam. Many large important cities and plenty of farmlands and important industrial areas built beside the river.
The reservoir flood storage capacity is 22 km2. The dam passed its first flood control test in July and August 2007, when heavy rain caused water level in the Yangtze river to rise above flood levels while other rivers overflowed their banks, killing hundreds and causing billions of dollars of damage. Barges navigated up and down the river and water flowed into irrigation canals as usual even though run-off water poured into the reservoir at 51000m3/s.
The three gorges dam regulates the river by releasing limited amount of water and trapping excessive rains in the huge reservoir. When the water level peaked boat traffic was haled and ship locks were closed and water was released at a rate of 48000m3/sec through 18 grant slanes. The rest of water remained baked up in the reservoir . The water peaked at 43m danger level downstream from the dam and then started to decline.
8. SEDIMENTATION CONTROL AND EROSION
Two hazards are uniquely identified with the dam one is that sedimentation projections are not agreed upon and the other is that the dam sits on a fault.
At current level 80% of the land in the area is experiencing erosion , depositing about 40 million ton of sediment into Yangtze annually. The relocation of people from the reservoir area will coarse farther deforestation and erosion due to agricultural needs.
Stopping the periodic and uncontrolled flooding of the river will lessen bank erosion . The dam will reduce the amount of silt transported Yangtze delta, eventually reduces the effectiveness of the dam of electricity degeneration. Perhaps more important is the deduction of downstream silt could results the sinking of coastal area.
Excessive sedimentation could block the down sluie gates. Critics claim that the reservoir with average 530 million ton of silt accumulate per year clogging the turbines entrance way . 151 million tons of sediments reach the reservoir every year . The huge sluice gates at the bottom of the 185m dam are opened to flush away sediment collected in the reservoir during floods.
The Three Gorges Dam is the largest hydro electric dam in the world and will supply a growing non coal energy source to supply power for growing industrial manufacturing sector of China. The dam was overwhelmingly beneficial and is an example to follow for future dam projects worldwide. The dam is also a symbol of China’s new future and role in world economy showing the contradiction that was raised up through the years of planning and construction that may have potentially hurt the dam’s overall impact on the Three Gorges Valley. The dam functions is basic but on a huge scale and its flood function is the primary purpose for the project . There is much scepticism hovering around the project . If the dam fails then a catastrophic event will occur.
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