An Update on The Three Gorges Dam
Harnessing the power of the Yangtze!
Introduction
No civil engineering blog would be complete without having covered the Three Gorges Dam.
An engineering marvel, it is visible from space and scheduled to be fully operational by 2011 (the dam body was complete in 2006 and currently undergoing installation of its 32 main generators).
Due to the controversial nature of the project (as discussed later), I selected Wikipedia as my primary source for this write-up.
The Wiki entry includes a variety of sources (including the Chinese government) along with criticism regarding environmental and socioeconomic impacts caused by the project.
Quick Facts
Construction Start Date: December 1994
Construction Cost (Estimate): $39 billion (yes, with a “b”)
Length x Height x Width (at base) = 7600 ft (1.44 miles) x 311 ft x 377 ft
Concrete Quantity Placed: 35.6 million cubic yards
Steel Quantity Used: 436,000 tonnes (enough to build 63 Eiffel Towers)
Soil Excavation: 134 million cubic yards
Power Generation Capacity (when completed): 22,500 Megawatts (for comparison, nuclear power plans have a summer peak capacity of 500-1300 MW)
The primary reason for constructing the dam is for flood control. Mnay important cities such as Wuhan, Nanjing and Shanghai are located adjacent to the river.
The dam reduces the probability of a major flood in downstream areas from once every 10 years to every 100. Reduced flood means less damage to downstream agricultural/residential land and the resulting economic impacts.
Main generator turbine.
Power Generation
The Dam has generated enough energy to cover roughly a third of the construction cost, according to the Chinese government.
Each of its 32 main generators (2 additional plant generators power the dam itself) contains a turbine measuring 34.11 ft in diameter, running at 94% avg efficiency (96.5% peak) and letting 750 cubic yards (1240 cy peak) of water through per second (cy/s).
Power generation is limited to the hydraulic head (height of water) behind the dam, so output peaks during flood season and operates closer to the average rate the rest of the year.
Power is distributed onto three grids (the East, Central and South China grides) to feed the nine provinces and two cities (including Shanghai) consuming energy from the plant.
At the time the project began, the plan was estimated to provide about 10% of electricity consumption in China; however, that number has now been reduced to 3% due to China’s explosive population and urban growth.
Environmental Contribution (Benefits)
China currently relies on coal as its primary source for electricity generation. The project would reduce coal consumption by 31 million tonnes and cut air emission of:
- 100 million tonnes of greenhouse gas
- 1 million tonnes of sulfur dioxide
- 370 thousand tonnes of nitric oxide
- 10 thousand tonnes of carbon monoxide
- significant amounts of mercury and dust/soot
These numbers do not the energy saved from:
- Eliminating the annual mining, cleaning and shipping of approximately 31 million tonnes of coal from northern China to the Eastern and Southern provinces.
- Increasing freight capacity along the Yangtze by a factor of 6 and reducing shipping costs by 25%, which will encourage more shipping along the river vs. highway transportation.
Environmental Impacts
The project has received criticism from abroad and domestically for its environmental impacts, including:
Affecting Biodiversity: The dam will increase the river’s reservoir upstream and destroy a large number of wetlands, which contain delicate ecosystems.
Increase Pollution: Because the river has traditionally been polluted into by cities and urban areas along the river, the dam will decrease the river’s flushing capacity and increase pollutant concentration in the water.
Effect on Local Culture and Aesthetics: The dam’s 375 mile long reservoir has flooded or will flood over 1300 archaelogical sites along the Yangtze, which has been a main feature of China’s natural beauty.
Down Stream Erosion and Upstream Sedimentation: Downstream erosion could make riverbanks more vulnerable to flooding and impact river ecosystems. Upstream sedimentation could impact power output of the dam (approx 480 million tonnes has been predicted to deposit in the dam reservoir).
Structural Integrity: The dam has few signs of damage (except for several hairline cracks forming during the initial filling of the reservoir) since beginning construction in 1994. However, it has not experienced a 10 or 100-year period flood, which would may exceed its load bearing limit and possibly lead to failure of the dam. Such a failure would be catastrophic to the downstream areas.
Earthquakes and Landslides: Although these events have been factored into the design of the dam, they may still cause damage to the structure and threaten its integrity.
Conclusion
The Three Gorges Dam has many benefits such as flood control, electricity generation and environmental contributions. However, these benefits come at a cost of wildlife habitat destruction, catastrophic failure of the structure and down stream/upstream effects.
