Engineering Challenges of the Three Gorges Dam Project
The Three Gorges Dam is the largest and most powerful hydroelectric project in the world. The final structure is 610 feet high and 1.3 miles wide. The reservoir from the dam extends to 410 miles upstream on the Yangtze River, the third longest river in the world, when the water is at its maximum level of 574 feet above sea level. The planning and management of this engineering wonder has been a challenge for the civil engineers involved. The idea of a dam along the Yangtze River was first proposed in 1919 by Sun Yat-sen in International Development of China. The modern plans from the dam were created during the 1980s, and the final project, costing an estimated $30 billion, was approved by the National People’s Congress in 1992. Cost recovery is expected to be achieved after the dam has generated 1,000 TWh of electricity, approximately ten years after full operation is started.
The dam structure is classified as a concrete gravity dam. A concrete gravity dam is one that forms a straight barrier across the river. The cross section of the wall is designed so that it resists the moment force applied by the water behind the dam. The Three Gorges Dam’s wall is 377.3 feet thick at the base and 131.2 feet thick at the top. Hydroelectric power is not the only function of the Three Gorges Dam, it is also meant to provide a flood control system for the Yangtze River, which floods regularly. Large cities in the rivers path, including Shanghai, are at risk to flood damage from the river.
The two functions needing to be performed by the dam – hydroelectric power generation and flood control – needed to be balanced by the design engineers. Flood control reservoirs usually maintain a low reservoir level so that flood waters can be retained when they occur. However, hydroelectric power generation reservoirs usually have high reservoir levels. Civil engineers working on the Three Gorges Dam project met these conflicting requirements by specifying the reservoir height differently at different times of the year. In the dry winter season, the reservoir is reduced to allow reserve capacity for the rainy season. Downstream farmers are also assisted by this timing because the water flow downstream is increased during the dry season.
Engineers working on the dam design also needed to allow for ships to be able to pass the structure. The reservoir allows 10,000 ton freighters to enter China’s interior. Prior to construction, boats were limited to 1,500 tons. To accommodate this function, the Three Gorges Dam has two sets of locks, each with five cells. It takes ships three to four hours to cross the lock system. There is also a ship lift that is under construction and is scheduled to have a capacity of 3000 tons. The lift would reduce the crossing time to 40 minutes.
The final challenge for engineers involved in the project was the landscape itself. The dam structure needed to be able to withstand earthquake forces, up to 7.0 on the Richter scale, since the site lies over a fault line.