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Shanghai Pudong 上海浦东

Yangpu Bridge

The Yangpu Bridge can be seen to the north crossing the Huangpu River in Shanghai. The coordinates of the bridge are 31°15'25"N   121°32'27"E on Wiki maps.

Pudong is surrounded by water. To the North, the Huangpu River will link with the Yangtze and to the east, the Yangtze will empty into the East China Sea. The average elevation of the area is about 13 feet (4 meters).

In July of 2003 a section of a tunnel under the Huangpu collapsed while under construction. One high-rise collapsed, two others tilted, and part of the sea wall that holds back the Huangpu also collapsed. In June of 2004, three stations on the new elevated train had to be closed for repairs because sinkage had misaligned their tracks.

Since 1920 Shanghai proper has sunk over 6 feet (2m). In the 1960s the sinkage had increased to such a rate that people noticed. Limits were placed on ground water usage at that time. If they hadn't, geologists predicted that Shanghai would have already been below sea level today. As it is, the sinkage has slowed, but not stopped. There were a flurry of news stories in 2003 sparked by a government report.

Here are the basics. Shanghai continues to sink. It's called subsidence in polite circles. The primary cause of the subsiding is the combined use of ground water for drinking water, industrial cooling and irrigation for farming. Strong limits were placed on ground water usage in the 1960s which limited it, but not to the levels planned by the government. People continued to dig wells but the Shanghai government has recently put even stronger controls on the practice. It did slow the rate. As of 2003 Shanghai was sinking, on average, 1.5 centimeters a year.

The secondary cause of sinking is the weight of skyscrapers which accounts for as much as 40% of the problem. In areas where there are high concentrations of high rises, such as the Jin Mao Tower, the rate doubles to 3 cm per year. Over the past 10 years, over 3000 buildings over 24 floors have been built and another 3000 are planned. On February 13, 2003 design plans were changed for the Shanghai World Financial Center to increase the height from 460m to 492m. This will make it the world's tallest, at least for the moment. It is due to be completed in 2008 and with 101 floors it will be 233 feet (71m) taller than the Jin Mao Tower. It will be quite near the Jin Mao Tower.

Shanghai isn't the only sinking city. Mexico City, San Francisco, Osaka, and of course New Orleans and Venice are sinking at rates that require intervention. Venice is perhaps the most famous, or was until Katrina. Both New Orleans and Venice, like Shanghai, were built on river delta land. That creates a soft spongy soil given to swampy marshes. When you take the water out, the soil compresses and the swamps dry up but the land level decreases. If you leave the water in and compress the soil with a ton of cement the water acts according the laws of physics and is sqeezed out.

In one news story an expert was quoted as saying that builders were aware of the problem and were using lighter materials such as steel to build their skyscrapers. So, if you use a half of a ton of cement and place it on the soil, the laws of physics still hold true. If you have 3,000 half ton blocks you are still going to have an effect on the substrate. In fact, world wide subsidence of coastal areas is a minor but real contributor to increased sea levels.

A third cause of sinking cities applies not only to Shanghai, but to all coastal areas. In February 2007, the Intergovernmental Panel on Climate Change of the United Nations estimated that ocean levels will increase 7 to 23 inches by 2100. The cities will be submerged rather than sunk, but the result is the same.

The coastal area surrounding the Yangtze River is already stressed by increased sea levels. Storm surge at high tide pushes salt water up the Yangtze with resulting damage to the ecology of the area. The low-lying basin surrounding Taihu Lake to the south-west is already at or below sea level. During storms, draining that area becomes impossible. In cities, such as Shanghai, storm drains and sewer lines back up. With increased sea levels, drainage would require pumping stations. While levees can protect an area from incursion from the sea and rivers, they also keep water in. In New Orleans, pumping stations are used to evacuate a small area when compared to the Taihu Lake basin which is roughly the size of southern Louisiana.

Storm surges due to typhoons increased dramatically after 1960 and caused repeated flooding in Shanghai. In response, Shanghai completely changed the face of the Bund by building a 30 foot levee or dyke along the Huangpu River. This is the most visible, but there are levees all along the coastal area and more are planned. Each time the sea level rises, the levees have to be raised.

Levees and dykes are also used to control coastal erosion. The eastern shore of Pudong juts into the North China Sea and is the target of currents. It is one of the high risk areas along the coast for erosion. Geologists and ecologists are just beginning to be able to measure the impact of protecting one area from erosion will have on erosion in adjacent areas. When you prevent water from spilling over in one place, it moves to another and that area suffers cumulative effects.

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Last update: February 2007
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