(Jun 20, 2007)

Mexico drainage tunnel could fail, causing floods

By Mark Stevenson
From AP

MEXICO CITY (AP) - There is a ''high possibility'' a huge underground drainage tunnel could soon fail, flooding parts of this mountain-ringed metropolis 5 meters (yards) deep in sewage, the national water agency said Tuesday.

Officials have been puzzled for years by the gradual decrease in capacity of the 6.5-meter-wide (7-yard-wide) tunnel built in the 1970s to drain waste water from the valley, which is home to 20 million people and has no natural outlet. They have speculated that the tunnel may be partially clogged or that its walls could be decaying.

But because it is constantly filled with water, officials have not been able to travel through the structure to inspect it - or perform much-needed maintenance.

''Because of a lack of maintenance in Mexico City's deep drain over the last 15 years, there is a high possibility that it could fail,'' according to a National Water Commission statement.

''A failure ... could cause severe floods reaching five meters in the city's historic center, the international airport'' and other boroughs on the city's east side, the statement said.

Poor drainage and flooding has been a historical problem for the city, especially during the rainy season that runs typically from late May through October.

Experts say a number of solutions could be used to solve the drainage problem, including unblocking the tunnel, building another branch of deep drainage, or pumping water out through a previously dug drainage ditch. But extremely heavy rainfall could temporarily overwhelm such systems.

The Mexico Valley, where the city is located, was largely covered by lakes when the Aztecs founded the city on an island in 1325. The Aztecs built dikes to try to keep out flood waters. The Spaniards who conquered Mexico in 1521 tried to drain the lakes, which have disappeared under the urban sprawl.

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