Apr 262011

An old joke relates how engineers initially rejoiced at successfully draining the former lake on which Mexico City was built (something the Aztecs had tried, but failed to achieve), only to discover that the city now lacked any reliable source of fresh water for its inhabitants (something the Aztecs had successfully managed by building a system of aqueducts).

Water has been a major issue for Mexico City since it was founded almost 700 years ago. Civil engineering works by the Aztecs included causeways and aqueducts connecting their island capital to the mainland as well as lengthy dikes separating the fresh water lakes from the brackish Lake Texcoco which surrounded the city.

The Spaniards did not maintain the Aztec civil works, deforested the surrounding hillsides, and started filling Lake Texcoco. This contributed to major flooding in 1555, 1580, 1604. The city was actually underwater (continuously!) from 1629–1634. During this period the Spaniards invested in several flood control efforts, but they were not successful. In 1788 they started construction of a massive canal to connect the basin to rivers north of the city flowing to the Gulf of Mexico. The open canal, which was up to 30 m (100 ft) deep in places, provided flood relief, but did not completely solve the problem, and flooding continued.

In the mid-1850s the government approved another massive flood relief scheme. Construction of the Gran Canal was delayed by numerous political and financial problems; it was not completed until 1900. The 58 km (36 mi) long canal included a 10 km tunnel, and carried lake water, storm water and sewage north to the Río Salado and eventually to the Gulf of Mexico. The scheme successfully drained most of the basin lakes, but summer flooding continued to be a problem for decades.

Mexico’s deep drainage system (drenaje profundo) was completed in the 1970s. It relies on a 68-km-long central tunnel (Emisor Central) which has a maximum depth of 250 meters below the surface. Built to allow for a flow of 170 cubic meters/second, subsurface subsidence under Mexico City had reduced its maximum capacity to barely 15 cubic meters/second by 2008.

Paradoxically for a city originally built on a lake and which experiences regular summer floods, Mexico City is desperately short of drinking water. The drilling of wells to obtain potable water from the aquifer under Mexico City is one of the main reasons for the ground subsidence which has reduced the effectiveness of the deep drainage system.

Many parts of Mexico City still experience serious drainage problems every rainy season. During the long dry season, many street drains plug with garbage (especially impermeable plastic bags). City motorists dread the first heavy rains of the year since much of the rainwater which falls is unable to find its way underground and backs up from blocked drains. City authorities have an annual campaign to try to clear all drains before the first rains, but are never completely successful.

Since 2007, jointly agreed programs to maintain and renovate the deep drainage system have been undertaken each year during the dry season by Mexico City and the administrations of adjoining states, in an effort to reduce the metropolitan area’s serious flood hazard. In the first four years (2007-2010), more than 42 km of tunnels have been renovated. The 2010-11 season of repairs to the drenaje profundo will be completed before the start of the rainy season (usually in late May or early June), according to city mayor Marcelo Ebrard.

Why are some parts of Mexico City sinking into the old lakebed?

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