May 30, 2024

This story was originally published by WIRED and is reproduced here as part of the Climate desk cooperation.

With its sprawling buildings and concrete, Mexico City might not look funny – but it is. Ever since the Spanish conquistadors drained Lake Texcoco to make way for more urbanization, the land has gradually compacted under the weight. This is a phenomenon known as subsidence, and the result is grim: Mexico City is sinking up to 20 inches per yearwreaking havoc on its infrastructure.

This includes the city’s subway system, the second largest in North America after New York City. Now satellites have allowed scientists to accurately measure the rate of sinking in Mexico City, and map where subsidence has the potential to damage railroads. “When you’re here in the city, you get used to buildings tilting a little bit,” said Darío Solano-Rojas, a remote sensing scientist at the National Autonomous University of Mexico. “You can feel the rails wobble. Riding the subway in Mexico City feels strange. You don’t know if it’s dangerous or not – you bird like it’s dangerous, but you don’t have that certainty.”

In a recent study in the journal Scientific Reports, Solano‐Rojas went looking for certainty. Using radar satellite data, he and his team measured how the elevation changed over the city between 2011 and 2020. Subsidence is not uniform; the rate depends on several factors. The most dramatic cases worldwide are due to the over-extraction of groundwater: Pump out enough liquid and the ground collapses like an empty water bottle. That’s why Jakarta, Indonesia, is sinking up to 10 inches per year. Over in California’s San Joaquin Valley, the land sank as much as 28 feet in the last century, due to farmers pumping out too much groundwater.

A similar draining of aquifers occurs in Mexico City, which is gripped by a worsening water crisis. “The subsoil is like a sponge: We get the water out, and then it deforms, because it loses volume,” says Solano‐Rojas. How much volume depends on the underlying sediment in a given part of the city – the ancient lake did not layer neatly equal proportions of clay and sand in every area. “It produces very different behaviors on the surface,” adds Solano-Rojas.

Subsidence rates across Mexico City vary significantly, from 20 inches per year to none at all, where the city is built on top of solid volcanic rock. This creates “differential subsidence,” where the land sinks differently, not just square mile to square mile, or block to block, but square foot to square foot. If a road, railway line or building sinks on a different side than the other, it will destabilize.

That’s how you get the tilted road traffic barriers at Acatitla Station, shown above. And below, the distortion of tracks at Oceanía station. If the ground in one of these places was sinking at a uniform rate, the tracks and road would also sink uniformly, and you might not have a problem. “We found that some of the segments of the subway system were moving faster” than they were designed for, Solano‐Rojas says. The study found that nearly half of elevated segments of the subway experience differential subsidence. This would imply that they would need to be serviced before the system’s typical threshold of 50 years, at which point a segment would require rehabilitation or repair to continue optimal operation.

Sistema de Transporte Colectivo, which operates the Mexico City subway, had no comment for this story after repeated inquiries.

A subway system is by nature a vast web of lines: Mexico City’s includes 140 miles from tracks that run underground in subways, above ground as you can see above, and on elevated platforms. “It goes from areas that are really stable to areas that are decreasing by 30 centimeters a year, or even close to 40 centimeters every year,” Solano-Rojas. “So the goal here was to see where the most damage could be.”

That damage comes in a few forms. As the land sinks, it can create gullies for rainwater to accumulate, causing flooding along railroad tracks. It could mess with the electrical system that powers the trains, says Solano‐Rojas.

And elevation changes can increase the grade of the rails. The subway’s trains are designed to operate at a maximum grade of 3.5 percent, Solano‐Rojas says, but some stretches of track are now twice that due to subsidence. “Trains can derail very easily if there’s a slight change in the alignment of the tracks,” said Manoochehr Shirzaei, an environmental security expert at Virginia Tech who studies subsidence but was not involved in the new paper. “Most of the infrastructure has certain thresholds; it tolerates a certain level of differential ground settlement. But often they don’t take into account the rate we see in Mexico City, for example.”

Solano‐Rojas and his colleagues found subsidence in the area of ​​an overpass near the Olivos station, which collapsed in 2021 while a Metro train was passing over it. “We did part of this analysis before 2021, and we detected that that area had differential displacements,” says Solano-Rojas. “We were like, ‘Oh, yeah, it looks like something could happen here in the future.’ We think it is no coincidence that we found it.” Solano‐Rojas was careful to say that the potential contribution of subsidence to the disaster would require further evaluation, and official investigations have construction errors cited and don’t mention subsidence.

For this study, the researchers looked at the subway infrastructure above ground, not the subway segments — basically the parts of the system they could visually verify. (The photo below shows the differential settlement of columns supporting a bridge.) But by providing the system’s operators with information about how quickly its infrastructure can degrade, their work can hopefully inform interventions. Engineers can add material under railway tracks, for example to restore lost height. However, strengthening subways can be much more challenging. “We don’t have a concrete solution for that,” says Shirzaei. “In most cases, when this happens, it just leads to closing the project and trying to open a new track.”

This is not just Mexico City’s problem. Earlier this year, Shirzaei and his colleagues found that the East Coast infrastructure is in serious trouble due to slower — yet steady — subsidence. They calculated that 29,000 square miles of the Atlantic coast are exposed to subsidence of up to 0.08 inches per year, affecting up to 14 million people and 6 million properties. About 1,400 square miles are sinking up to 0.20 inches per year.

Differential subsidence threatens not only railways, the researchers found, but all kinds of other critical infrastructure, such as embankments and airports. A metropolis like New York City has the added problem of sheer weight imprint on the ground, which alone leads to collapse. The Bay Area too, is sinking. On both coasts there is subsidence which greatly exacerbates the problem of sea level rise: The land goes down just as the water rises.

Wherever in the world this happens, people must stop over-extracting groundwater to slow subsidence. New-fangled systems are already relieving pressure on aquifers. It is getting cheaper and cheaper to recycle toilet water into drinking water, for example. And more cities are the deployment of “sponge” infrastructure — lots of green spaces that allow rainwater draw into the underlying aquifer, which essentially re-inflates the soil to stave off subsidence. Such efforts are becoming increasingly urgent as climate change exacerbates droughts in many parts of the world, including Mexico City, putting increasing pressure on groundwater supplies.

With increasing satellite data, cities can get a better handle on the subsidence they cannot immediately avoid. “I really feel that governments have a chance to use these kinds of studies to have a more structured plan of action,” says Solano-Rojas.

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