Evrim Yazgin
Cosmos science journalist
A study of the 28 most populous cities in the US shows that all are sinking. The most common cause is groundwater extraction. But the authors present possible solutions.
The fastest-sinking city is Houston, with more than 40% of it descending more than 5mm a year, and 12% sinking twice as quickly. Some localised spots are going down as much as 5cm per year.
Other fast-sinking zones include parts of Las Vegas, Washington DC, San Francisco and the area around New York’s LaGuardia Airport.
“As cities continue to grow, we will see more cities expand into subsiding regions,” says Leonard Ohenhen, a researcher at Columbia University in New York and lead author of a paper published on the research in Nature Cities. “Over time, this subsidence can produce stresses on infrastructure that will go past their safety limit.”
The 28 cities are home to 34 million people in total – 12% of the US population.
At least 20% of the urban area of every one of these cities is sinking by 2 to 10mm each year. At least 65% of the urban area is sinking in 25 of the cities. The sinking is not confined to coastal areas with many inland cities experiencing just as much, if not more, subsiding.
Differences in the sinking rate between parts of an urban area can cause stress on infrastructure and building foundations.
“A lot of small changes will build up over time, magnifying weak spots within urban systems, and heighten flood risks,” Ohenhen explains.
“The latent nature of this risk means that infrastructure can be silently compromised over time with damage only becoming evident when it is severe or potentially catastrophic,” says Manoochehr Shirzaei from Virginia Tech. “This risk is often exacerbated in rapidly expanding urban centres.”
Cities are at risk of sinking as the demand for freshwater increases and the extraction of water from underground aquifers is faster than they are being replenished.
Building weight and construction are also causing a downward pressure and adding to sinking, according to some studies.
Other cities around the world are at risk of sinking. A recent study showed that major New Zealand cities are suffering the combined consequences of sinking due to groundwater extraction and sea-level rise from global warming caused by the continued emission of greenhouse gases. As the global climate warms, ice sheets at the poles are melting causing a rise in sea levels.
About 10% of New Zealand’s urban area is sinking by more than 3mm a year.
Many other coastal communities around the world, including island nations, are already feeling these effects.
The new study also found that 8 of the US cities had seen 90 significant floods since 2000. It is possible that increasingly extreme flooding is a result of sinking land.
But the authors stress that solutions can be found if cities act quickly.
Flooding can be mitigated with land raising, drainage systems and creating areas to absorb floodwaters like artificial wetlands. Structures can also be strengthened and land motion integrated into building codes, as well as limiting new building in the areas of most threat.
“As opposed to just saying it’s a problem, we can respond, address, mitigate, adapt,” says Ohenhen. “We have to move to solutions.”
