28 US cities are sinking – but one is going down at an alarming rate

An alarming new report reveals that right across the country, 28 US cities are sinking, showing the kind of geologic subsidence that could impact buildings and infrastructure sooner rather than later. While some of it is natural, 80% is the the direct result of human activity.

A large team of researchers including geoscientists from Virginia Tech and Columbia University used satellite-based radar measurements to assess areas of shifting land, and found that from coast to coast, there was significant movement in both large and small urban hubs. And in 25 of the 28 cities, at least 65% of the urban area is sinking.

"Even slight downward shifts in land can significantly compromise the structural integrity of buildings, roads, bridges, and railways over time," said Leonard Ohenhen, former Virginia Tech graduate student and the study’s lead author.

One place, however, is outpacing the others – Houston, Texas. The large southern city is in a bad way overall, but the study revealed that 42% of land area is currently sinking by more than 5 mm (0.2 inches) each year, with 12% subsiding by more than 10 mm (0.4 in) a year. That may not seem like a whole lot if you're only looking at 12 months of data, but over a decade, 10 mm of subsidence poses a significant structural threat to the built environment, particularly if that sinking is uneven.

“Unlike flood-related subsidence hazards, where risks manifest only when high rates of subsidence lower the land elevation below a critical threshold, subsidence-induced infrastructure damage can occur even with minor changes in land motion,” the researchers noted in the study.

Texas had the three most rapidly sinking cities of the 28, with Fort Worth and Dallas behind Houston. It's no real surprise, given the state's climate, growth, intensive groundwater use for the population as well as agriculture and industry, and oil and gas extraction.

The three cities sit on clay-rich soil, which is prone to naturally expanding when wet and shrinking quickly when dry, which makes the land beneath these urban places particularly susceptible to subsidence. Adding to this, pumping groundwater lowers underground water pressure, which sees clay soils contract and compact – which, as you can probably guess, is not good news for what's on the surface. What's more, the weight of infrastructure and drought conditions further impact these clay-rich soils.

While Houston is still sinking, it's nothing compared to past patterns where unregulated groundwater pumping saw, at its peak, the city subside nearly 5 cm (2 in) each year. This led to an overhaul of the system to mitigate the damage.

Other cities high on the list of 28 include Chicago, New York, Seattle, Denver and Detroit. Overall, the urban areas house around 34 million people. And scientists are concerned that because it's not as immediately impactful as a natural disaster like a hurricane or flood, it's easier (and cheaper) to ignore it.

"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," said Manoochehr Shirzaei. "This risk is often exacerbated in rapidly expanding urban centers."

Other areas of concern for researchers included around New York’s LaGuardia Airport, where the land was sinking more rapidly than elsewhere in the state, and sections of Las Vegas, Washington, DC. and San Francisco.

And it's not just infrastructure to worry about; any loss of ground leaves area more vulnerable to flooding – an extreme weather event that is only expected to be more prevalent as the planet heats. New York, Chicago, LA, Phoenix, Houston, Philadelphia, San Antonio and Dallas have, collectively, experienced more than 90 flooding events since 2000.

Last month, another study delved into the geological processes behind North America's subsiding problem, which is worth a read if you're not yet catastrophe-ed out.

However, the US is certainly not alone, with China facing the same issue, as well as Venice – which has so far lost around 23 cm (9 in) – and Mexico City, where some areas are shockingly sinking at a rate of around 50 cm (1.6 ft) each year.

But as Houston's mitigation measures show, intervention is key. The scientists of the latest study point out that measures such as land raising, better drainage and even constructing artificial wetlands can lower floodwater impact.

“As opposed to just saying it’s a problem, we can respond, address, mitigate, adapt,” said Ohenhen. “We have to move to solutions.”

The researchers are the latest group of scientists urging law and policymakers to act now rather than wait for the consequences down the line.

"Targeted mitigation through strategic dam planning, managed aquifer recharge, and resource extraction policies may be useful to control, pause, or even reverse subsidence," they wrote. "Ultimately, a robust and sustainable mitigative and adaptive framework should encompass continuous monitoring, stakeholder collaboration, and flexible management plans."

The whole 28 cities, in order from most rapid ground changes, are:

1. Houston, TX
2. Fort Worth, TX
3. Dallas, TX
4. Chicago, IL
5. New York, NY
6. Denver, CO
7. Seattle, WA
8. Columbus, OH
9. Indianapolis, IN
10. Charlotte, NC
11. Detroit, MI
12. San Antonio, TX
13. Las Vegas, NV
14. Washington, DC
15. Nashville, TN
16. San Francisco, CA
17. Portland, OR
18. San Diego, CA
19. Philadelphia, PA
20. Austin, TX
21. Oklahoma City, OK
22. Phoenix, AZ
23. El Paso, TX
24. Boston, MA
25. Los Angeles, CA
26. Memphis, TN
27. San Jose, CA
28. Jacksonville, FL

The study was published in the journal Nature Cities.

Source: Virginia Tech