Twenty-eight major cities in the United States, including New York, Los Angeles and Washington, D.C. are seeing their urban areas sink by some 2–10 millimeters per year, with Houston being the fastest-sinking city, new research has shown.
Massive ongoing groundwater extraction was found to be the most common cause of the sinking, which may push infrastructure “past their safety limit,” researchers warn in a study published in the journal Nature Cities.
The study, by researchers from Virginia Tech and several other U.S. universities, used satellite data to map out vertical land movements in 28 of the most populous cities, those where populations exceed 600,000 people.
The researchers found that in 25 of the 28 cities studied, two-thirds or more of the urban area is sinking. In every city studied, at least 20 percent of the area is sinking, while in 25 cities, at least 65 percent is sinking. Around 34 million people live in these affected areas.
According to the study, eight cities—Chicago, Dallas, Houston, Los Angeles, New York, Philadelphia, Phoenix and San Antonio—account for more than 60 percent of those people living on subsiding land. These eight cities have had more than 90 significant floods since 2000, likely driven in part by the lowering topography, the researchers noted.
“As cities continue to grow, we will see more cities expand into subsiding regions,” said the study’s lead author, Leonard Ohenhen, a researcher at the Columbia Climate School in New York City, in a statement.
“Over time, this subsidence [sinking] can produce stresses on infrastructure that will go past their safety limit.”
The structural integrity of buildings, roads, bridges and dams can be profoundly affected by even small movements, he said.
“Even slight downward shifts in land can significantly compromise the structural integrity of buildings, roads, bridges, and railways over time,” said Ohenhen.
New York, Chicago, Seattle, Denver and five other cities were found to be sinking about two millimeters (around 0.08 inch) per year.
Several cities in Texas had some of the highest measured rates of subsidence, sinking at about five millimeters (around a fifth of an inch) per year and as much as 10 millimeters (just under half an inch) per year in some parts of Houston, the fastest-sinking city in the study.
Over 40 percent of Houston’s urban area is sinking more than five millimeters per year, while 12 percent of the area is sinking at twice that rate.
Some localized areas are sinking as much as five centimeters (about two inches) per year, with two other cities in Texas, Fort Worth and Dallas, being not far behind. Other fast-sinking localized zones include areas around New York City’s LaGuardia Airport, as well as parts of Las Vegas, Washington, D.C. and San Francisco.
The study also found that the local areas in some cities are sinking at different rates, with some parts even sinking while other areas rising. Uplift in certain local areas was found to be more than compensating for the overall sinkage in three cities, which include Jacksonville in Florida, Memphis in Tennessee and San Jose in California.
These uneven vertical movements can lead to dangerous tilting, whereas if an entire urban area is shifting either up or down at the same rate, the risk of the stresses put on infrastructure is minimized, the researchers noted.
According to the study, only around one percent of the total land area in the 28 cities lies within zones with differing rates of motion that could impact buildings and other infrastructure. However, these zones are typically the densest urban areas and currently have around 29,000 buildings.
The most hazardous cities in this regard were found to be San Antonio, where the researchers say one in 45 buildings are subject to high risk, as well as Austin (one in 71), Fort Worth (one in 143) and Memphis (one in 167).
The study also found that groundwater removal for human use was the cause for 80 percent of the overall sinkage. After the water is extracted, the pore spaces formerly occupied by water can eventually collapse and lead to sinkage at the surface. The study points out that in Texas, the issue is made worse by the pumping of oil and gas.
“A lot of small changes will build up over time, magnifying weak spots within urban systems and heighten flood risks,” Ohenhen warned.
Manoochehr Shirzaei, an associate professor at Virginia Tech who worked with Ohenhen, said in a statement: “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,” noting that “this risk is often exacerbated in rapidly expanding urban centers.”
The researchers say that continued population growth and water usage coupled with climate-induced droughts in some parts will likely exacerbate sinkage in the future. The study recommends implementing mitigation measures, such as groundwater management and enhanced infrastructure resilience planning to account for varying sinking rates, to prevent worsening infrastructure risks.
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