All of the biggest U.S. cities are sinking

(Press-News.org) A new study of the 28 most populous U.S. cities finds that all are sinking to one degree or another. The cities include not just those on the coasts, where relative sea level is a concern, but many in the interior. Furthermore, using newly granular data, the study finds that some cities are sinking at different rates in different spots, or sinking in some places and rising in others, potentially introducing stresses that could affect buildings and other infrastructure. Massive ongoing groundwater extraction is the most common cause of these land movements, say the authors, though other forces are at work in some places.

The study was published this week in the journal Nature Cities.

“As cities continue to grow, we will see more cities expand into subsiding regions,” said lead author Leonard Ohenhen, a postdoctoral researcher at the Columbia Climate School’s Lamont-Doherty Earth Observatory. “Over time, this subsidence can produce stresses on infrastructure that will go past their safety limit.” 

Rapidly subsiding coastal metropolises such as Jakarta, Venice and New Orleans have already drawn major attention, and multiple recent studies have shown that many places along the U.S. East Coast and elsewhere are subsiding. But most studies have relied on relatively sparse data spread over wide areas to paint a broad picture.

Looking at all U.S. cities with populations exceeding 600,000, the new study uses recent satellite data to map out vertical land movements down to the millimeter in grids measuring just 28 meters (about 90 feet) square. The authors found that in 25 of the 28 cities, two-thirds or more of their area is sinking. Overall, about 34 million people live in affected areas.

The fastest-sinking city is Houston, with more than 40% of its area subsiding more than 5 millimeters (about 1/5 inch) per year, and 12% sinking at twice that rate. Some localized spots are going down as much as 5 centimeters (2 inches) per year. Two other Texas cities, Fort Worth and Dallas, are not far behind. Some localized fast-sinking zones in other places include areas around New York’s LaGuardia Airport, and parts of Las Vegas, Washington, D.C. and San Francisco.

In addition to measuring surface-elevation changes, the researchers analyzed county-level groundwater withdrawals for the affected areas. Correlating this with land movements, they determined that groundwater removal for human use was the cause for 80% of overall sinkage. Generally, this happens as water is withdrawn from aquifers made up of fine-grained sediments; unless the aquifer is replenished, the pore spaces formerly occupied by water can eventually collapse, leading to compaction below, and sinkage at the surface. In Texas, the problem is exacerbated by pumping of oil and gas, the paper says.

The researchers say that continued population growth and water usage combined with climate-induced droughts in some areas will likely worsen subsidence in the future.

In some areas, natural forces are at work. In particular, the weight of the towering ice sheet that occupied much of interior North America until about 20,000 years ago made the land along its edges bulge upward, somewhat like when one squeezes air from one part of a balloon to another. Even today, with the ice long gone, some of these bulges are still subsiding at rates of 1 to 3 millimeters each year. Affected cities include New York, Indianapolis, Nashville, Philadelphia, Denver, Chicago and Portland.

Even the sheer weight of buildings may be taking a toll. One 2023 study found that New York’s more than 1 million buildings are pressing down on the Earth so hard that they may be contributing to the city’s ongoing subsidence. A more recent separate study found that some buildings in the Miami area are sinking in part due to disruptions in the subsurface caused by construction of newer buildings nearby.

The new study found that eight cities (New York, Chicago, Los Angeles, Phoenix, Houston, Philadelphia, San Antonio and Dallas) account for more than 60% of the people living on sinking land. Notably, these eight cities have seen more than 90 significant floods since 2000, probably driven in part by lowering topography.

Another key finding: some cities are seeing differential motion, with adjoining localities sinking at different rates, or even sinking while other areas rise; the upward motion possibly caused by quick recharge of aquifers near rivers or other water sources. (Uplift in certain areas is actually more than compensating for overall sinkage in three cities: Jacksonville, Fla.; Memphis, Tenn. and San Jose, Calif.)

Differential motion is a problem because, as the authors point out, if a whole urban area is moving up or down evenly at the same rate, that minimizes the danger of stresses to building foundations and other infrastructure. But if structures are subjected to an array of uneven vertical movements, they can experience dangerous tilting.

“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 authors write.

The study found that only about 1% of the total land area in the 28 cities lies within zones where differential motion could affect buildings, roads, rail lines and other structures. However, these areas tend to be in the densest urban cores, and currently contain some 29,000 buildings. The most hazardous cities in this regard are San Antonio, where the researchers say 1 in 45 buildings are subject to high risk; Austin (1 in 71); Fort Worth (1 in 143) and Memphis (1 in 167).

The upshot for individual structures in these areas is unclear; it probably would require an even finer-grained study, said Ohenhen. An earlier study of 225 U.S. building collapses between 1989 and 2000 found that only 2% were directly attributable to subsidence. However, the factors behind 30% were designated unknown, suggesting that subsidence could have played a larger role, says the new study.

The paper concludes that cities should use this new information to focus on solutions. They say that in many places, flooding can be mitigated with land raising, enhanced drainage systems and green infrastructure such as artificial wetlands to absorb floodwaters. Cities susceptible to tilting hazards can focus on retrofitting existing structures, integrating land motions into building codes, and limiting new building in the areas of most threat.

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

The study was coauthored by researchers from Virginia Tech, the Netherlands Organisation for Applied Scientific Research, University of California Berkeley, Texas A&M University, University of Colorado Boulder, Brown University and United Nations University.

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Lead author contact: Leonard Ohenhen olo2103@columbia.edu

More information:  press@climate.columbia.edu


 

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