The 28 largest cities in the U.S. are slowly sinking, according to a new study. That could increase the likelihood of flooding and impact the structural integrity of buildings, roads, bridges and dams in areas where millions of people live.
The study’s authors say Canadian cities could also be at risk due to sinking land, also known as subsidence.
“Cities experiencing rapid subsidence often face uneven ground settlement, which leads to cracked roads, tilted buildings, broken pipelines, and damaged foundations,” lead author Leonard Ohenhen told CTVNews.ca by email. “Importantly for coastal cities, when combined with sea-level rise, subsidence can greatly accelerate/increase relative sea-level rise, overwhelming flood defenses and drainage networks.”
Published Monday in the journal Nature Cities, the study found that urban areas in cities like New York, Houston and Seattle were sinking between two to 10 millimeters per year, largely due to groundwater extraction. Other factors included oil and gas extraction, tectonic activity, and natural sediment compaction in coastal areas.
“The weight of buildings and urban infrastructure can indeed exacerbate subsidence, particularly in cities built on compressible clay or peat soils,” Ohenhen added.
Ohenhen is a researcher at Columbia University’s climate school. He co-authored the study with Manoochehr Shirzaei, an associate professor of geophysics and remote sensing at Virginia Tech.
Shirzaei says similar risks exist in Canada but are less widespread. Toronto and Montreal, for example, lie in areas affected by what’s known as glacial isostatic adjustment, which is causing land to slowly rise in reaction to glaciers that melted long ago. However, this could still lead to structural issues.
“Vancouver, located on a tectonically active margin, may face subsidence from seismic activity, soil compaction, or sea-level rise,” Shirzaei told CTVNews.ca by email. “Groundwater pumping in urban or agricultural areas of Canada, especially southern Ontario and parts of British Columbia, could also cause localized subsidence if not well managed.”
‘Serious risks to urban areas’
The study used satellite-based radar measurements from 2015 to 2021 to map subsidence rates in 28 of the most populous cities in the U.S. In 25 of the cities, at least 65 per cent of the urban area was sinking; in the other three cities, at least 20 per cent of land experienced subsidence.
The most widespread sinking was observed in Texas cities like Dallas and Houston. High-subsidence variability was also noted in New York, Las Vegas, and Washington, D.C.
Lower-ground elevations can increase the risk of flooding, but over time, subsidence can also crack and destabilize buildings, foundations and infrastructure. Because of how slowly the land is sinking, the study’s authors caution the effects can be easy to miss until major damage has occurred.
“We are already witnessing such impacts in cities like Jakarta, Indonesia; Norfolk (Va.), and in other regions where chronic flooding and infrastructure failures have become major socioeconomic challenges,” Ohenhen said. “In Mexico City infrastructure hazard is a major issue in some areas.”
Ohenhen and Shirzaei say cities can mitigate the risks by using monitoring tools and better managing groundwater resources. Zoning regulations could also be updated to restrict development in high-subsidence areas, which should also be targeted with drainage system improvements.
“Subsidence poses serious risks to urban areas,” Ohenhen said. “We must be proactive not reactive in our response.”
