That sinking feeling: SFO is subsiding in the mud

Built on mud, runways are sinking nearly a half inch every year, scientists found.

Ladies and gentlemen, San Francisco Airport has begun its descent.

Every year, the airport is sinking an average of nearly 10 millimeters, almost a half an inch, according to new satellite data.

There’s no need to fasten seat belts or secure carry-on items. The measurement is so tiny that it almost sounds amusing. But there’s concern that the subsidence, which is experienced unevenly around the airport, could crack runways.

“Differential sinking along a runway increases the risk of the runway getting damaged, increases the cost of maintenance, and increases the risk to users,” said Oluwaseyi Dasho, an environmental hazard specialist at Virginia Tech who presented the new study this week at the American Geophysical Union meeting in San Francisco.

“Even though it’s just a minute portion of the runway, if it causes a crack, that would render the whole runway unusable,” he said.

Built on mud that compacts over time, the airport could sink a foot in 30 years. The analysis found that other coastal airports also are sinking, such as Ronald Reagan National Airport in Washington D.C. and Tampa International Airport in Florida.

By comparison, Los Angeles International Airport is sinking much more slowly — 1.7 millimeters, or around 0.07 inches, per year. The Oakland and San Jose airports were not part of the study.

The new finding is consistent with 2019 research by researchers at UC Berkeley and Arizona State University that found that much of the San Francisco Bay’s shoreline is sinking about 2 millimeters a year, roughly the thickness of a nickel.

That study also discovered that Treasure Island and Foster City are sinking at a much faster rate, about the same as SFO..

The sinking land at the airport and other low-lying areas is due to the compaction of man-made landfill and soft bay mud on which they are built, according to Roland Burgmann, a UC Berkeley professor of earth and planetary science and co-author of the 2019 study.

Meanwhile, the sea level around the Bay is rising at about one-tenth of an inch per year due to global warming. There’s concern that the subsidence, coupled with rising water, could make the airport more prone to natural disasters.

In response, SFO is planning a protection system — comprised of a barrier consisting of steel piles and steel sheet walls topped with concrete caps — installed along the airport’s shoreline to keep the water at bay. It would help fend off up to 42 inches of future sea level rise, in addition to the existing 100-year flood event protection that is required by the federal government. Construction could start in 2025.

The elevation loss measured by the Virginia Tech researchers was detected using radars on European Space Agency Sentinel-1 satellites, along with advanced data processing techniques.They measured 16 coastal airports over seven years, from 2015 to 2022.

Those same satellites have previously revealed the sinking of San Francisco’s luxury skyscraper Millennium Tower, also known as the Leaning Tower of San Francisco. Additionally, large portions of San Francisco’s Mission Bay are also showing signs of subsidence.

The Virginia Tech team recommended continuous monitoring, so problems can be quickly fixed.

They also urged a closer look.

“Some engineering solutions can be deployed,” said Virginia Tech geophysicist Manoochehr Shirzaei. “But we need to know where to start. More data will help prioritize the sections of the runways that need attention.”