“If you’re an average citizen in a coastal city, I think it’s important that you understand what the vertical land motion component does and how it can change the susceptibility to flooding, even from one neighborhood to the next,” said Brett Buzzanga, lead author of the study and a coastal scientist at NASA’s Jet Propulsion Laboratory.
Comparatively, millimeters of subsidence in New York City might not sound like much, but it can exacerbate flooding with rising sea levels. Over the past 20 years, sea levels have increased by 4.4 millimeters per year in Manhattan, some of that due to sinking land, the study stated. That can have a grave effect when extreme weather strikes: One study estimated that about $8 billion of damage from Superstorm Sandy in 2012 could be linked to sea level rise.
“There’s more damage that can be done with higher sea levels and lower land together,” said Buzzanga. “It’s kind of adding to the background state on which these storms act. There’s more water to slosh around.”
Buzzanga and his colleagues employed a new technique using satellite data to obtain high-resolution measurements and compare neighborhoods. The study improves on GPS data from ground instruments or airplanes that can also collect high-resolution data, but point by point.
“We highlight with really unprecedented detail the differences throughout the region and that some spots can be sinking faster than others and that can cause more flooding in some spots,” said Buzzanga. “This is going to be the same in any city.”
New York is one of several coastal cities seeing noticeable land subsidence. Other research showed Norfolk and Virginia Beach, for example, are sinking at more than 3.5 millimeters per year, on average. Some areas around New Orleans have experienced 40 millimeters of sinking per year.
Land can sink for several reasons. In New York City, much of it can be linked to the retreating of glaciers from the last Ice Age, when ice sheets weighed down the land and caused the crust to stretch and sag beneath areas such as the northeastern United States. New York City, however, was on the periphery of the sagging area and bulged upward. As the ice sheets began retreating, the land began slowly leveling out, and drooping areas began rising while bulging areas began sinking.
Think of the change like pressing your finger (acting as the ice sheet) down on a balloon and then lifting it (ice sheet retreat).
But human activities can add extra stress to our land, causing it to sink or rise beyond these deep natural processes. The study found the sinking hotspots at LaGuardia, Arthur Ashe Stadium and along select roads were landfill sites in the past. Buzzanga said those sites are “going to compress much faster than … something built on more solid ground would.”
Arthur Ashe stadium was even renovated with a special lightweight roof to reduce subsidence.
The study also found some unexpected areas where land was rising. For instance, in Brooklyn, East Williamsburg’s Newton Creek showed about 2 millimeters of uplift per year. The study notes that the area has a massive engineering project recovering and treating polluted groundwater from the creek’s aquifer, which could cause land to rise. But rising land can also destabilize infrastructure.
Tom Parsons, a geophysicist at the U.S. Geological Survey who was not involved in the study, said he was impressed by the resolution of the subsidence and uplift data the authors produced.
Their conclusions also aligned with what Parsons and his colleagues previously found — infrastructure built on top of artificial fills at or near the coast appear to sink at higher rates.
This study confirms the utility of satellite data “in helping major cities manage multiple issues such as where large buildings are built, the use of artificial fill, and where protective measures are needed against future inundation,” Parsons said.
Steven D’Hondt, a professor of oceanography at the University of Rhode Island who was not involved in the new research, added that the settling of places built on landfill may be exacerbated by the weight of buildings and runways. But the sinking caused by this added infrastructure weight usually stops within a few years following the construction, so it’s not a primary contributor to land sinking.
Buzzanga said he and his colleagues plan to use their newly developed algorithm to make such vertical displacement data available across North America, which could help inform a community’s flooding risk and city planning.
“Every city in the world would benefit from such a refined analysis of vertical land motion throughout the city,” said D’Hondt.
