Sections of the US Atlantic coastline from North Carolina to Massachusetts are sinking into the ground at an alarming pace, exacerbating flood risks for coastal communities according to a new study. While rising seas driven by climate change remain a serious concern, researchers have found that land subsidence caused by natural processes and human activities is occurring even faster in some areas. Major cities like New York and Baltimore as well as Naval bases in Virginia are especially vulnerable.
Accelerating Subsidence
The study published this week in Proceedings of the National Academy of Sciences combined GPS data with tidal records going back decades to provide one of the most comprehensive analyses yet of vertical land motion along the East Coast. The results showed that sections of the coast from Cape Hatteras to north of Boston are sinking at rates up to 16 millimeters per year – more than 5 times faster than sea levels are rising in the region.
While Miami and areas along the Gulf Coast have received more attention for increased flooding driven by sea level rise, the analysis indicates over 50,000 square kilometers of land and coastal infrastructure along the Atlantic is also at growing risk.
Report co-author Tal Ezer, professor of ocean, earth and atmospheric sciences at Old Dominion University said, “Ongoing subsidence has had a huge impact that really changed the hot spots in terms of making flooding more severe and causing infrastructure damage along the East Coast.”
Threats to Infrastructure
Critical infrastructure in major metropolitan areas like New York City and Washington D.C. as well as Naval installations in Virginia’s Hampton Roads region sit on coastal land that is sinking.
According to Benjamin Hamlington, a research scientist at NASA’s Jet Propulsion Laboratory:
“We can think of subsidence as an added factor that’s increasing the risk for coastal infrastructure but which hasn’t been fully taken into account in the past. This demonstrates the importance of using precise measurements and comprehensive monitoring to correctly estimate flood potential and develop mitigation strategies.”
The Hampton Roads area, which houses the largest Naval base in the world, is experiencing land subsidence of up to 10.5 millimeters per year according to estimates. Regional studies indicate relative sea level rise could be as much as 7.31 feet over the next 80 years.
The threat is already being felt – Norfolk Naval base reports 14.5 additional days per year of coastal high tide flooding compared to 50 years ago.
According to Wired, museums, subways stations and other infrastructure in New York City is also increasingly vulnerable:
New York City is sinking by about 1.5 inches per decade. As a result, La Guardia Airport’s runways flood during extreme high tides and rainstorms. The Metropolitan Transportation Authority has resorted to elevating fan plants and vent shafts, as well as sealing subway entrances.
Key Contributors to Land Subsidence
While glacial isostatic adjustment has caused portions New England to gradually sink since the end of the last ice age, human activity is exacerbating the problem in coastal areas built on unconsolidated fill.
Groundwater extraction, underground excavations, and load stress from heavy built environments concentrate settlement that would normally occur over centuries into several decades.
Jonathan Overeem, a co-author on the recent study said accurate, regularly updated mean sea level trends and vertical land motion measurements are crucial for local governments planning climate change mitigation. However, while the technology exists, many municipalities have limited access to the necessary data.
Outlook Going Forward
Researchers say the combined effect of rising seas and sinking land doubles flood exposure in sections of the US Atlantic coast. Moving forward, coastal communities will need to factor land subsidence into their climate resilience plans.
Strategies such as tidal gates, seawalls and pumping systems have already been implemented in some areas but carrying out these massive infrastructure projects across vulnerable cities like New York and Hampton Roads poses major financial and engineering challenges. Ultimately the prognosis for these regions depends on how swiftly measures can be enacted before impacts become unmanageable.
More extensive monitoring networks paired with advanced high-resolution models that assimilate updated measurements of vertical land motion and sea level rise trends will be essential tools for local decision makers. Accurately quantifying subsidence rates down to the neighborhood level allows for targeting of at-risk infrastructure. It also enables cost-benefit analysis of various adaptation options versus managed retreat from the most hazardous zones.
With billions in coastal assets at stake, researchers say getting a better handle on subsidence trends needs to be an urgent priority up and down the Atlantic seaboard.
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