New York City faces accelerating inundation risk from sea-level rise, subsidence, and increasing storm intensity from natural and anthropogenic causes. A recently published study led by USGS examines the previously unquantified contribution to subsidence from the cumulative mass and downward pressure exerted by the built environment of the city.
The study, The Weight of New York City: Possible Contributions to Subsidence From Anthropogenic Sources, published this month by Earth’s Future, examines a previously unquantified contribution to subsidence from the cumulative mass and downward pressure exerted by the built environment of the city.
The authors show that the combination of construction densification and sea-level rise will likely lead to increasing inundation hazard to the city’s more than 8 million residents.
To quantify the hazard, the authors introduce a subsidence mapping concept, adding specificity to soil types and conditions across the city. They present satellite data that show that the city is sinking 1–2 millimeters a year, with some areas subsiding much faster.
For instance, areas in Lower and Midtown Manhattan and parts of Brooklyn and Queens show some of the greatest calculated subsidence, caused by combined pressure on silty clay and fill soil from dense high-rise construction. These soil types typically exhibit what is known as “long-term secondary settlement under load” that can continue indefinitely, meaning that some aspects of the study’s subsidence maps may predict future sinking. The authors note that mitigation strategies may need to be included with the construction of additional high-rise buildings, to offset their contribution to future flood risk.
“The weight of these cities and their occupants contributes to coastal subsidence, which in some areas is accelerating due to other factors,” said USGS Geophysicist Parsons, lead author of the study. “Add to that rising sea levels due to global warming, and the weight of cities becomes a significant concern to those living in coastal areas.”
This study is related to Parson’s previous published work concerning the weight of San Francisco and its 7.75 million residents. In that study, Parsons found that the city’s weight puts considerable pressure on the ground and active faults below, contributing not only to coastal subsidence but potentially influencing tectonic activity as well.