The Broadsheet

That Sinking Feeling - Scientists Predict That Lower Manhattan Could Get a Lot Lower

A new report by a team of scientists based at U.S Geological Survey (USGS) and the Graduate School of Oceanography at the University of Rhode Island indicates that the ground of Lower Manhattan might sink (or “subside”) by as much as two feet before the end of this century.

A peer-reviewed study appearing the May issue of the scholarly journal “Earth’s Future,” published by American Geophysical Union (a professional organization of Earth, atmospheric, ocean, hydrologic, space, and planetary scientists) draws this inference from an analysis of the weight and density of buildings in Lower Manhattan, combined with the composition of the soil beneath the local ground. “We calculate a previously unquantified contribution to subsidence from the cumulative mass and downward pressure exerted by the built environment of the city,” the authors note.

In their report, “The Weight of New York City: Possible Contributions to Subsidence From Anthropogenic Sources,” authors Tom Parsons (a PhD research geophysicist at the USGS), along with Meng Wei, Steven D’Hondt, and Pei-Chin Wu (respectively, PhD holders/faculty members at the Graduate School of Oceanography, and a PhD candidate at the same institution) worked from a “sample point” near the center of the community, then analyzed the content of nearby subsurface material. They document that soil beneath Lower Manhattan falls almost entirely into two categories. At the shorelines of the Hudson and East Rivers, there is “artificial fill” dating from the Holocene (the current geological epoch, which began approximately 12,000 years ago). This is the residue of human efforts to reclaim land from the water, and expand Manhattan. In the interior, the soil is primarily “glacial lake deposits,” dating from the Pleistocene (the epoch that began roughly 2.6 million years ago, spanned the last ice age, and ended with the Holocene). These deposits contain “varved silt” made up mostly of sand and clay.

While there is solid bedrock underneath both of these zones, its depth in Lower Manhattan can range to as much as 150 feet. It is this soft layer, between street level and immobile substratum below that is prone to subsidence.

As the report notes, “Clay rich soils and artificial fill are especially prone to significant building settlement and clay models show the largest potential subsidence, ranging from −75 to −600 mm with a median of −294 mm at a sample point in lower Manhattan.” These values convert to a range of approximately three to 24 inches, with a media value of slightly more than 11 inches.

As the report also notes, “New York City faces accelerating inundation risk from sea level rise, subsidence, and increasing storm intensity from natural and anthropogenic causes.” While subsidence is a separate phenomenon from sea-level rise, the two may converge in Lower Manhattan, where extensive development at sea-level puts the community at risk of rising waters.

A separate report released in September by the Army Corps of Engineers notes that Lower Manhattan faces some of the highest risk of any area within the New York region, acknowledging that, “the average tidal range is greatest at the Battery, in southern Manhattan,” at 4.4 feet.” And these fluctuations are slated to become more intense, with Army Corps projections for the Battery ranging “from an increase of 0.7 feet for the low scenario, increase of 1.8 feet for the intermediate, and up to 5 feet for the high scenario through 2100.”

A further 2022 report, “Global and Regional Sea Level Rise Scenarios for the United States,” from the National Oceanic and Atmospheric Administration, anticipates that local sea levels will rise 19 inches between 2005 and 2060, compared to a nationwide average of ten to 12 inches. The same study predicts that the local occurrence of “extreme water level events” (defined as water cresting five feet, six inches above the highest ordinary tide) will happen four to five times a year during that interval.

If the worst-case scenario outlined for ground-level subsidence in “The Weight of New York City” coincides with similarly dire predictions for sea-level rise, this could drastically limit the benefit envisioned for various resiliency measures now planned for Lower Manhattan, since the elevation of flood walls would be effectively reduced even as water levels surged.

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