Science Seminar Series

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Tuesday, October 19, 2021, 1:30pm Pacific/4:30pm Eastern

Tom Parsons, USGS Pacific Coastal and Marine Science Center

The Weight of Cities: Urbanization Effects on Subsidence

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Abstract: Across the world, people increasingly choose to live in cities. By 2050, 70% of Earth’s population will live in large urban areas. Upon considering a large city, questions arise such as, how much does that weigh? What are its effects on the landscape? Does it cause measurable subsidence? Here I calculate the weight of San Francisco Bay region urbanization, where 7.75 million people live at, or near the coast. It’s difficult to account for everything that is in a city. I assume that most of the weight is buildings and their contents, which allows the use of base outline and height data to approximate their mass, which is cumulatively 1.6•10¹² kg. I build a series of finite element models to study effects of pressure exerted by the weight distribution. Within the elastic realm, I look at compression, flexure, isostatic compensation, stress change, dilatation, and fluid flow changes. Within the nonlinear realm I show example calculations of primary and secondary settlement of soils under load.  The combined modeled subsidence from building loads is at least 5-80 mm, with the largest contributions coming from nonlinear settlement and creep in soils. A general result is closing of pore space and redirection of pore fluids. While the calculated subsidence of the Bay Area is relatively small compared with other sources of elevation change such as pumping and recharge of aquifers, all sources of subsidence are concerning given an expected 200-300 mm sea level rise at San Francisco by the year 2050.

I also examine New York City, which faces accelerating inundation risk from sea level rise, subsidence, and increasing storm intensity from natural and anthropogenic causes. I calculate a previously unquantified contribution to subsidence from the mass and downward pressure exerted by the built environment of the city and apply that load distribution onto a finite element model. Complex surface geology requires multiple rheological soil models to be applied; clay rich soils and artificial fill are calculated to have the highest post-construction settlement as compared with more elastic soils. Mapping shows areas in Lower and Midtown Manhattan and parts of Brooklyn and Queens with 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 long-term secondary settlement under load that can continue indefinitely, meaning that some aspects of these maps predict future subsidence.

Natural Bridges State Beach, shown here in the middle of the photo, is nestled amongst coastal neighborhoods in the southwestern part of Santa Cruz, California. View is looking south. The offices of the USGS Pacific Coastal and Marine Science Center are just a few hundred feet north, outside the bottom portion of the photograph.

Photo by Laura Torresan