ASU Scientist: Sinking Ground Will Worsen Rising Seas In San Francisco Bay Area

Published: Wednesday, March 7, 2018 - 5:30pm
Updated: Thursday, March 8, 2018 - 12:49pm
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(ASU/Manoochehr Shirzaei)
Areas of San Francisco Bay Area at risk from sea level rise. Yellow shows where projected rises in sea level will flood the shoreline by the year 2100. Red shows where subsidence would combine with sea level rise to increase the area prone to flooding.
(ASU/Manoochehr Shirzaei)
Foster City and San Francisco Airport are at heightened risk from flooding by 2100. Blue areas indicate local land sinking; yellow shows what's at risk from sea-level rise; and red reveals the impact of both processes combined.

A new study suggests official flood risk plans for the San Francisco Bay Area may underestimate inundation due to sea level rise over the next century by nearly 4 to 91 percent.

The research, which adds the effects of local land subsidence to flood calculations, appeared March 7 in the open-access journal Science Advances.

Other coastal cities could face similar effects, even under best-case scenarios.

"What we put here is an average of what we should expect. There will be a worse scenario — a much worse situation than this — if there is a storm and high tide in the Bay Area," said Arizona State University's Manoochehr Shirzaei, a member of NASA's Sea Level Change Planning Team.

In recent memory, hurricanes such as Harvey, Irma and Maria have shown how warmer seas driven by global climate change can produce stronger storms, heavier rainfall, higher storm surges and record flooding.

As land subsides due to groundwater pumping, landfill compaction and other processes, the impacts of such intensified floods could intensify.

Current flood forecasts account for large-scale motions, such as crustal plate movements or the slow rebound of land masses after heavy glaciers melt away. But such studies have generally not taken into account smaller-scale subsidence.

To estimate its effects in the Bay Area, Shirzaei and co-author Roland Bürgmann of University of California, Berkeley, cross-referenced a variety of satellite data, including synthetic aperture radar interferometry, global navigation satellite systems and high-resolution LIDAR elevation maps.

Like radar, LIDAR (Light Detection and Ranging) uses echoes to calculate distances. But where radar uses sound waves, LIDAR relies on laser light pulses.

Radar interferometry works by bouncing high-frequency electromagnetic waves off a target area. The distance traveled by each wave group affects its round-trip time and phase — the position of its crests and troughs relative to the radar detector. If the distance to target differs during the satellite's next pass — say, due to subsidence — then parts of that image will be out of phase with the first.

"When we compare backscatter radar signal to satellite, to many, many acquisitions that we collect — over hundreds of acquisitions are used in this particular study — we can measure how fast or slow land moves toward satellites or away from satellites," Shirzaei said.

The Bay's coastal areas mostly experience subsidence rates of less than 2 millimeters, or about a twelfth of an inch, each year.

But in areas built on artificial landfill and certain mud deposits, such as the San Francisco International Airport and Foster City, the sinking exceeds five times that rate. When combined with estimated sea level rise, that means water will cover nearly half the airport's runways and taxiways by the year 2100.

Landlocked Arizona could also experience changes. Rapid subsidence wrought by groundwater pumping might already be altering flood patterns in Phoenix and Tucson.

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