Sinking Ground Adds to Sea-Level Rise Risk in Bay Area

(CN) – A new study finds sea-level rise isn’t the only thing the San Francisco Bay Area has to worry about: sinking land will greatly exacerbate the flooding risk.

In the report, published Wednesday in the journal Science Advances, researchers use detailed measurements of subsidence – the gradual sinking or caving in of land – around the San Francisco Bay between 2007 to 2011 from satellite-based synthetic aperture radar, or InSAR. The team mapped out shorefront areas that will be affected by various estimates of sea-level rise by 2100.

The San Francisco Bay shoreline, where yellow indicates areas where a projected rise in sea level (SLR) will result in flooding by 2100. Red shows where local land subsidence (LLS) will combine with SLR to increase the flood-prone areas. (ASU/Manoochehr Shirzaei)

Depending on how quickly the seas swell, twice as many areas could be at risk of inundation compared to what had been estimated for sea-level rise alone, according to the findings.

“The estimates are significant,” first author Manoochehr Shirzaei, a member of NASA’s Sea Level Change planning team, told Courthouse News. “When we examined different sea-level rise scenarios, we found that for the San Francisco Bay shoreline by the year 2100, some 20 to 160 square miles face a risk of flooding.”

After adding the impact of submerging ground along the shoreline, the team found that the area threatened by rising sea levels expanded to between 48 and 166 square miles.

“We are only looking at a scenario where we raise the bathtub water a little bit higher and look where the water level would stand,” said senior author Roland Burgmann, a professor of earth and planetary science at the University of California, Berkeley. “But what if we have a 100-year storm, or king tides or other scenarios of peak water-level change?

“We are providing an average; the actual area that would be flooded by peak rainfall and runoff and storm surges is much larger.”

The findings will help local and state agencies prepare for the future and provide improved hazards maps for emergency response agencies and cities.

“Accurately measuring vertical land motion is an essential component for developing robust projections of flooding exposure for coastal communities worldwide,” said co-author Patrick Barnard, a research geologist with the U.S. Geological Survey in Menlo Park, California.

“This work is an important step forward in providing coastal managers with increasingly more detailed information on the impacts of climate change, and therefore directly supports informed decision-making that can mitigate future impacts.”

The lower estimates of flooding stem from conservative projections for sea-level rise by 2100 – about 1.5 feet. However, those projections are now being questioned with Greenland and West Antarctic ice sheets melting quicker than many researchers expected. Some extreme estimates now peg the rise as high as 5.5 feet.

That said, the sinking – which the team found to be as high as 10 millimeters per year in certain areas – makes less of a difference in extreme scenarios, Burgmann noted. Most of the Bay Area is subsiding less than 2 millimeters a year.

“The ground goes down, sea level comes up and flood waters go much farther inland than either change would produce by itself,” said Shirzaei, a former UC Berkeley postdoctoral fellow who is now an assistant professor at Arizona State University.

The research was funded by the National Science Foundation, NASA and the Point Reyes Bird Observatory.

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