Coastal challengesSinking ground in San Francisco Bay exacerbates flooding from rising sea levels

New research shows that sections of the San Francisco Bay shoreline are sinking at rates of nearly half an inch (10 millimeters) a year.

But knowledge of where the ground in the Bay Area is sinking, and by how much, is not included in the official planning maps that authorities use to assess the local flooding risk from rising sea levels.

The new findings appear in a paper published in the journal Science Advances. The lead author is Manoochehr Shirzaei, assistant professor in Arizona State University’s School of Earth and Space Exploration and a member of NASA’s Sea Level Change planning team. His co-author is Roland Bürgmann of the University of California, Berkeley.

ASU says that the scientists measured how much the land along the bay’s shoreline has sunk with interferometric imaging using synthetic aperture radar from Earth orbit to detect elevation. The technique is sensitive enough to measure year-to-year changes in local ground elevation as small as a millimeter. Their study used data from 2007 to 2011.

“Although we found that most of the bay’s shoreline is sinking by less than 2 millimeters a year, in several areas we discovered subsidence rates of 10 millimeters a year and more,” Shirzaei said.

He points to San Francisco Bay’s Treasure Island as an example of land that is subsiding at a relatively high rate. The island, located in the bay midway between San Francisco and Oakland, was created as landfill for the 1939 Golden Gate International Exposition, but is now mostly a mix of historic buildings. The northwest corner of Treasure Island is sinking at a rate of half an inch to three-quarters of an inch (13 to 19 millimeters) a year.

It is, the scientists said, typical of most areas of land subsidence in the Bay Area. Many are former landfills that are slowly compacting, while others are places where streams and rivers are depositing successive layers of mud as they flow into the bay.

Not on FEMA’s planning maps
The scientists note that state, county, and municipal administrations currently use maps prepared by the Federal Emergency Management Agency (FEMA) to make plans for dealing with possible flooding.

FEMA’s Increased Flooding Scenario Maps brochure and FAQ use data from the agency’s San Francisco Bay Area Coastal Study to estimate how the coastal floodplain would change with a 1-foot, 2-foot, and 3-foot rise in bay water levels.