Sinking ground in San Francisco Bay exacerbates flooding from rising sea levels

When Shirzaei and Bürgmann examined different sea-level rise scenarios, they found that for the San Francisco Bay shoreline by the year 2100, some 20 to 160 square miles (52 to 414 square kilometers) face a risk of flooding.

But that was just from rising sea levels. When they added the effects of sinking ground along the shoreline, they found the area threatened by rising water was notably larger: 48 to 166 square miles (124 to 430 square kilometers).

There are many estimates and models for sea-level rise,” Shirzaei said, “but they all fall short because they don’t take into account land elevation changes.”

These estimates are conservative, the researchers add. Extreme high tides, major storms and periods of high runoff from rain and snowmelt in the rivers that feed into the bay can raise its water level temporarily above that projected from rising sea levels.

“Flooding from sea level rise is clearly an issue in many coastal urban areas,” co-author Bürgmann said. “This kind of analysis is probably going to be relevant around the world, and could be expanded to a much, much larger scale.”

High economic impact
When formerly dry land becomes flooded, it causes saltwater contamination of surface and underground water and it accelerates coastal erosion and wetland losses. 

But the scientists spotlight two areas of sinking ground on the bay’s shoreline where flooding would have a high economic impact: San Francisco International Airport and Foster City. 

The airport, built mostly on landfill, handles more than 200,000 annual landings and sees 56 million passengers passing through it each year. The airport is both a destination for travelers to the Bay Area and a hub for domestic and international flights. The scientists’ data show that when land subsidence is combined with projected rising sea levels, water will cover nearly half the airport’s runways and taxiways by the year 2100. 

Foster City, which lies halfway between San Francisco and San Jose, was built in the 1960s, partly on engineered landfill. It has become home to a growing number of high-tech and IT companies because real estate is limited and expensive in the Bay Area to the north and south. However, the parts of the city that were built on landfill are subsiding and, coupled with rising sea levels, significant parts of the city will be at risk of flooding by 2100.

Looking forward
The researchers note that the FEMA maps of the Bay Area need to be updated with the measurements of land subsidence as well as recent projections of rising sea level so that local authorities can make better flood resilience plans.

Unfortunately, Shirzaei said, sinking ground along the shoreline greatly magnifies the effects of sea level rise because the processes work together to worsen the situation.

The ground goes down, sea level comes up, and floodwaters go much farther inland than either change would produce by itself.”

— Read more in Manoochehr Shirzaei and Roland Bürgmann, “Global climate change and local land subsidence exacerbate inundation risk to the San Francisco Bay Area,” Science Advances 4, no. 3 (7 March 2018) (DOI: 10.1126/sciadv.aap9234)