Sinking coastal land exacerbates Bay Area flooding risks, UC Berkeley study says

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It’s not just warming oceans and melting glaciers that are causing sea levels to rise and are threatening to flood coastal areas — a new study led by UC Berkeley researchers found that the subsidence, or sinking, of coastal land around the San Francisco Bay Area exacerbates the expected sea level rise and flooding risk to local populations.

Shorelines that are built on mud will compact over time, leading to a sinkage of less than 2 millimeters per year, according to the study. Even faster sinking rates exist on areas that were built either on Holocene mud deposits or man-made landfills. These areas are sinking at rates as fast as 10 millimeters per year and include portions of Treasure Island, Foster City, San Francisco and San Francisco International Airport.

Researchers at UC Berkeley and Arizona State University projected that the total area in the San Francisco Bay at risk for inundation, or flooding, when factoring in the effect of local land subsidence, is 125 to 429 square kilometers, as opposed to the projection of 51 to 413 square kilometers when considering rising sea levels alone.

Warming oceans and the melting of land based ice are the big contributors, but then more locally ocean circulation patterns and relative land motion are also important,” said U.S. Geological Survey Pacific Coastal and Marine Science Center research geologist Patrick Barnard in an email.

As sea levels rise because of climate change and other factors, flooding risks also increase. Coastal flooding is the biggest socioeconomic impact of rising sea levels in the 21st century, according to the study. Former studies predicted that by 2100, more than 480,000 people and $100 billion worth of property in California will be at risk of flooding.

Though melting glaciers and rising ocean temperatures have been proposed as possible causes of rising global sea levels, this study narrows in on the San Francisco Bay Area, looking specifically at how local land subsidence implicates the relative rates of sea level rise in the area.  

“This is the first study that really integrates … deformation (of land) with the sea level projections and then applies that to a model that ultimately allows us to determine how much flooding to expect,” said UC Berkeley researcher and lead study author Roland Bürgmann.

The researchers updated the projections of sea level rise using old technology in “revolutionary” ways, according to Bürgmann. Scientists used data from a global navigation satellite system and an interferometric synthetic aperture radar to help them measure elevations on the Earth’s landscape to within 1 millimeter of accuracy and to produce the sea level estimates.

Rising sea levels, among other factors such as saltwater contamination, accelerated coastal erosion, wetland losses and flooding, can lead to “unprecedented” socioeconomic effects, according to the study.

“The likely approach that people will take is to build walls around (their cities),” Bürgmann said, especially when people are unwilling or unable to relocate.

He added that this flood wall technique has already been implemented in New York City and New Orleans.

There is an opportunity for areas that have yet to be developed to be made into vegetated, naturally evolving wetlands that grow with the rising sea level, according to Bürgmann. This would not be a solution, however, for areas where housing developments, warehouses and airports already exist.

Bay Area communities now have a wealth of scientific information to determine flood risk and allow them to move beyond the planning stages,” Barnard said in an email.

Contact Alicia Kim at [email protected] and follow her on Twitter at @aliciackim.

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