First rising tides, now sinking shorelines: Study finds new trouble for Bay Area

SAN FRANCISCO -- Rising tides aren't the only problem the Bay Area faces as sea levels climb. Those high waters may advance on a sinking shoreline.

A study published Wednesday finds that flooding along San Francisco Bay could be far worse -- sometimes twice as bad as current models suggest -- because much of the bayfront is slipping downward at the same time that global warming is driving ocean levels upward.

While scientists have known that this one-two punch means seawater will push farther inland, exacerbating flooding that is expected to cost billions to remedy, the new study out of UC Berkeley and the University of Arizona is the first to use satellite radar to quantify just how much the sinking land may contribute to the toll.

Such spots as Foster City, San Francisco International Airport and Treasure Island -- places built on landfill that continues to settle -- are dropping as much three-quarters of an inch per year, the research shows. This continuing retreat comes on top of projections that bayfront water levels will increase by 2 to 6 feet by the end of the century.

``In addition to sea-level rise itself, which I think people are quite aware of now, land subsidence is making that problem that much worse,'' said Roland Burgmann, a co-author of the paper and a UC Berkeley professor of earth and planetary science.

When Burgmann and lead author Manoochehr Shirzaei, a professor at the University of Arizona, factored in the sinking shoreline, they found that between 48 and 165 square miles of land around San Francisco Bay is at risk of being underwater by the end of the century. The exact amount depends on how quickly sea levels rise, a calculation muddled in part by the unknowns of future greenhouse gas emissions.

Prior models of sea-level rise have downplayed the impact of the sinking land -- or estimated it with less precise methods.

Flood-risk maps prepared by the Federal Emergency Management Agency, which communities use to assess their vulnerability to rising water, don't take into consideration many of the effects of climate change, much less the role of the dropping shoreline in sea level rise.

Consequently, cities, counties, developers and home buyers don't always have a full picture of the threats they face in future decades as well the more immediate dangers of big storms and surging tides.

``A lot of areas don't know they're at greater flood risk than they used to be,'' said David Lewis, executive director of Save the Bay, who has worked to address problems associated with sea-level rise. ``And you can't adequately protect people against the risk of floods if you don't know where the risk is.''

Burgmann and Shirzaei hope their findings on land subsidence will be incorporated into future flood models and maps.

Using satellite data from 2007 to 2011, the scientists found that most of the Bay Area's shoreline is sinking at a rate of just under a tenth of an inch per year. Much of these losses -- which are expected to continue -- are simply the result of soft, squishy soils. In areas where rivers and streams carry mud to the bayfront, the ground is giving out slightly more.

Trouble spots include parts of Union City, Hayward, Redwood City, San Francisco and South San Francisco, according to the study.

The worst of the subsidence is occurring in areas constructed of engineered fill, where the underlying sand, gravel and dirt continues to compact. Treasure Island, built to create new land for the Golden Gate International Expo in 1939, is sinking between a half inch and three quarters of an inch per year, the study found.

Parts of San Francisco International Airport are slipping by nearly half an inch annually, a rate that would put half the runways and taxiways underwater by 2100, according to the study.

Foster City, built in the 1960s to provide new real estate for the region's growing suburban workforce, is also plunging close to a half inch per year, the study found.

The Bay Area has begun to plan for sea level rise, albeit slowly. Communities are raising levees and seawalls and restoring marshlands, which help absorb the influx of water.

In 2016, voters in the nine-county region approved a $500 million tax measure to help fund such initiatives. The first grants are expected to be doled out next month.

``California is pretty progressive and there's a lot of planning going on now,'' said Patrick Barnard, a research geologist for the U.S. Geological Survey, who helps model sea-level rise. ``But it's in the early stages.''

The San Francisco Bay has risen a little more than half a foot since 1900. As the planet continues to warm, causing arctic ice to melt and ocean water to expand, the rate of sea-level rise is expected to grow exponentially.

A recent state-commissioned report estimated that a rise of 3.4 feet by 2100 was the most likely scenario if significant action isn't taken to slow global warming. If considerable steps are taken to halt the emissions of heat-trapping gas, water levels would rise by an estimated 2.4 feet.

Across California, rising seas could inflict as much as $100 billion in property damage, with roads, homes, businesses and utilities submerged, according to studies. Nearly 500,000 people could be at risk of flooding.

While the new study focuses on the Bay Area, Burgmann and Shirzaei say their modeling of sea-level rise can be applied to other coastal communities, allowing residents and leaders to get a better idea of how much flooding is likely to occur this century.

The study was published in the journal Science Advances.