Warning on risk of flood disasters

Climate change to intensify catastrophes, scientists say

The one-day deluge that soaked San Francisco in 4 inches of rain last October is just a damp teaser of what is coming when it rains torrentially off and on for 30 days, according to a harrowing study.

As much as 60 inches of rain (not snow) in a warming Sierra Nevada storm could wash down the delta and flood every river and stream in the Bay Area. The rising waters could then meet with a coastal storm surge that raises the level of San Francisco Bay by several feet to wash out lowlying roadways and overwhelm urban areas from the East Bay to San Jose.

These are the inferences from a research article in the journal Science Advances that was published Friday under the headline “Climate change is increasing the risk of a California megaflood,” by UCLA climate scientist Daniel Swain and colleague Xingying Huang of the National Center for Atmospheric Research.

“The kinds of scenario we

are talking about is ‘What if the worst storms that you can remember in California all occurred back to back in one month?’ ” Swain said in an interview.

Storms seem hard to fathom now, with California in its third year of drought. But climate change is making both drought and rainstorms more prolonged and intense, climatologists believe.

The dense 13-page treatise uses the Great Flood of 1862 as its model. An overwhelming series of storms turned the entire Central Valley into an inland sea and washed out what is now Los Angeles and the cities of Orange County. That flood caused an estimated 4,000 deaths as the equivalent of 10 feet of rain fell over a span of 43 days, according to references.

The study says that climate change is dramatically increasing the risk of megastorms like the one that spawned the 1862 flood, so that an event that would have occurred only once every two centuries is becoming one that might occur around three times each century. Just as worrisome, the consequences could be worse now because of the combined conditions of human-induced global warming, sprawling development and wildfires.

Recent analysis has suggested that “such an event would likely produce widespread, catastrophic flooding and subsequently lead to the displacement of millions of people, the long-term closure of critical transportation corridors and ultimately to nearly $1 trillion in overall economic losses,” the study states.

For Swain, the worst storm in his memory came in December 1995, with rain blown from the Pacific on wind so strong it nearly destroyed the more than century-old Conservatory of Flowers in Golden Gate Park.

“It is one of my most distinct childhood memories,” said Swain, who grew up in San Rafael and was 6 at the time. “Windows rattled. The power was out.”

For many people, the worst storm was the atmospheric river of last October. Atmospheric rivers, which occur during California’s winters, are long bands of water in the sky that fall as rain or snow over the course of several days. But Swain noted that wet as the storm was, it followed a long dry spell and was followed by dry weather afterward. The ground could absorb the runoff.

“It brought the single wettest day on record,” but it was an isolated event, he said. In the scenario he is modeling, based on his study of the flood of 1862, “we would see a storm like last October’s in the middle of winter, followed by more atmospheric rivers, and that is the problem,” he said. “A sequence of five or six strong and extreme atmospheric rivers. They just keep coming.”

And when they do, there will be more rain than snow, because of the warming winter climate, in the middle elevations of between 4,000 and 7,000 feet. That means less snowpack and more runoff. That runoff will hit a mountain landscape denuded by wildfire, so there will be nothing to slow it and no trees or

“The rain itself will flood every river and stream significantly worse than before.”

Daniel Swain, UCLA climate scientist

undergrowth to keep the hillside from coming down with the rainfall.

To compound the downfall, rising temperatures are exponentially increasing the capacity of clouds to hold water vapor. This increases “the propensity of the atmosphere to act as a giant sponge,” Swain said. He expects extreme storms in the Sierra Nevada to create 200% to 400% more runoff. Rain accumulation in lower elevations could be 30 to 35 inches, with 10 to 15 inches in the driest parts of the state.

“The rain itself will flood every river and stream significantly worse than before and in new places that have never flooded this century,” he said.

The report released Friday investigates a “plausible worst case scenario.” It addresses the big picture, statewide and does not get into regional or urban specifics Swain addressed in the interview. But that report is coming. He has funding from UCLA and the California Department of Water Resources and is already at work on part two.

“For that, we will simulate the flooding itself,” he said. The follow-up report should be delivered in a year or so. Get ready.

Provided by California State Library 1862 Water fills K Street in Sacramento during the Great Flood of 1862, the result of several rainstorms over a 43-day span. Climate change makes such disasters more likely, scientists say.

Brontë Wittpenn / The Chronicle 2021 Floodwaters engulf a car near the Russian River in the Sonoma County town of Forestville during October’s “atmospheric river.” Such storms are expected to become much more common.

Brant Ward / The Chronicle 1995 A storm shattered glass panes at the Conservatory of Flowers in S.F.’s Golden Gate Park in December 1995. Scientists say such storms could come in quick succession in the future.