Entire California cities are cut off from each other.
By the time the waters recede, dozens are dead and the damage is in the billions.
It might be hard to wrap your head around a doomsday flooding scenario as California struggles through one of the worst droughts ever recorded. But a growing number of scientists say climate change — the same catastrophe that’s drying up the West — is also increasing the risk of nightmarish flooding across much of the state.
Among the most vulnerable: Sacramento, Modesto, Stockton, Fresno and other cities of the Central Valley, the flood plain stretching from Red Bluff to Bakersfield.
A new study by climatologists at UCLA found that by the end of the 21st century, California’s most powerful atmospheric rivers could produce up to four times as much rain than any storm ever recorded.
Atmospheric rivers form as high-powered winds drag a fire hose of tropical moisture across the Pacific Ocean, pointing directly at California for days on end. Just a handful of these storms account for the most of the precipitation the state receives during its brief rainy season in winter and early spring.
Northern California most recently got a taste of how powerful these storms can be in 2017, when a series of them tore a hole through Oroville Dam’s flood-control spillway, eventually prompting the evacuation of 180,000 people.
The UCLA researchers say global warming is increasing the size and severity of these storms. They say the precipitation they bring will fall as rain instead of snow. While snow gradually melts, the climate-driven rains will come sloshing into California’s rivers in a rush, making floodwaters harder to control.
The study warns that just one of these mega-storm events — dubbed Biblical “ArkStorms” — could kill untold numbers of Californians, displace millions of others, close major highways for weeks and lead to $1 trillion in economic losses.
They could dwarf the historic flood of 1862 — when, after a month of nonstop rain — newly-elected Gov. Leland Stanford had to take a rowboat to his inauguration in Sacramento. The Central Valley became a massive inland sea.
Since then, of course, California has built billions of dollars worth of levees and dams, weirs and river bypasses, to safeguard its cities and divert flood waters away from population centers.
But will this infrastructure — much of it constructed decades ago, and designed based on weather patterns of that era — be enough to protect Californians from the climate-driven mega-floods of tomorrow?
UCLA climate scientist Daniel Swain, a co-author of the paper, warns California’s flood-control systems may not be up to the task.
“Whatever the risks were 50 or 100 years ago,” he said, “they are about double that now.”
How Sacramento was jolted into flood defense
Climate change was hardly a top-of-mind issue for most Californians 25 years ago. But shortly after New Year’s Day in 1997, the Sacramento Valley got a dose of what could happen in a heavy atmospheric river storm, with tragic consequences.
An unusually warm belt of precipitation drenched much of the Valley with rain. Parts of the Sierra foothills got 40 inches.
For a while it was unclear whether the levees around Sacramento would hold back the raging Sacramento and American rivers. Just north, in Yuba and Sutter counties, levees broke along the Feather River north of where it merges with the Sacramento River. Eight people died and damage was estimated at $1.8 billion.
The next big jolt came in 2008. Inspectors discovered that the levees guarding Sacramento’s Natomas basin, just north of downtown, were prone to seepage and could falter during a heavy storm.
Officials realized that Natomas lacked what’s generally considered the minimum standard of defense: 100-year flood protection. That’s the ability to withstand a flood that has a 1-in-100 chance of occurring in any given year.
The city imposed a moratorium on new constructionin Natomas. It wasn’t lifted until 2015.
In the meantime, state and local governments started bearing down on flood risk.
In the wake of Hurricane Katrina, the Legislature in 2007 passed a law requiring urban areas to have 200-year flood protection — enough defense to handle a flood with a 1-in-200 chance of occurring in a year.
In the Sacramento area, federal and state agencies, along with the Sacramento Area Flood Control Agency, have strengthened levees in Natomas and elsewhere in the region. The federal government built a new flood-control spillway at Folsom Dam that will enable dam operators to start spilling water from the reservoir more quickly if a big storm is coming. The wall of the dam is also being raised for added reservoir capacity.
More work is underway. Among other things, officials are overhauling the two major bypasses that sit near downtown Sacramento and serve as a sort of safety valve for the floodwaters that could inundate the city. When the Sacramento and American rivers get too high, the bypasses shunt water onto levee-lined farm fields in Yolo County.
By 2030, much of the Sacramento Valley will have achieved 200-year flood protection mandated by the Legislature. Nearly $6 billion will have been spent in the Sacramento area alone, according to spokesman Tyler Stalker of the Army Corps of Engineers.
“I’m totally confident that the system will be able to pass the 1862 flood,” said Joe Countryman, a member of the Central Valley Flood Protection Board, a state agency.
“Sacramento is going to be very, very safe from flooding.”
Swain doesn’t share Countryman’s confidence. The UCLA climatologist said it’s becoming impossible to say that a city has sufficient protection against a 200-year storm, for example, when records for previous storms only go back as long as people were keeping track. At the same time, climate change is obliterating previous estimates of how much damage a 200-year storm can bring.
“We’ve seen this all around the country where the Army Corps said, you know, this (city) has 200- or 300-year flood protection or even 500-year flood protection,” Swain said. Then a far worse flood than anyone imagined comes along and overwhelms the city.
The UCLA researchers say the “flood sequences” envisioned in their study could generate 200 percent to 400 percent more runoff by the end of the century.
Yet Countryman, an engineer who retired from the Army Corps, said the UCLA study depicts a “Never-Neverland” storm that would beyond a doubt overwhelm the Sacramento region’s flood-control capacity.
“If you got a flood twice the 1862 flood, we would not be able to manage it,” he acknowledged.
But he said his agency’s climate models suggest the flooding greater Sacramento could expect would be 10 percent to 15 percent more dangerous than what what the region has experienced historically — much less than what UCLA is predicting.
San Joaquin Valley has higher flood risk
The experts do, however, agree that the San Joaquin Valley is in much worse shape.
The Valley “is most at risk to this climate-driven flooding,” said Barry Nelson, a water-policy consultant to environmental organizations.
The San Joaquin Valley’s cities are less prosperous — and have less political clout — than Sacramento. As such, they haven’t been able to make similar massive flood-control investments.
Swain and Countryman agree that the Valley’s risk is considerably higher.
For one thing, the San Joaquin Valley is only just now starting to upgrade its antiquated flood-protection infrastructure, putting it decades behind communities to the north in the Sacramento Valley.
For another, both the modeling done by the UCLA study and Countryman’s flood board show much the same thing when it comes to how climate change is going to dramatically change precipitation patterns for that region.
The Sierra Nevada mountains rise higher in the south than in the north. That means more rain falling at higher elevations of the southern Sierra as the climate warms — creating more danger for the reservoirs of the San Joaquin Valley below.
“With that snow line rising,” Countryman said, “it may double or triple the flows that we’ve seen historically from rainstorms in the San Joaquin.”
The reservoirs would be more easily overwhelmed. And below their dams, the levees along the rivers aren’t able to contain the gush of water that would spill out, said Jay Lund, director of the Center for Watershed Sciences at UC Davis.
“In the San Joaquin Valley,” he said, “one of the terrible things that they’ve done is they’ve really constrained the channel capacities out of the major reservoirs.”
He said the levees along the rivers in much of the San Joaquin Valley have only a fraction of the capacity of those guarding the American and Sacramento rivers — making a catastrophic flood in cities such as Merced, Modesto or Stockton much more likely.
Nelson said it’s past time to begin work to make these places safer by expanding the river channels. Environmentalists say expanding flood-plains along rivers would also have the additional benefit of creating habitat for the region’s imperiled native fish species.
If there’s any good news, it’s that the San Joaquin Valley is less populated than much of California. There’s more undeveloped space to create these “multi-benefit projects” that help the ecosystem and protect against catastrophic flooding in urban areas, said Tim Ramirez, a member of the Central Valley Flood Protection Board who’s focused on the San Joaquin Valley.
“The San Joaquin is rich with potential,” he said. “And I think we’re trying to get the funding to catch up to capitalize on that opportunity.”
Preparing for a climate change catastrophe
In the meantime, the experts said Californians must prepare for the worst.
“There’s going to be a big storm, and it’s probably going to come sooner rather than later,” Ramirez said. “And even if we build up everything, … we’re still at risk here. And we still need to be prepared.”
The Central Valley does have one thing going for it even with climate change making its storms worse, said Lund, the UC Davis watershed expert.
The atmospheric river storms that strike the Central Valley and the Sierra for days or weeks on end tend not to be like hurricanes that send massive amounts of rain in just a few hours, quickly overwhelming levees and inundating towns.
“The nice thing about floods in the Central Valley ,” he said, “is they’re usually pretty gradual.”
And unlike what California experienced with the great flood of 1862, the state has massive reservoirs now that can capture much of the rainfall and dole it out over longer periods. “Because we have those big reservoirs,” Lund said, “it gives people a chance to get out of the way.”
But Swain, the UCLA climate researcher, warns that Californians shouldn’t get complacent or feel like their flood protection systems will keep them safe.
The sorts of mega-storms envisioned in his study wouldn’t just hit one region. Instead, they’d blast Los Angeles and Fresno and San Francisco and Sacramento in one big, weeks-long deluge.
It would make the damage from an earthquake look minor in comparison, he said.
“You wouldn’t just have one disaster zone in one city to deal with,” Swain said. “You’d have a bunch of simultaneous problems emerging everywhere all at once.”
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