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‘Megaflood’ warnings resonate in Sonoma County, where historic floods have hit home

“Megafloods” could cause intense, long-duration rainstorms that would likely inundate existing hot spots and raise smaller creeks and streams around Sonoma County.|

Key points from new study: Climate change is increasing the risk of a California megaflood

We may be in the midst of pervasive drought, but California has a history of extreme, episodic “megafloods,” occurring five to seven times per millennium. The most recent one, the Great Flood of 1861-1862 occurred in December and January of that year and created a 300-mile long, 20-plus mile wide inland sea in the Sacramento-San Joaquin valleys.

Climate change contributes to the increased strength of atmospheric rivers — the long bands of moisture-laden vapor that flow toward California and produce the heaviest rain — increasing cumulative rainfall and hourly intensity, resulting in substantially more runoff and the risk of megafloods.

Climate change already has doubled the chances of a megaflood occurring over the past decade and is expected to continue to increase their frequency as the temperature continues to rise. By 2060, events that occurred once every 200 years could occur about three times a century.

A “double-whammy effect” is described in the Central Valley related to rising snow levels in the Sierra Nevada as the region warms so that winter storms bring more rain than snow, instantly creating more runoff than previously had been the case. Peak runoff could be 200% to 400% higher than in historic megafloods. Rain falling on preexisting snow intensifies that effect, a kind of “triple whammy,” co-author Daniel Swain said.

Source: Climate change is increasing the risk of a California megaflood in Science Advances.

There are images from the record 1986 Russian River flood that those who lived through it recall.

Navigating the chest-high, milk chocolate water that invaded neighborhoods along the lower river corridor.

The low-flying choppers that spent days plucking stranded residents to safety.

The sheer breadth of the river as it spread across the land, filling homes and buildings, and wrenching free stairways, hot tubs, cabins and propane tanks — some of which were aflame as they rushed downstream.

Roads were cut off. The power was out. Access to drinking water was, for many, limited to emergency supplies hauled in.

And there was the unrelenting rain, six days of it — a drenching, soaking, profuse rain that, on the day the flood started, just “didn’t stop,” the late Mike Reilly, a former county supervisor, had said. It was “like somebody just took a big bucket and kept dumping it right on top of you.”

It was the worst flood in modern history on the lower Russian River, an area prone to breaching its banks as the accumulated runoff from a 1,485-square-mile watershed squeezes through a narrow passage in the Coast Range, en route to the Pacific Ocean.

The floodwaters reached 49 1/2 feet on Feb. 18, 1986, cementing The Valentine’s Day Flood’s place in the history books. It broke the record by 1 1/2 feet (the current runner-up was Jan. 10, 1995), and it was 17 1/2 feet over flood stage.

Now climate scientists say that one of the big mega-disasters awaiting California is a sequence of 1986-style storm systems occurring back-to-back-to-back-to-back across the state over the course of about 30 days.

The result would be water streaming off the mountains, turning the ground to mud and raising even the smallest creeks around the state until floodwater reached into spaces never before even dampened by flood.

Californians could expect millions of people to be displaced, long-term closure of transportation corridors and economic losses of $1 trillion or more, according to a new study.

In Sonoma County, low-lying roadways that flood routinely during wet winters would likely be well under water, while water levels at other hot spots — the lower Russian River, Sonoma Creek, the Laguna de Santa Rosa and Petaluma River — would reach new heights, as would smaller streams and creeks most folks normally don’t even know exist.

Dams and levees around the county would be tested, as well. Flood channels might overflow. Washouts and mudslides would threaten, especially where wildfires have stripped the land of vegetation and burned the soils.

And along the shoreline of San Pablo Bay, where Highway 37 already gets washed over from time to time, swollen bay waters and storm surge from expected high winds would likely produce bigger problems than ever, West Coast climate scientist Daniel Swain said last week.

Even though Caltrans eventually plans to elevate the highway, “I’m not sure they’re going to raise it high enough,” he said.

Swain is co-author of the new study, released to considerable interest Aug. 12 and published in the journal Science Advances.

It describes a growing risks of calamitous “megafloods” in the age of a warming climate.

He and co-author Xingying Huang acknowledge the irony of its publication in the midst of historic drought. But they say, too, that the Western states’ focus on drought mitigation and wildfire likely means the very real threat of catastrophic flood is underappreciated.

Swain pointed to the record rainstorm that just last October dumped more than 8 inches on Santa Rosa in a 48-hour period and 14¼ inches on Venado, west of Healdsburg.

Two and a half years earlier, the flood of February 2019 caused substantial damage in the lower Russian River and in Sebastopol, where the Laguna de Santa Rosa overflowed, swamping The Barlow marketplace and nearby areas.

“We have seen hints of this,” Swain said. “They just haven’t been catastrophic because they’ve occurred in the midst of drought. But what happens when these happen in sequence?”

In fact, climate change already has doubled the chances of a “megaflood” occurring in California, compared with a century ago, the authors say.

And as temperatures warm, so does the risk of increasingly moisture-dense atmospheric rivers and long-duration storm series capable of producing severe flooding across a broad area of California, the study says.

A megastorm, previously something that might have happened once in 200 years, could occur closer to three times a century by 2060, the study says.

“Our research now suggests that this is actually something that we probably are all going to have to deal with at some point in our own lifetime and potentially more than once, depending on the warming trajectory that we take,” said Swain, a climate scientist with the Institute of the Environment & Sustainability at UCLA.

He and Huang say there is evidence in riverbed sediment and elsewhere of historic large-scale megafloods occurring five to seven times every 1,000 years or so, most recently during the Great Flood of 1861-1862.

The state received so much precipitation in December and January that winter, beginning with mountains of snow followed by warm rain, that an inland sea formed in the Sacramento and San Joaquin valleys extending 300 miles long and 20 miles wide.

Floodwaters inundated modern-day Los Angeles and Orange counties as well as most of California’s lowlands, including Petaluma, Sonoma, Santa Rosa and other communities, killing people and livestock and driving people from their homes.

The new study uses computer modeling to evaluate the features and implications of a multiweek, intensive storm sequence similar to 1861-’62, but set in the years 1996-2005 and in 2071-2080, after temperatures have continued to rise, as predicted, given carbon targets set by the world’s nations, Swain said.

Their work builds on a 2010 project called ARkStorm, led by the U.S. Geological Survey to assess flood preparedness based on a megaflood scenario across California.

ARkStorm 2.0, Swain and Huang’s work, predicts megastorms of increasing intensity and frequency as temperatures go up, resulting in cumulative rainfall totals that would eclipse anything on record.

Areas of Sonoma County, for instance, could see 30-day rainfall totals upward of 40 inches in the future, according to their analysis.

But hourly rates are expected to increase even faster than cumulative totals, creating prime conditions for mudslides, debris flows and flash floods. The problems would be particularly acute in places like Sonoma County and neighboring counties, where smaller rivers and stream systems react quickly to heavy rainfall.

The study also found that most megafloods would align with El Niño years, when warm ocean currents are more likely to produce wet winters. Thus, the ground would likely already be saturated, or close to it, and the water table full when the megastorm arrived, contributing to excessive runoff, Swain said.

In the Central Valley, the authors predict a “double whammy effect,” as climate change raises the elevation at which rain becomes snow in the Sierra Nevada. Instead of accumulating as snow pack, more of the precipitation would fall as rain and, thus, quickly accumulate as floodwater in the valley below. The potential for a kind of “triple whammy” rises when rain falls on snow, melting it and contributing to the rapid runoff, Swain said.

Sonoma County and the North Bay won’t be contending with that, but in its flashy, responsive river systems flooding is still an issue, as history shows.

We’ve seen flooding not only multiple times in the lower river, which help give Sonoma County status as California’s largest “repetitive flood loss” community, but in downtown Healdsburg, where Foss Creek tends to rise swiftly, and at Highways 12 and 121, in Schellville, thanks to Sonoma Creek.

Flood risk in the Payran Road area of Petaluma has been reduced in recent decades thanks to $40 million in new flood control infrastructure installed between 1997 and 2008, but all creeks, and the Petaluma River, a tidal slough responsive to conditions miles downstream in San Pablo Bay, remain vulnerable.

The Federal Emergency Management Agency bases flood insurance rates on flood hazard maps like this one for Sonoma County, taken from the county's recently updated Multi-Jurisdictional Hazard Mitigation Plan, adopted in December 2021. FEMA is expected to update its flood maps for the area in fall 2022. (Sonoma County/FEMA)
The Federal Emergency Management Agency bases flood insurance rates on flood hazard maps like this one for Sonoma County, taken from the county's recently updated Multi-Jurisdictional Hazard Mitigation Plan, adopted in December 2021. FEMA is expected to update its flood maps for the area in fall 2022. (Sonoma County/FEMA)

But given the storm predictions — potential for 10 to 14 days of rain out of 30, under the historical megastorm scenario— any stream or creek is at risk of flooding and creating a hazardous situation, Swain said.

The region’s primary reservoirs, Lakes Mendocino and Sonoma, perform important flood control functions during wet winters under the management of the Army Corps of Engineers. Officials said Warm Springs Dam, completed three years before the 1986 Valentine’s Day Flood, spared downstream residents 5 vertical feet of floodwater.

Nick Malasavage, chief of operations and readiness for the Army Corps’ San Francisco District, said everything is vulnerable at some point. But he said Coyote and Warm Springs dams were built and are maintained contemplating “so-called unthinkable events.”

While both reservoir dams are constructed of compacted earth, Malasavage said he “was satisfied and comfortable” that they could handle substantial rain of the sort pondered by Swain and Huang.

“I’m not dismissing this new study by any means, but I know we have different compartments in our program in water management, asset management and dam safety that really focus on those components,” he said.

Marty Ralph, a research meteorologist at Scripps Institution of Oceanography at UC San Diego, also referenced the aid of Forecast Informed Reservoir Operations pioneered at Lake Mendocino — a strategy that allows for a more flexible schedule of water releases, basing releases and retention of reservoir storage on the outlook for incoming storms.

Forecast-informed operations mean water managers can hold onto storage, even in the midst of winter, if there are no moisture-laden storms on the horizon, and they can discharge water if a storm is incoming and likely to replace what’s released.

Sonoma County Emergency Management Director Chris Godley said the megaflood concept is not new to local emergency officials, but there’s a recognition of greater need to ensure flood prevention measures are up to the challenge.

He also noted that creeks that have never flooded before would spill their banks, given sufficient rain and runoff.

Creeks around the county “could be new areas of concern for us, if the intensity of the rainfall exceeds levels we’ve seen historically,” he said.

This Sonoma County flood awareness map shows mapped and unmapped streams, creeks and channels that could be subject to flooding under after intense rainfall. The map was taken from the county’s recently updated Multi-Jurisdictional Hazard Mitigation Plan, adopted in December 2021. (Sonoma County)
This Sonoma County flood awareness map shows mapped and unmapped streams, creeks and channels that could be subject to flooding under after intense rainfall. The map was taken from the county’s recently updated Multi-Jurisdictional Hazard Mitigation Plan, adopted in December 2021. (Sonoma County)

In Santa Rosa, where Emergency Preparedness Manager Neil Bregman took local partners on a long-planned tour of flood prone areas Tuesday, it’s already clear new infrastructure is needed in the southwest area of the city, where recent development has expanded into the flood plain — once three inland seas, according to Godley.

But with Matanzas and Santa Rosa creeks converging right at Santa Rosa City Hall, any major flood would put downtown under at least 2 to 3 feet of water, he said.

Bregman said the value of the new study and others like it is, in part, that it exposes potential “blind spots.”

“I want to know where those are now,” he said. “We need to begin to recalibrate our understanding. This is an opportunity.”

The Sonoma County Board of Supervisors in July allocated nearly $1.2 million to county Emergency Services and Sonoma Water for a 2-year Drought Response and Flood Control Coordination Project.

It would try to improve coordination between a number of entities with flood management responsibilities in the county, including municipalities, U.S. Army Corps of Engineers, which operates the region’s two flood control and water supply reservoirs (Lakes Mendocino and Sonoma), multiple flood control districts, and Sonoma Water, which maintains 75 miles of flood control channels in the county.

It also gives the county a chance to investigate how flood control measures line up against the potential hazards to come, Godley said.

“Will it now be sufficient? This could eventually have real impacts for people,” he said.

Herb Genelly III, of Petaluma, was a 25-year-old firefighter medic with the Guerneville Fire Department when the 1986 flood hit. He spent days, soggy and exhausted, tending to emergencies as the water rose and then stayed high after the storm.

What most people don’t realize, he said, is that a rising flood spreads out, like liquid in a full funnel, covering lots of ground and causing more destruction than most imagine.

“I can’t wrap my head around the kind of damage that would occur if any of these comes to fruition,” he said of the megaflood predictions.

But within days of the study’s release, he already had talked to his folks, both of whom are in their 80s and live on the river outside Monte Rio, about the threat of a major flood and the need to plan ahead for evacuations.

Genelly said he’s worried the people tuned to the risk of wildfire and trained to flee when the time comes will not have the same sense of urgency about flooding, which can proceed slowly, and then get really bad really fast.

“You’re talking about getting up into levels with people who have never experienced that type of flooding,” he said.

Swain said the next phase of his and Huang’s work involves working with the state Department of Water Resources to create a spatially explicit map detailing where flooding is predicted to occur, how deep it would be and how long it would persist. They also want to work with the USGS and other partners to model debris flow and coastal inundation, he said.

During a briefing on the study last week, Swain told reporters that he hoped the study would help California be better prepared when the next megaflood hit, and “it is a matter of ‘when’ rather than ‘if,’ ” he said.

“It will be a disruptive, damaging and destructive event, no matter what. But there is still a lot we can do to head off some of the very worst consequences, and being thoughtful and proactive about that is really what we want to accomplish,” he said.

You can reach Staff Writer Mary Callahan at 707-521-5249 or mary.callahan@pressdemocrat.com. On Twitter @MaryCallahanB.

Key points from new study: Climate change is increasing the risk of a California megaflood

We may be in the midst of pervasive drought, but California has a history of extreme, episodic “megafloods,” occurring five to seven times per millennium. The most recent one, the Great Flood of 1861-1862 occurred in December and January of that year and created a 300-mile long, 20-plus mile wide inland sea in the Sacramento-San Joaquin valleys.

Climate change contributes to the increased strength of atmospheric rivers — the long bands of moisture-laden vapor that flow toward California and produce the heaviest rain — increasing cumulative rainfall and hourly intensity, resulting in substantially more runoff and the risk of megafloods.

Climate change already has doubled the chances of a megaflood occurring over the past decade and is expected to continue to increase their frequency as the temperature continues to rise. By 2060, events that occurred once every 200 years could occur about three times a century.

A “double-whammy effect” is described in the Central Valley related to rising snow levels in the Sierra Nevada as the region warms so that winter storms bring more rain than snow, instantly creating more runoff than previously had been the case. Peak runoff could be 200% to 400% higher than in historic megafloods. Rain falling on preexisting snow intensifies that effect, a kind of “triple whammy,” co-author Daniel Swain said.

Source: Climate change is increasing the risk of a California megaflood in Science Advances.

Mary Callahan

Environment and Climate Change, The Press Democrat

I am in awe of the breathtaking nature here in Sonoma County and am so grateful to live in this spectacular region we call home. I am amazed, too, by the expertise in our community and by the commitment to protecting the land, its waterways, its wildlife and its residents. My goal is to improve understanding of the issues, to find hope and to help all of us navigate the future of our environment. 

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