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