Sonoma County sustained $5 billion damage over 40 years from storms known as atmospheric rivers

Sonoma County sustained more than $5 billion in damage from megastorms known as atmospheric rivers over 40 years, the hardest hit among 414 counties in 11 western states, according to a new study.

The flood-prone county’s economic toll was tops by far, with Lewis County, Oregon in second place with $3 billion in damage, according to the study, which calculated the cost of 1,603 atmospheric rivers from 1978 to 2017.

The 1995 storm that inundated Guerneville for three days, pushing the Russian River to its second highest flood level since 1940, was ranked the most destructive atmospheric river in the study period.

Insured losses in Sonoma County exceeded $50 million from the January 1995 storm that made landfall in Southern California and did $3.7 billion in total damage.

All told, atmospheric rivers - a global phenomena that play a key role in the world’s water supply, unleashing floods ?and ending droughts - did ?$42.6 billion in damage, about $1.l billion a year, according to the study published this week in journal Science Advances.

The damage toll will likely rise as a warming climate makes the megastorms wetter, longer and wider, giving western states an “increasingly volatile precipitation regime” as the population expands, the authors said.

“It’s crazy,” said Jerry Knight, owner of the River Theater in Guerneville and a veteran of five major floods in the lower Russian River since the early 1970s.

In 1995, the storm-swollen river crested at 48 feet, rising to the top of the bar in Knight’s live music venue, a fixture on Main Street that was built as a movie theater in 1947.

“I could have jumped in and gone swimming around the dance floor,” he said.

Instead, Knight launched one of his five hovercrafts and set about securing propane tanks, which he said would become “floating bombs” if they broke loose and were swept out to the ocean.

“Sonoma County is ground zero for atmospheric river flood damage,” said Marty Ralph, director of the Center for Western Weather and Water Extremes at the Scripps Institution of Oceanography, one of the study’s five authors.

Five other California counties - Marin, Napa, Monterey, Sacramento and Los Angeles - had more than $1 billion in damage from atmospheric rivers.

Atmospheric rivers actually hit harder up near the California-Oregon border than they do in the Bay Area, but the damage is higher in counties with dense populations, Ralph said.

The cost estimates are based on 40 years of data from the National Flood Insurance Program, which includes a formula for estimating uninsured losses.

An atmospheric river that swamped Guerneville and other lower river communities this year in late February, as the river rose to its highest level since 1995, did an estimated $155 ?million in damage but was not included in the study period.

Supervisor Lynda Hopkins said Sonoma County is naturally flood prone because the Russian River’s vast watershed of nearly 1,500 square miles of forests, farm and urban lands funnels into the river’s relatively narrow channel.

Hopkins said she is looking forward to more advanced forecasting of atmospheric rivers, allowing residents more time to prepare for floods.

Atmospheric rivers are massive columns of water vapor that dwarf the flow of the Mississippi River, delivering close to half of Sonoma County’s drinking water, said Jay Jasperse, chief engineer at Sonoma Water, the agency that supplies water to 600,000 Sonoma and Marin county residents.

“History has shown when we don’t get many atmospheric rivers we are almost always in drought,” he said.

When there are too many, or when they come two or three in a row, “that’s when we get our floods,” he added.

Jasperse said he was keenly aware of the forecast that atmospheric rivers will intensify with climate change.

“It’s not just today’s problem, it’s tomorrow’s problem consistently,” he said.

Atmospheric rivers are also capable of running with the wind across the entire continent, dropping heavy rain and snow from coast to coast.

A severe atmospheric river, following months of record-breaking rainfall, damaged spillways on the Oroville Dam in the Sierra Nevada foothills, causing the evacuation of more than 180,000 residents and more than $1 billion in damage in February 2017.

But half of the 1,603 atmospheric rivers in the study accounted for no insured losses.

You can reach Staff Writer Guy Kovner at 707-521-5457 or guy.kovner@pressdemocrat.com. On Twitter @guykovner.