California at risk of ‘megaflood’ where ‘every major population center would get hit at once’

California’s ‘Great Flood’ in 1862 swamped the state’s Central Valley up to 300 miles long and nearly 60 miles wide

Californians already live with increased risk of wildfires and drought linked to the climate crisis, not to mention the threat of a major earthquake on the San Andreas Fault. But now scientists are warning of potential diaster from “the Other Big One”.

New research, led by scientists at University of California, Los Angeles (UCLA) and the National Center for Atmospheric Research, has found that the state faces a catastrophic flood that could inundates vast regions with water flows hundreds of miles long and tens of miles across.

“Every major population center in California would get hit at once — probably parts of Nevada and other adjacent states, too,” said Daniel Swain, UCLA climate scientist and co-author of the paper, in a news release.

Historical climate change has already doubled the likelihood of a “megaflood” in California, the researchers found, and the likelihood increases as global temperatures continue to tick up.

Around a decade ago, scientists began to examine modern-day flood risk in California, motivated by a major disaster which took place in 1862. During that “Great Flood” - when no flood management practices were in place - waters swamped the state’s Central Valley up to 300 miles long and nearly 60 miles wide.

Around half a million people lived in California back then, compared to 40 million today. A similar event today would leave major cities like Los Angeles and Sacramento under water, even with flood control measures. The damage estimate is $1trillion, the most costly ever.

Researchers found that, even with weeks of notice from meteorologists and climatologists on such a “megaflood”, it would not be possible to evacuate the 5 to 10 million people who would be displaced by flood waters.

While it helped inform flood planning in some regions, the exercise was limited due to lack of organized resources and funding, Dr Swain added.

Such a vast flood would also put California’s major freeways under water and hamper relief efforts. The researchers also say that such a disaster in California - the world’s fifth largest economy - would ripple out globally.

Though California has escaped such devastating flooding to date, climate models and the paleoclimate record — which looks back through thousands of years of geological evidence — shows that such an event typically happened every 100 to 200 years. And this is before taking into account the impacts of human-caused climate change of the past 150 years.

The new findings from the ongoing research project – dubbed “ArkStorm 2.0” to reflect the biblical scale – account for how the climate crisis will exacerbate flooding. The study was published on Monday in the journal Science Advances.

“In the future scenario, the storm sequence is bigger in almost every respect,” said Dr Swain. “There’s more rain overall, more intense rainfall on an hourly basis and stronger wind.”

The study predicts that storms later this century will create 200-400 per cent more run-off in the Sierra Nevada mountain range, which stretches through California and up the western US.

The climate crisis is likely to bring more rainfall to California, and increased heat means that the Sierras will experience more rain than snow.

Execessive run-off could lead to devastating landslides and debris flows, particularly in steep areas that have previously been burned by wildfires.

The study compared two extreme scenarios - one that would occur about once-per-century from the recent historical record, and another in 2081-2100 based on projected impacts of the climate crisis.

Both scenarios would involve multiple storms over the course of a month and fueled by atmospheric rivers, so-called “rivers in the sky” which carry heavy rain loads.

Researchers also simulated how the storms would affect parts of California at a local level.

“There are localized spots that get over 100 liquid-equivalent inches of water in the month,” Dr Swain said, referring to the future scenario.

“On 10,000-foot peaks, which are still somewhat below freezing even with warming, you get 20-foot-plus snow accumulations. But once you get down to South Lake Tahoe level and lower in elevation, it’s all rain. There would be much more runoff.”

Dr Swain pointed out that with constant headlines about wildfires and the “megadrought” in the American West, the threat of extreme flooding had faded into the background.

ArkStorm 2.0 aims to help the state prepare for such an event, with the study was funded by the California Department of Water Resources.

“Modeling extreme weather behavior is crucial to helping all communities understand flood risk even during periods of drought like the one we’re experiencing right now,” said Karla Nemeth, the department’s director.

“The department will use this report to identify the risks, seek resources, support the Central Valley Flood Protection Plan, and help educate all Californians so we can understand the risk of flooding in our communities and be prepared.”

The research team plans to run more advanced flood simulations to further research and help in preparations to respond to such a scenario, including where flooding could be the most damaging and what the state can do to mitigate it.