The threat of a catastrophic ‘megaflood’ has doubled for California


Even today, as California grapples with a severe drought, global warming has doubled the likelihood that weather conditions will be as devastating as the Great Flood of 1862, according to a UCLA study released Friday.

In that flood 160 years ago, 30 consecutive days of rain triggered a monster flood that roared across much of the state and changed the course of the Los Angeles River, moving its mouth from Venice to Long Beach.

If a similar storm strikes today, the study said, up to 10 million people would be displaced, with major Interstate freeways like Interstates 5 and 80 closed for months, and parts of Los Angeles, including Stockton, Fresno and Los Angeles. The population center would be submerged – bigger than any $1 trillion disaster in world history.

Climate scientist Daniel Swain, a co-author of the study published Friday in the journal Science Advances, said it is likely to be “larger in almost every respect” than what scientists have come to call the “arkestorm scenario” of 1862.

“Overall there is more rain, more intense rainfall and stronger winds on an hourly basis,” he said.

The paper is the latest piece of research to describe the whiplash effects of a heating planet, where rising temperatures allow the atmosphere to absorb and store more and more moisture. This atmospheric “thirst” can result in either extreme drought and aridity or a massive release of water in the form of an atmospheric river.

The study used a combination of new, high-resolution weather modeling and existing climate models to find that the risk of “megafloods” increases as global temperatures rise. It also simulated how a long series of storms would locally affect parts of California over the course of a month by atmospheric rivers in a projected climate of 2081–2100. They found that some places would receive more than 100 inches of rainfall.

Graphic illustration showing moisture plumes in an atmospheric river that rises above coastal mountains and the Sierra Nevada, pouring rain and snow.

(Paul Duginsky / Los Angeles Times)

At 10,000-foot peaks, which will still be somewhat below freezing despite global warming, “you get more than 20 feet of snow accumulated,” Swain said. “But once you get down to the level of South Lake Tahoe and lower in altitude, it’s all rain.”

Swain and co-author Xingying Huang project that century-end storms will generate 200% to 400% more runoff in the Sierra Nevada Mountains, as precipitation increases and more precipitation falls as rain, not snow.

The increased runoff can lead to massive landslides and debris flows, especially in mountainous and mountainous areas that have been stripped of vegetation by forest fires.

“Whiplashing shifts” in extreme weather could also challenge the sustainability of California’s massive collections of old dams and dams, causing flooding in major cities.

The study also found that “megaflood” risk is likely to increase further this century with each additional degree of global warming.

ARKstorms are also known as the “Other Big One” after the nickname of an expected major earthquake along the San Andreas Fault.

But unlike an earthquake, an arcstorm event will result in a disaster area spanning thousands of square miles, complicating emergency response efforts and triggering economic and supply chain disruptions that will be felt globally.

Researchers are now working with the California Department of Water Resources to develop maps of where floods may be worst and preparedness strategies to reduce potential loss of life and property.

However, some of his initial proposals are sure to create tension between flood risk management plans and water conservation projects.

Alexander Gershunov, a climate scientist, said more frequent cycles of drought and deluge by atmospheric rivers – Pacific-based storms that are hundreds of miles wide – pose problems for West Coast reservoir managers to balance mandates for water storage and flood control and Opportunities will present both. at the Scripps Institution for Oceanography in San Diego which is not affiliated with the study.

That’s because, he said, “California will have to rely on potentially dangerous atmospheric rivers and flood waters for water resource production in a warmer climate.”

Preparing for large-scale flooding, the UCLA study says, “this could mean releasing water from reservoirs beforehand, allowing water to flood floodplains, and allowing water to drain.” away from population centres.

Earlier this week, however, Governor Gavin Newsom called on state agencies to begin preparing for a hot, dry future with strategies including expanding water storage and water recycling capacity.

That’s because, he said, new data suggests that California will lose 10% of its water supply by 2040.

“It’s a good move to retain as much water as we can, except that could collide with the need to prepare for a catastrophic flood,” Swain said.

Currently, people are focusing on the risk of wildfires, plagues and earthquakes, Swain said. “But devastating floods are a risk that is quietly, but steadily rising in the background.”

“Eventually, it’s going to come back to bite us,” he said.


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