A disastrous megaflood could bring more than 8 feet of water to parts of California, scientists say

Devastating wildfires and droughts are not the only natural catastrophes that California will continue to face. According to new research published on Friday, a disastrous megaflood could bring so much water to some areas of the state that it could completely drown entire stop signs on a neighborhood street. 

Scientists say it's part of an investigation into a "plausible worst case scenario." Their research, published in Science Advances, focused on two extreme flooding scenarios: one based on recent historical climate data and another that's based on the projected climate for the end of this century, from 2081-2100. 

Using climate models and high-resolution weather models, scientists found that California should brace for possible impact in the coming decades.

The historical model, known as ArkHist in their study and based on data from 1996 to 2005, a megaflood could bring a maximum of 85 inches to California's Sierra Nevada. Under this scenario, the state would also see higher precipitation intensities, with coastal areas having eight out of 30 days and mountain areas having 14 out of 30 days be "heavy precipitation." Overall, broad regions could expect more than a foot-and-a-half of precipitation, with widespread areas in the Sierra Nevada and some spots in the Coast ranges, Transverse Ranges and Cascade Range seeing more than double that.

UCLA climate scientist and research co-author Daniel Swain said in a UCLA press release that in the future modeling, "the storm sequence is bigger in almost every respect." 

"There's more rain overall, more intense rainfall on an hourly basis and stronger wind," he said. 

Under the future model, which is based on a scenario of the continued rapid growth of greenhouse gas emissions and global warming, precipitation would accumulate from more than two feet to more than two-and-a-half – essentially double what the state would see under the historical model. 

Coastal areas would face 16 days and mountain areas would face 20 out of 30 days of heavy precipitation, with some areas of the Sierra Nevada and southern Cascades seeing a full month. There is also a 220% increase in heavy precipitation hours.

This future modeling could be disastrous for some localities. 

"There are localized spots that get over 100 liquid-equivalent inches of water (8.3 feet) in the month," Swain said in a UCLA press release. "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."

In his own analysis of the research, Swain said that the primary concern for these findings is the increased runoff into rivers and streams, which increases the risk of floods. 

Their research found that under the future high emissions scenario, runoff is 200 to 400% greater than historical values – numbers that will have massive implications for the Sacramento and San Joaquin River flood plains. These areas, Swain said, are the home of ancient flood deposits, as well as millions of Californians. 

"Flood risk during an event like either of these scenarios will bring widespread and severe flood risk to nearly the entire state," Swain said, "but the extreme increases in projected surface runoff in the Sacramento and San Joaquin basins are of particular concern given the confluence of high pre-existing risk in these regions and a large population that has never experienced flooding of this magnitude historically."

Both of the scenarios paint a grim forecast but climate change and humans continuing to feed global warming through greenhouse gas emissions are only going to make the outcome worse, the researchers said. 

They found that for every 1ºC of global warming, the annual likelihood of an event based on their historical modeling increases rapidly. As of this year, climate change has already increased the likelihood of such an event by about 105% compared to 1920. 

And if the world continues on a path of high emissions over the next 40 years, they said, the likelihood increases by about 374%. 

"Currently proposed emission reduction targets would likely result in an additional 1-1.5ºC of warming beyond what we've already seen," Swain said on his website. "So it is highly likely, at this point, that California will experience further large increases in megastorm events capable of producing megaflood conditions." 

Such an event would be rare and devastating, but not unlike something California has seen before. The Great Flood of 1861-1862, researchers noted, turned the Sacramento and San Joaquin valleys into a "vast in-land sea nearly 300 miles in length." Experts believe floods of that severity happen five to seven times every 1,000 years. 

It was that storm that led to the U.S. Geological Survey creating ARkStorm 1.0 in 2010, a system that built a hypothetical storm system of similar severity to determine what its impact would be present-day. That research found that a massive event such as the Great Flood would create "widespread, life-threatening flooding" and cause a total economic loss surpassing $750 billion in 2010 dollars, or $1 trillion in 2022. Such a cost would mark "the most expensive geophysical disaster in global history to date." 

The system used in the latest study was ARkStorm 2.0, "a new severe storm and flood scenario reimagined for the climate change era."

And while the world must work to minimize global emissions and thus the risk of these kinds of megastorm events, there also must be a focus on adaptation because there will be some drastic change to at least some extent, scientists said. 

The risk, researchers said, has been "broadly underappreciated." 

"All of this suggests that California really needs to be planning for an increasing risk of catastrophic flooding – risk that was widely underestimated even absent climate change, but now those risks are rising further," Swain tweeted. On his website analysis, he said that the state's water and flood management policies and infrastructure need to be "substantially revamped for our brave new 21st century climate." 

In:
• Climate Change
• California
• Flood