Meet the Megaflood: How 'The Other Big One' Could Devastate S.D. - Voice of San Diego

Science/Environment

Meet the Megaflood: How 'The Other Big One' Could Devastate S.D.

A 2011 report envisions hundreds of billions in statewide damage, and researchers say it’s not even the worst-case scenario

You’ve heard of The Big One. Now meet The Other Big One – a massive statewide flooding catastrophe that could cripple California for months or years.

Researchers think a megaflood is just as likely to hit the state as a 7.8 magnitude earthquake, and has the potential to cause three times as much damage. San Diego County alone would suffer $25 billion in losses under a doomsday “atmospheric river” scenario created in 2011 by dozens of researchers during which a series of heavy storms would slam the northern and central parts of the state.

Nearly the entire neighborhood of Mission Beach would under water. Large numbers of residents would lose power and sewage service. Landlines, internet connections and cell phones could stop working for 15-20 percent of customers. And both I-15 and I-5, the two main routes to the north, wouldn’t return to full service for months.

The scenario assumes that California would be hit by a series of storms just as it was during the Civil War when the Great Flood of 1861-1862 boosted L.A.’s annual rainfall to 66 inches, forced the relocation of the state capital and produced a massive inland sea in the Central Valley.

Does the scenario the researchers modeled still hold up six years after it was released? Absolutely, the researchers say, with one putting it this way: “This past winter reassured me of this again and again.”

Michael Dettinger, a research hydrologist with the U.S. Geological Survey and one of the authors of the megaflood report, said it’s clear that climate change is exacerbating the risk of a such an event. Nonetheless, the 2011 report doesn’t appear to have focused many minds or prompt any major changes in how we prepare for disasters in California.

‘I Don’t Think the City Will Ever Rise from the Shock’

In the fall of 1861, shortly after Abraham Lincoln became president and the Civil War began, parched farmers and ranchers in California prayed for rain. When the winter came and the rain didn’t let up for 43 days, they prayed for it to stop.

Thousands died in the resulting statewide floods, which submerged Sacramento for months. “I don’t think the city will ever rise from the shock,” one observer moaned. “I don’t see how it can.”

The damage extended far past the capital all the way to San Diego, where residents scrambled to rooftops to escape the floods. Only one home in the San Diego River floodplain managed to survive when the flood current ran through its open doors and windows.

“One-quarter of the state’s estimated 800,000 cattle drowned in the flood, marking the beginning of the end of the cattle-based ranchero society in California,” writes climatologist B. Lynn Ingram in a 2013 Scientific American article. “One-third of the state’s property was destroyed, and one home in eight was destroyed completely or carried away by the floodwaters.”

It was caused by a series of storms that barreled into the state via a phenomenon that Californians learned about during last winter’s drought-busting soggy months — an “atmospheric river,” or plume, of endless moisture from the Pacific Ocean.

For the scenario created in 2011, known as ARkStorm (a clunky acronym for Atmospheric River 1,000 Storm), 117 researchers and other specialists led by the U.S. Geological Survey imagined what would happen if California is hit by a series of winter storms dumped about as much rain as those in 1861-1862.

The main difference between then and now: 39 million additional people live in the state.

Predicting the Flood Next Time

To use the terminology we’ve come to understand in the wake of the recent Houston floods, the ARkStorm megaflood scenario envisions a 500- or 1,000-year flood for much of the state — one that has a one in 500 or one in 1,000 chance of happening each year. (This does not mean that this kind of flood happens every 500 or 1,000 years. It could happen three years in a row or once in 5,000 years.)

Researchers in 2011 estimated that their envisioned megaflood could cost the state $725 billion in damage, or more than $800 billion in 2017 dollars: “25 percent of buildings in the state could experience some degree of flooding in a single severe storm. Only perhaps 12 percent of California property is insured, so millions of building owners may have limited or no ability to pay for repairs. That degree of damage would threaten California with a long-term reduction in economic activity, and raise insurance rates statewide — perhaps nationwide or more — afterwards.”

More than $300 billion in property damage is expected, including $25 billion in San Diego. The scenario envisions especially severe coastal flooding in Mission Beach, Del Mar (where the race track would be inundated), Coronado and Imperial Beach, while “drastic shoreline change” through erosion could hit Imperial Beach, La Jolla, Del Mar, Solana Beach and Carlsbad.

Here are other potential local effects envisioned by the megaflood scenario:

 Flooding would strike many highways in San Diego County, and it would take more than a month to restore I-15 and I-5 to full service.

 An estimated 15 percent of people in San Diego wouldn’t have power at the end of the storm period, and some may not get it back for a week or more. Similar percentages of customers would lose sewage service and landline, internet and cell phone service.

In the statewide picture, “power, food chains and agriculture could and would be disrupted, and — as the literal end of the line for many of these product flows — San Diego would surely feel the impacts,” Dettinger said.

And it could be worse. The megaflood scenario the researchers envisioned doesn’t put us at around ground zero. “There is nothing that prevents San Diego from being the focus of an ARkStorm, rather than L.A.,” Dettinger said.” In such an event, flooding is the major threat, but winds and resulting structural damages, transportation and power outages are likely to occur at levels beyond our memories.”

Like Last Winter, But Compressed

We saw plenty of rain last winter thanks to a steady stream of “Pineapple Express” storms, enough to end the drought and give California its wettest water year in 122 years of recorded records (not including the 1861-1862 storms).

But our recent winter water wonderland still doesn’t measure up to the projected megaflood. “The ARkStorm scenario is much more intense and severe than what we saw,” Dettinger said. “Over the course of the 80 days from early January through March, the total amount of precipitation that fell in Northern California was just about equal to the ARkStorm total. But it just came in 80 days, instead of the 23 days of ARkStorm. Imagine how things would have gone if all that precipitation had fallen in a third of the time!”

Things could actually be worse than the megastorm scenario envisions. “These major storms have happened throughout history and prehistory, and these types of storm that we’re looking at is completely plausible for today. But we dialed back on it,” said Dale Alan Cox, a U.S. Geological Survey researcher who led the ARkStorm project. “It has to be believable, so it’s not a worst-case scenario.”

Houston’s Lesson: Worst Cases Can Be Real Cases

In light of the ongoing disaster in Houston, Derringer said we can learn a few lessons:

 “We have to consider worst cases in our planning and mitigation of these storm hazards. Sticking to more reasonable numbers is just exposing us when the worst comes.”

 “Worst cases can recur. Just because southern Louisiana experienced the ‘worst’ with Katrina or in last year’s storms doesn’t mean that a Harvey can’t show up this year. Just because we had a historic winter last year doesn’t mean that we can ignore its likes again in our future.”

But Derringer couldn’t say whether the megastorm scenario research has made California safer.

“Frankly,” he said, “it’s hard to figure out the extent to which ARkStorm has specifically improved our readiness. I won’t deny it.” San Diego emergency managers did use the scenario in an exercise, Cox said.

Still, Derringer said, “there is no reason that ARkStorm couldn’t come to pass in the real world. It’s just a roll of the dice that we haven’t yet seen.”

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