Is the ARkstorm Coming?

Storm Clouds & Los Angeles River

Following this latest atmospheric river/bomb-cyclone storm cycle, and SoCal's record-setting rainfall totals, I have a new appreciation for what the Army Corps of Engineers accomplished in the Los Angeles Basin in the late 1930's.

Since the storm began on Saturday, a truly massive amount of water fell across Southern California (UCLA saw nearly 12 inches of rain in 48 hours). Yet the basin's floodwater infrastructure did its job. We did not see people floating down Los Angeles boulevards in canoes. We did not see fields of homes and businesses submerged in water.

But perhaps wait a moment before you feel entirely reassured.

To function properly, the Army Corps floodwater system depends on the grace of Mother Nature. Without sufficient breaks between large storms, debris and sediment accumulates and clogs the basin's flood channels and reservoirs. Sediment, in particular, is becoming a major issue due to the effects of climate change.

LA County Public Works hopes to remove over 15 million cubic yards of sediment by 2038. But in an era of stronger storms and larger debris flows, it's not clear they can keep up. Additionally, much of the original 1930's infrastructure is no longer considered structurally sound and needs replacing.

The biggest threat is a sustained pattern of atmospheric river events coming one after another. This is the so-called "ARkstorm" scenario.

The ARkstorm already happened, if you're wondering. In December 1861, a series of large storms dumped an estimated ten feet of rain on parts of California. According to Wikipedia, the Great Flood of 1862 was the largest recorded flood in California, Oregon, and Nevada, and devastated much of the Western U.S.

It's important to note that the ARkstorm was not, in fact, a single storm. It was a parade of large AR storms, lasting a staggering 43 days.

So: can it happen again?

There is no reason to think it couldn't, nor to believe our current flood infrastructure could handle such a deluge. If anything, our warmer, wetter, more energetic atmosphere makes such an event more likely (though still, probabilistically speaking, rare).

For now, climate researchers like UCLA's Daniel Swain are quick to emphasize there's no indication an ARkstorm pattern is imminent. But Swain does believe climate change is increasing the risk of a California megaflood—and he's concerned we aren't prepared for it.

"Preparation" probably doesn't mean trying to upgrade the floodwater system to handle such an extreme event. That would be prohibitively expensive, and it still wouldn't guarantee success. A better goal, in my opinion, would be to design systems which are intended to fail gracefully.

Either way, it's going to be an interesting century.

— February 6, 2024

Andy Lewicky is the author and creator of SierraDescents

George Karpouzis February 8, 2024 at 8:09 pm

Imagine a storm like that dumping 40 feet of snow on the baldy bowl! We would possibly ski into the summer !

Andy February 9, 2024 at 8:20 am

If ten feet of rain falls on the Basin, that might be kind of a Mad Max on skis scenario. Would make for good video! ;)

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