Los Angeles Is Probably Too Good at Preventing Big Floods

For the past few days, Southern Californians have found themselves living with a kind of weather one doesn’t typically associate with the region: rain. Days of rain. The kind of rain that, in the worst cases, causes flooding and landslides, and in the best cases enforces a kind of unwilling solitude. (A friend in the Los Angeles area recently sent me a video of her German Shepherd, yowling discontentedly at the falling water.) One gauge at the University of California, Los Angeles recorded more than a foot of rain in 24 hours, making it what the National Oceanographic and Atmospheric Administration calls a “thousand-year” rainfall event.

The cause is a slow-moving atmospheric river that has essentially parked itself over the region. And yet, even with all that rain, California still won’t be able to escape the drought that’s gripped the West for years. California, it turns out, might be a bit too good at flood control.

Undeveloped floodplains act like natural speed bumps for fast-moving floods — the water spreads out over the plain, slows down, and eventually seeps into the dirt to become groundwater. But land is at a premium in Southern California, especially in the L.A. area, and for decades the prevailing wisdom was to contain rivers with concrete so they wouldn’t flood, opening the floodplains to housing development. What we still call the L.A. River is now, essentially, a concrete drain.

This creates two problems. First, there’s no natural barrier that can slow down flood waters during events like this week’s storms. And second, there’s nowhere for that water to go other than into the rivers and out to sea.

“Most of the rivers in Southern California are channelized, or concrete-lined, and most of the floodplains are developed,” Jay Famiglietti, a hydrologist at Arizona State University’s Global Futures Lab and a former member of the California State Water Board in Santa Ana and Los Angeles, told me. “When we built all that infrastructure, we really were thinking more about flooding and not thinking about saving that water.”

That’s because California has historically been pretty well-supplied with water from the Colorado River and Sierra Nevada snowpack that would melt in the spring and summer, delivering water through mountain streams. But as Ian James wrote in the Los Angeles Times last week, California is in a "snow drought," with about only about half the historical average snowpack for this point in the season. While the storms might help shore up the Sierra Nevada snowpack a bit, it still won’t be enough.

Warmer air carries more water, but it also means that less snow falls at lower elevations than before — where once we might have seen a few inches of snow, we now see a few inches of rain. During an atmospheric river in particular, so much water is dumped over such a short time period that protecting people from flooding becomes the primary concern.

“It's a difficult balance between protecting people against flooding, which I think we do a really good job at, and replenishing groundwater,” Famiglietti said. But, he told me, Californians need to start having some difficult conversations about the possibility of moving people off the floodplains — what planners called “managed retreat” — and returning riverbanks to their natural state so that floodwaters can spread out into the plains, slow down, and refill aquifers that the state can rely on throughout the year.

Managed retreat is a difficult topic — nobody ever wants to leave their home. And then there’s the problem of money; local, state, and federal government agencies would probably have to fund all those moves and the subsequent restoration of the rivers and floodplains. “We need to be spending a lot more money,” Famiglietti told me. “We’re talking trillions of dollars. And who pays for it, the federal government or the state? I don’t have the answer.”

There are some plans to reshape the L.A. River, spearheaded by none other than Frank Gehry — but Famiglietti says that instead of restoring the river’s banks and opening up floodplains, the plans call for even more development. A coalition of environmental groups opposes the plan, saying Gehry’s idea, which includes the construction of “platform parks” that would span over the canal, saying that it “stands to do particular ecological harm, create real estate speculation, and precludes future opportunities for climate resilience.”

Going in the opposite direction — less development and less concrete — is sort of antithetical to our ideas of progress. But the atmospheric rivers will keep coming, Famiglietti stressed, and it’s difficult to plan a city around unpredictable rain events.

“We did the right thing at the time [when we built flood infrastructure],” Famiglietti told me. “But now, in a sense, we have to be thinking about moving backwards.”