In early February, meteorologists warned Southern Californians that a supercharged storm was headed their way, potentially bringing catastrophic flash flooding. The predictions came true, and then some: Up to 10 inches of rain fell on parts of Los Angeles over a 24-hour period, shattering all-time precipitation records.
That meant as much as 21 million acre-feet of water — or one and a half times the entire annual volume of the Colorado River — fell on the 36,000-square-mile greater LA metro area.
When trillions of gallons drench a typical concrete-dominated cityscape, the water runs off rooftops and slides onto impermeable concrete driveways and into streets, turning them into virtual rivers. It cascades into stormwater drains and concrete-cased arroyos — picking up dirt, garbage, oil and other substances — before rushing into larger rivers and, ultimately, the sea. When it rains as much as it did in February, the chances of a system failure — drains clogging, gutters overflowing, flood-control structures collapsing — rise, setting the stage for an urban flooding catastrophe.
But this time, most of LA’s stormwater system held up. Not only that, but instead of sending all that water straight to the sea, the city managed to capture more than 8 billion gallons of it in reservoirs and groundwater aquifers for future use.
As much as 21 million acre-feet of water — or one and a half times the entire annual volume of the Colorado River — fell on the 36,000-square-mile greater LA metro area.
That’s partly due to stepped-up efforts to make the city a bit more spongelike, by retrofitting impermeable concrete that was designed to repel water and instead transforming it into a more absorbent landscape. That not only takes a load off drainage infrastructure, it also helps keep the water — and the pollutants in it — out of the Pacific Ocean.
These retrofits are neither cheap nor easy, but with human-caused climate change threatening to make both storms and droughts more severe, opportunities abound to make Western cities just a bit more spongelike.
🅐 Impermeable roofs, driveways and parking lots collect and shed vast quantities of stormwater, turning streets into rivers.
🅑 About 136,000 gallons of water run off a typical 5-acre big-box store parking lot during a 1-inch rainstorm.
🅒 Billions of gallons of runoff gain enough force and volume to destroy flood-control and storm-runoff infrastructure.
🅓 The runoff picks up trash, lawn pesticides and fertilizers, oil and other pollutants and debris and carries it into waterways and out to sea.
🅔 A rainwater catchment system on an average-sized 1,700-square-foot roof could capture more than 1,000 gallons during a 1-inch storm for irrigation, toilet-flushing or other uses.
🅕 Water absorbed by the ground ends up in aquifers and the soil for later use by people and vegetation.
🅖 Green spaces absorb water, sustaining trees and other vegetation — combating the urban heat-island effect.
🅗 Infiltration basins capture runoff and allow the ground to absorb it, filtering out pollutants and recharging aquifers.
🅘 Permeable pavement absorbs water, reducing chances that streets become rivers.
🅙 Green roofs absorb precipitation, sustaining plants that can provide a green space in the sky and cool the structure, while blue-green roofs capture and retain rainwater for use in the building.
🅚 Less runoff flowing into the sea means less pollution, too.
Data visualization by Jennifer Di-Majo/High Country News
SOURCES: Los Angeles County, Los Angeles Department of Water and Power, U.S. Environmental Protection Agency, The Conversation, Invisible Structures, Wired, Los Angeles Times.
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This article appeared in the September 2024 print edition of the magazine with the headline “Sponge cities.”
