Los Angeles proved how spongy a city can be


Earlier this month, the future fell on Los Angeles. A long band of moisture in the sky, known as an atmospheric river, dropped 9 inches of rain in three days on the city – more than half the amount of rain the city usually receives in a year. This is a type of extreme rainfall that will become more intense as the planet warms.

However, city water managers were ready and waiting. Like other urban areas around the world, L.A. has been transforming into a “sponge city” in recent years, replacing impermeable surfaces like concrete with permeable surfaces like dirt and plants. It has also created “spreading plains”, where water accumulates and seeps into the earth.

With traditional dams and all the new spongy infrastructure, between February 4 and 7 the metropolis collected 8.6 billion gallons of stormwater, enough to provide water to 106,000 homes for a year. Overall for the rainy season, LA has stored 14.7 billion gallons of water.

LA, which has long been dependent on snowmelt and river water coming from far away, is trying to produce as much water as it can locally. “There will be a lot more rain and a lot less snow, which will change the way we melt snow and capture aquifer water,” says Art Castro, manager of watershed management at the Los Angeles Department of Water and Power. “Dams and spillways are workhorses for capturing local stormwater for flood protection or water supply.”

Centuries of urban-planning dogma dictate using gutters, sewers and other infrastructure to drain rainwater out of a metropolis as quickly as possible to prevent flooding. However, given the increasingly devastating urban flooding around the world, this is clearly no longer working, so planners are now finding clever ways to capture stormwater, treating it as an asset rather than a liability. . “Urban hydrology problems are caused by thousands of small reductions,” says Michael Kiparsky, director of the Wheeler Water Institute at UC Berkeley. “No road or roof in itself causes large-scale changes in the hydrological cycle. But add millions of them into one area and this happens. Maybe we could solve that problem with a thousand Band-Aids.

Or in this case, sponge. The trick to making a city more absorbent is to add more parks and other green spaces that allow water to seep into underlying aquifers – porous underground material that can hold water – which a city can draw on in times of need. Can get. Engineers are also greening the medians and roadside areas to absorb water that normally runs off roads into drains and ultimately into the sea.

As the American West and other regions dry up, they are looking for ways to produce more water themselves rather than importing it by aqueduct. (By the way, that strategy involves recycling toilet water into drinking water so that cities reduce water use in the first place.) Plus, climate change is fueling heavier rain storms, naturally enough. : For every 1 degree Celsius of warming, the atmosphere can hold 6 to 7 percent more water, meaning there is often more moisture available for storms to dump as rain. In fact, studies have shown that West Coast atmospheric rivers, such as the ones recently hitting LA, are getting wetter.