… 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. 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.