The Absurdity of Flushing Rainwater Away

For decades, Swiss municipal engineering operated under a simple, immaculate imperative: when rain fell on paved towns, it was to be flushed down the drain as swiftly as possible. Clean, orderly, and thoroughly shortsighted. Now, after another scorching summer left urban concrete baking under relentless sun, the nation's water managers are coming to a delayed realization. Sweeping every drop of rainwater out of sight might not have been the masterstroke it seemed.

The response to this self-inflicted dryness is the sponge city concept, put on display at the Wolkenwerk development in Zurich Oerlikon. Rather than banishing rainwater underground, the 70-meter residential towers route runoff through terraced rooftop gardens. With substrate layers up to 70 centimeters deep, these green spaces absorb moisture, irrigate flora, and direct the overflow into a central pond. During severe storms, the complex can hold back 280 cubic meters of water—the equivalent of six tanker trucks.

At a media briefing in Zurich, representatives from the Association of Swiss Wastewater and Water Protection Experts made the case for treating rainwater as an asset rather than a hazard. Total annual rainfall in Switzerland remains largely unchanged, but it arrives in violent bursts separated by prolonged heatwaves. Green roofs absorb up to 70 percent of annual precipitation, while large tree canopies can lower nearby temperatures by four to seven degrees Celsius through natural evaporation.

Yet, for all the praise lavished on Oerlikon's high-rise terraces, Switzerland's enthusiasm for ecological urban planning exhibits its usual blend of financial comfort and structural inertia. A few flagship developments look handsome in press brochures, but the vast majority of urban space remains sealed tightly under asphalt. Even a sponge eventually dries out during extended heatwaves if it receives no additional water. A clever rooftop garden cannot magically compensate for decades of unyielding concrete.

Written by Thomas Nussbaumer thomas.nussbaumer@alpineweekly.com