A storm this week lashed rooftops and gardens at the Buerger Center at Children's Hospital of Philadelphia (CHOP), dropping thousands of gallons of rain before moving east.

The water percolated through the garden soil or flowed down collection pipes before settling into giant holding tanks hidden in the parking garage.

Within days, the water was pumped back up and began irrigating the gardens.

What the water didn't do is flow onto streets and overwhelm the city's aging stormwater system.

That advanced stormwater installation has earned CHOP's Buerger Center for Advanced Pediatric Care the Philadelphia Water Department's Stormwater Pioneer award.

"We chose CHOP because it was a really perfect example of just not taking the bare minimum requirements, but because it integrated the system into the whole space," said Victoria Lenoci, a manager at the Water Department. "It also shows that a stormwater system doesn't have to be a burden."

The Water Department began giving out the Pioneer award in 2014 for exceptional stormwater designs that exceed expectations.

Lenoci said that, in 2006, the city began requiring all new construction more than 15,000 square feet in size to have internal stormwater systems capable of handling at least 1.5 inches of rain. About 70 projects are approved each year, she said.

Mayor Kenney and Councilwoman Jannie Blackwell recognized CHOP's accomplishment during a ceremony Wednesday on the sixth-floor rooftop garden, which also functions as a therapeutic setting for children and their families.

Kenney called it a "unique environment."

And most of it was disguised beneath his feet.

Doug Carney, CHOP's senior vice president, said the garden and aesthetics represent the stormwater system's "sizzle." The hidden engineering behind it all is "the steak," he said.

The goal is to keep rainwater from gushing into one of three city wastewater-treatment plants.  These plants can get overwhelmed during major storms, with untreated water flowing into waterways.

"Monday night's storm?  We captured all of it," Carney said.  "The system itself can hold 114,000 gallons of water. So it will capture 100 percent of the water that hits the site."

Carney said water flows down 15-inch pipes into a series of eight fiberglass tanks, which hold tens of thousands of gallons each. The tanks are behind walls within the five-level-deep parking garage. The system, in operation since the building opened in 2015, is designed to handle the volume of water engineers calculate would come from a storm big enough to occur only every 100 years.

The collected water is used to irrigate plants on a 2.3-acre ground-level plaza and a 16,000-square-foot rooftop garden. "So, for August, which is a dry month," Carney explained, "we might be watering the plants exclusively with recycled water."

Carney said the system is designed to release water slowly into the city's stormwater system.

"Managing the stormwater in any project is a technical problem that has to be solved," Carney said. "So that aspect of this is not unusual.  What makes it unusual — or remarkable — is how we did it in a way that will actually reduce our costs and create an environment that is not only pleasant and green but consistent with our mission to heal children."

The system was designed by Pennoni Associates, Turner Construction, and Nelson Byrd Woltz.