Philadelphia keeps stormwater out of sewers to protect rivers

Tropical Storm Girard unleashed a torrent of water on West Philadelphia in March.

Missed that news? It was not a real storm, but it might as well have been.

Philadelphia Water Department employees turned on a fire hydrant on West Girard Avenue for three hours, enough to fill up two longish ditches with 35,000 gallons of water - an experimental simulation of 3.5 inches of rain. Then they watched it disappear.

The ditches are rain gardens, brimming with grasses, flowers, and monitoring equipment, and they are part of an underground revolution.

This week marks the fifth anniversary of a consent agreement signed with state environmental regulators, and the city says it has met its target of keeping more than 600 million gallons of rain out of the aging sewer system each year.

It has done so with hundreds of water-absorbing "tools": plant-studded green roofs, parking lots made of permeable pavement, stormwater trenches, and rain gardens such as the two on West Girard.

The reason for the ambitious program? As in many older communities, most of the city is served by combined sewer systems, meaning that rainwater is directed into the same pipes that carry the flow from showers, sinks, and toilets.

That is OK in dry weather, but with any amount of significant rainfall, the combined stream of sewage and rainwater starts to overflow into creeks and rivers, running afoul of the federal Clean Water Act.

Some cities have opted to comply with the law by building giant underground tanks to hold stormwater temporarily before sending it off to sewage treatment plants. Philadelphia proposed, instead, to soak as much rain as possible into the ground, the way nature intended, said Christopher S. Crockett, a deputy commissioner for the water department.

"Instead of creating expressways for the stormwater to get into our system, we're trying to create little traffic jams for the stormwater to stay on the site and get into the ground," Crockett said.

But the job is less than 10 percent completed.

The city says it has exceeded its five-year pledge to "green" 744 acres, defined as acreage where the various rain gardens and other infiltration tools can absorb at least one inch of rain.

In the next five years, the agreement calls for the city to triple that amount. And by 2036, the target is 9,564 acres - the equivalent of keeping nearly eight billion gallons out of the sewer system each year. Even then, there would still be occasional sewage overflows.

How much is eight billion gallons? Imagine a giant tank covering JFK Plaza. That amount of liquid would fill such a tank more than a mile high.

Still, the water already being kept out the sewers, conservatively estimated at 600 million gallons a year, is more than a drop in the bucket. That amount would still fill an imaginary JFK Plaza tank more than 500 feet high.

"It's one of the largest-scale implementations of greened acres that we've seen in the nation," said Jon Capacasa, director of the water protection division for the U.S. Environmental Protection Agency's mid-Atlantic region.

Collecting data

The water department has water-pressure sensors deployed at dozens of sites to see how well it works. And at a handful of spots, scientists are busy gathering more detailed data.

At the West Girard location, among others, Villanova University researchers are measuring near-surface water pressure, temperature, humidity, and other indicators.

Readings are captured every five minutes, and Cara Albright, who is working on her doctorate in water resources engineering, comes by periodically to download it onto her laptop. Water-pressure readings are translated into rates of flow.

"We basically try to measure everything that's coming into the site and everything that's leaving the site," she said.

The gardens are planted with purple coneflower, black-eyed Susan, Shenandoah switchgrass, and other hardy species, said Chris Bergerson, a water resources engineer for the city water department.

"They have to be able to withstand periods of drought, and they have to be able to survive when inundated," he said.

Elsewhere, Swarthmore College is leading an effort to measure underground water pressure at various levels down to seven feet below the surface, in order to calculate how much water is getting to the water table. Measurements are taken beneath "greened" sites as well as under regular patches of grass.

Philadelphia as role model

It is too soon to report hard numbers, but the differences between greened and un-greened sites appear "dramatic," said team member Claire Welty, a groundwater hydrologist and engineering professor at the University of Maryland-Baltimore County.

"The whole country is looking to Philadelphia to see how to do it," she said.

Crockett said it also is too soon to see measurable improvements in the four water bodies into which the combined sewers discharge their flow: the Delaware River and Schuylkill, and the Cobbs and Tacony Creeks.

But the end result will be cleaner water as well as more natural, stable flow rates.

Currently, a heavy rain takes the Tacony from ankle-deep to more than 6 feet deep in some places, Crockett said. That leads to flooding and erosion, and it wreaks havoc on wildlife.

The benefits go beyond clean water, Crockett said. The added greenery keeps the city cooler in the summer, along with absorbing greenhouse gases that can contribute to climate change. Then there is the improved quality of life when a vacant lot is turned into an inviting urban oasis, he said.

It is not cheap, with an estimated long-term price tag of more than $2 billion. Some of that comes from business owners, who now pay sewer bills based in part on the amount of impervious cover on their sites - rather than the old method based strictly on water usage. Rates have soared for some.

The water department says the alternative to the green approach, building large underground holding tanks, would cost even more.

And such tanks likely would be built with outside labor, whereas the weeding, drain-clearing, and other maintenance required for the green approach is done by hiring city residents.

"You're really getting many, many benefits in one," said the EPA's Capacasa.

The tally of green installations changes from week to week, at www.phillywatersheds.org, but is well into the hundreds - some built by the city, others by private landowners.

That is because for all development on sites measuring more than 15,000 square feet, the builder is required to install enough rain gardens, green roofs, and the like to handle the first 1.5 inches of runoff.

Source: Philly.com

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