Historically, big cities have managed stormwater with big pipes. Rainwater running off buildings and parking lots has been directed to drains, collected, and moved as fast as possible to local streams, rivers, or oceans.
There are several problems with this approach — not the least of which is, most cities still handle rainwater with the same sewers their citizens flush their toilets into. "Combined sewage and stormwater flows" cause big trouble for wastewater treatment plants. In New York City, as in most cities on the Atlantic coast, serious rainstorms flood sewage plants, forcing the operators to dump untreated sewage into surface waters.
Now, New York City is trying a different approach. The city is starting to implement what stormwater experts are calling "Best Management Practices" — detaining stormwater in vegetated basins at its source and letting the water percolate slowly into the ground.
The New York Times has this report (see: "New York Plants Curbside Gardens to Soak Up Storm-Water Runoff," by Matt Flegenheimer). "In what officials have billed as one of the most ambitious programs of its kind in the United States, New York City has, with little fanfare, embarked on a roughly 20-year, $2.4 billion project intended to protect local waterways, relying in large measure on 'curbside gardens' that capture and retain storm-water runoff. Begun as a pilot program under Mayor Michael R. Bloomberg — about 250 of the gardens are already in the ground — the initiative is set for a major expansion that will bring thousands of gardens to neighborhoods across the Bronx, Brooklyn and Queens in the coming months."
The gardens are slated to cost $20,000 to $25,000 each, the paper reports. But similar measures have performed well in other cities, including Philadelphia — which, like New York, has inherited a combined sewer system to handle stormwater and sewage together, and which experiences frequent overflows during rainstorms.
And if $25,000 — or $2.4 billion over 20 years — seems like a lot of money, consider the scale of the problem. New York Riverkeeper's web page "Combined Sewer Overflows" says, "More than 27 billion gallons of raw sewage and polluted stormwater discharge out of 460 combined sewage overflows ('CSOs') into New York Harbor alone each year. Although water quality in New York Harbor and throughout the Hudson River Estuary has improved significantly over the last few decades, many parts of the waterfront and its beaches are still unsafe for recreation after it rains. As little as one-twentieth of an inch of rain can overload the system. The main culprit is outmoded sewer systems, which combines sewage from buildings with dirty stormwater from streets."
According to the Stormwater Infrastructure Matters ("S.W.I.M.") Coalition, there are hundreds of "combined sewer outfall" locations around New York City, but a few big locations account for most of the dumping. "There are a handful – 15 out of the 434 CSO outfalls in the five boroughs – of CSO locations that are responsible for 50% of the total volume of CSO pollution that is released each year," the organization reports (see map). That's almost 14 billion gallons of dirty water.
That's a lot of water, and it's going to take a lot of curbside rain gardens to slow it down. But meantime, Brooklyn residents are enjoying the side benefits. Said one bystander to the Times: "Finally, the 'hood is going green."