Flood Philly

Why Philadelphia floods so easily when it rains

Correction appended

Ever feel like rainwater starts to collect quickly in Philly? That rain might be falling for just a couple of hours and it’s already collecting on the sidewalks and streets, turning a routine walk or drive into an adventure?

It’s not just you. Philadelphia is a city prone to flooding thanks to its geography and age. The flooding has to do with the rivers, its old drainage system, and even how residents remade the city some 200 years ago.

Creeks, drains and litter

Flooding happens after water starts collecting in Philly after substantial rainfall or when the rivers overflow. The latter tends to happen only after severe weather events like hurricanes and massive storms. But It’s the rain we’re focusing on here, kind of like the type we’re seeing today or on any number slate gray October and November days when rain falls for a few hours.

Mahbubur Meenar, a professor of community and regional planning at Temple, says that much of the flooding we see happens because of the city’s drainage system. In about 2/3 of the city, stormwater and wastewater — whatever comes out of your house or office building — drains through the same system. This happens because, well, the city is old. It’s so old, and so ingrained in the city’s infrastructure that it would be prohibitively expensive, if not impossible, to change.

On normal days, the drainage system works fine. Wastewater goes through and is treated before making its way to one of the rivers. But rain throws a wrench into the process. It flows into the same drains and mixes with the wastewater. The extra water can rise and flow onto the streets. Litter and fallen leaves don’t help, either. They can gather in the drains and make it more likely for flooding.

Another variable: Especially around Center City there are few natural resources that can capture water, i.e. streams and creeks. Nearly all of them have been filled in and turned into sewers. Dock Street is probably the best known example. That brick street in Society Hill used to be a creek. Dozens more have experienced the same fate, mostly in Center City and the neighborhoods closest to it. Check it out. The red lines indicate former bodies of water that have been filled in:

Creeks Flood Philly
PhillyH2O.org

If those creeks were still around, they could collect rainwater. Without them, stormwater lingers on the streets and has to go somewhere else — and in Philadelphia that’s through the drains where wastewater is already going.

“Depending on all these things,” Meenar said, “the road gets flooded.”

The city’s efforts    

So what is the city doing about it? For starters, the Streets Department is tasked with cleaning up the roads after big storms. The process of prevention falls to the Water Department.

To some extent, there’s not much the Water Department can do. It can’t restore all of Philadelphia’s creeks or overhaul the city’s infrastructure, particularly in the oldest parts of the city where stormwater and wastewater drain together. But the Water Department is working on green stormwater infrastructure to combat the problem. There have been some inroads throughout the city’s neighborhoods — things like green roofs, rain gardens and even man-made wetlands. They are designed to collect stormwater.

The primary purpose of these measures actually has to do with keeping our rivers clean. Stormwater that hits Philly’s streets can pick up chemicals harmful to our rivers and to us if it ends up in our drinking water. By storing the stormwater for a while, it can be released into a system where it will be properly treated, rather than flowing directly into the Schuylkill or Delaware.

The secondary effect for green stormwater infrastructure is that it helps prevent flooding. Not all of the water is rushing into drains at once.

“They try to store water as long as possible and then slowly release it to the drain,” Meenar said.

So that’s how the City is dealing with flooding from rainstorms. Besides rain and severe storms, of course, Philadelphia’s 3,000 miles of leaky pipes can cause flooding, too. That’s an entirely different problem, though.

This article has been updated to correctly reflect the approximate amount of stormwater/waste water draining systems and add that creeks had been turned into sewers.

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