Experts say flat topography, impermeable clay-based soil and building on a low lying coastal plain all contributed to the significant flooding issues in Houston over the past month.
Samuel Brody, professor of Marine Studies at Texas A&M-Galveston, said the city’s rapid expansion and development had a role in making it difficult for water to subside.
“On top of that, [our population is] 6-plus million people and with that, all the roads, rooftops, parking lots, which is pavement, concrete and no water can absorb into the soil,” Brody said.
He called the amount of rainfall from Harvey “biblical,” saying it was difficult to predict the severity of the storm. He added, however, some of the damage could have been prevented by implementing a tool called open space protection. Open space protection is any piece of land that has green space like grass, shrubs, trees or other vegetation that can soak up water and mitigate flooding.
But, he said, not all city planners are on board.
“It’s hard to do because the perception is that protecting open space and this ecological infrastructure replaces homes and economic opportunity and economic growth, but if those homes are flooded year after year, the value is rendered valueless in a lot of ways,” said Brody. “Would it be better to take those chronic, repetitive loss areas, convert it to open space, or restore it back?”
Pittsburgh is unlikely to see the 20 inches of rainfall in two days that Houston experienced. But Kyle Siler-Evans, an engineer at the Rand Corporation, said the region’s hills and valleys present a different challenge.
“What that’s going to do is push flood water to the low lying points very quickly,” said Siler-Evans. “So you would tend to have much more localized flooding in a place like Pittsburgh, but potentially much more severe.”
Rainwater runs from hilltops into a valleys where it pools and starts to flood, Siler-Evans said. While there won’t be floods everywhere, he said homes in those valleys would be at highest risk, especially, those near the city's rivers.
In Pittsburgh, sewer overflow is a major problem. Siler-Evans said even a small amount of rain can cause sewers to spill over.
“When we get a tenth of an inch of rainfall, somewhere in the system there is untreated sewage mixed with storm water that overflows into the river,” said Siler-Evans. “In a typical year we get 10 billion gallons of these sewer overflows.”
Siler-Evans recommends implementing green infrastructure to stop or delay the entrance of storm water into the sewer system.
“In Pittsburgh we do have a fair amount of [green infrastructure] that can help soak up some of that water,” said Siler-Evans. “However, if that green space is on a steep hill, you’re not gonna get as much benefit from it as green space on a flat area.”
Earlier this year, Alcosan gave $9 million in grants to support 32 projects spread throughout Pittsburgh, to help keep storm water from the sewer system.
Green infrastructure, such as bioswales, rain gardens, green roofs and permeable pavement, helps divert rainwater from the sewer system, instead allowing it to be absorbed.
Etna created a green infrastructure plan to help mitigate flood water in 2014 after being hit hard by Hurricane Ivan in 2004. The Allegheny County Conservation District created a homeowner’s guide for storm water, which is available online.
Siler-Evans said Pittsburgh is trying to follow the lead of Washington D.C. and Philadelphia by planning large-scale green infrastructures to mitigate storm water.
“Urban flooding is, I think, one of the most important issues this country is going to face over time, whether it’s tidal flooding in Miami or river flooding in Baltimore and Philadelphia or basement flooding in Chicago,” Brody said.