Their presentations were also a reminder that despite the upheavals in the Middle East and the growing efforts to minimize America's carbon footprint, water, not oil, could have a bigger effect on how Philadelphians live.
"We're always arguing about which is more important, carbon or water, water or carbon. It's really a fake choice," said Charles Waldheim, a landscape architecture professor at Harvard University. As he explained, it's impossible to separate rising sea levels from the climatic changes fueled by carbon emissions.
Many expect those changes to lead to increased rainfall in the Northeastern United States, which would make it harder for a city like Philadelphia to deal with the water that surges into its sewers after a heavy storm. More rain could make flash floods more common and increase pollution in the Delaware River, the main source of the city's drinking water.
As in many older cities, Philadelphia's underground sewer pipes collect both waste and rainwater and channel them together to treatment plants. But when it rains hard, facilities can't keep up and have to release untreated sewage into the city's rivers, city Water Commissioner Howard Neukrug told the symposium.
If Philadelphia expects to capture that excess flow, it needs to build a giant collector pipe around its perimeter, from the Betsy Ross Bridge to the Fairmount Dam, at a cost of about $10 billion. That project would gobble up so much revenue, Neukrug argued, it would leave the city with no money for anything else.
Instead, his department is focusing on alternatives, such as conservation. The city is pushing landowners to convert asphalt lots to porous surfaces that can absorb rain and slow the runoff into the city's century-old sewers.
"The goal is to make one-third of the city's land impervious within 25 years," Neukrug said. The city is converting 500 paved acres of its own to parkland.
Yet even if Philadelphia does its part to keep the rivers clean, that doesn't mean its neighbors will.
Neukrug said he was concerned about sprawl in the pristine woodlands of northeast Pennsylvania, which help protect the water supply for Philadelphia, New York, and their suburbs. As more roads and buildings are cut into the woods to support the new fracking industry, sediment could clog the Delaware. Some fear the fracking process could also sully groundwater reserves.
If Philadelphia has to worry about too much water, plenty of other places have to worry about too little.
"Water demand already exceeds supply in many parts of the world," noted James Corner, chairman of PennDesign's landscape architecture department. "We will have to deal with mass famine, death, and war" as a result.
Kongjian Yu, a Chinese landscape architect, told the symposium that two-thirds of Chinese cities were already short on water. China's thirst will only grow as its urban areas expand. Unless Beijing makes an effort to preserve open land for drainage, it will face the same storm-water problems as Philadelphia.
Most of the world's water problems are man-made, noted Doug Jerolmack, an environmental science professor from Penn who studies the Mississippi delta around New Orleans. Because dams and levees block the natural accumulation of sediment in the delta, the city sank below sea level, making it vulnerable.
It will cost billions to reengineer the delta, Jerolmack said, but he argued that this was necessary if Louisiana's southern coast is to remain habitable. Without intervention, he predicted, New Orleans will be underwater by 2100.
The Netherlands faces a similar engineering task, Dutch planner Hans Venhuizen said. A third of the country would be lost without seawalls and pumping stations to keep land from subsiding, or sinking.
Nearly all the speakers acknowledged that it is a pipe dream to expect the "gray infrastructure" of levees and pumps to keep cities safe. The Japanese thought their 34-foot-high, double-lined seawalls were a match for any tsunami, yet its northern coast now lies in ruins.
Because Philadelphia is so far upriver, being inundated by rising sea levels is one water issue the city doesn't have to worry about, Neukrug said.
At least for now.
Contact staff writer Inga Saffron
at 215-854-2213 or firstname.lastname@example.org.