Water burst over the banks of the Schuylkill and flooded Manayunk and Center City last week when remnants of Hurricane Ida moved through the region, forcing overflow pipes to spew diluted sewage into the river, just as Philadelphia’s aging stormwater system is designed to do.

Built mostly in the late 19th and early 20th centuries, the system struggled to hold back precipitation flowing down from places such as Phoenixville off the Schuylkill in Chester County and Skippack in Montgomery County, off the Perkiomen, which drains into the Schuylkill. Both towns, and those around them, were inundated by eight inches of rain.

Philadelphia lies at the bottom of the 135-mile long river’s vast drainage system. Because of climate change, its stormwater system needs to handle bigger storms more swollen with precipitation from not only the Schuylkill, but also a range of other waterways that all ultimately drain into the Delaware. Modernizing the system will be costly.

“Whenever we have a major flood event, we get very excited, and then we forget all about it. This is a wake-up call,” said Robert Traver, a civil and environmental professor of engineering at Villanova, and director of the school’s Urban Stormwater Partnership.

“Climate change is making these events occur more frequently,” said Franco Montalto, a professor in Drexel’s Department of Civil, Architectural & Environmental Engineering. “Of course, there were always extreme precipitation events ... but now the impacts are more extreme.”

Officials and experts hope money from Washington will help. President Joe Biden’s proposed $4.5 trillion infrastructure package calls for $56 billion in grants and low cost loans for upgrading and modernizing wastewater and stormwater systems. A vote on a $1 trillion bipartisan bill in the Senate is expected this month. How much money will eventually make its way to Philadelphia is not clear.

At issue for Philadelphia is how it copes with the older part of its two separate sewer systems that handle sanitary waste and storm runoff.

In the more modern system, generally built after the 1940s, two pipes exit each home or business. One pipe contains sewage, and flows directly to a treatment plant. Another pipe collects stormwater runoff from yards and roof gutters and channels it into waterways.

However, 60% of the city is covered by the older, combined sewer system, which has a single pipe carrying both stormwater and sewage from streets, houses, and businesses directly to a wastewater treatment plant. The system works fine during dry weather. However, during big storms, the combined system is overloaded and must divert the mix of sewage and stormwater into local rivers and creeks.

» READ MORE: The secret scourge of climate change? More raw sewage in Philadelphia’s waterways.

An Inquirer analysis shows that there are 168 combined outfalls along the Schuylkill and Delaware Rivers, and Cobbs, Tacony, and Pennypack Creeks. One outfall at Penn Treaty Park that flows into the Delaware dates to 1888. Most were installed from the 1920s through 1960s. Those outfalls are all located downstream from the intakes that provide the city with its drinking water.

During Ida, the tidal Schuylkill crested at about 16.13 feet at 9 a.m. on Sept. 2, according to a U.S. Geological Survey gauge near 30th Street. Major flood stage is 14 feet. Multiple overflows emptied sewage into the river.

Though the river level is back to normal, the Water Department still had designated the water quality “red” as of Friday. Red, the worst of three designations, indicates that bacteria levels are high and the water is not suitable for recreational uses such as boating, fishing or wading.

A decade ago, under order from the federal government, the Water Department launched Green City, Clean Waters, a 25-year, multi-billion dollar program, to implement “green” infrastructure to help manage stormwater and reduce combined sewer overflows.

So far, the department has installed more than 2,800 green tools throughout the city, including rain gardens, green roofs, rain barrels, gutters, and natural areas designed to either slow the runoff or have it soaked up by plants and soil. The city estimates that the effort has kept more than 2.7 billion gallons of polluted water out of waterways.

But those tools aren’t enough under the force of big storms. So the Water Department announced in January plans to use a $100 million low interest loan from the Pennsylvania Infrastructure Investment Authority’s PennVest program to build a preliminary treatment facility that will increase capacity at the Northeast Water Pollution Control Plant just south of the Betsy Ross Bridge.

The 18,000-square-foot plant will remove inorganic materials — trash, sand and grit that wash down storm inlets — and allow the plant to increase its wet-weather treatment capacity from 435 million gallons to 650 million gallons per day. It will help meet requirements under the federal Clean Water Act and the Pennsylvania Clean Streams Law.

PennVest is funding an additional $6.72 million for new green storm water projects in the Lawncrest neighborhood, bordered by the Tookany and Tacony Creeks, both of which get flooded with diluted sewage during big storms.

Brian Rademaekers, a spokesperson for the Philadelphia Water Department, said the agency is developing an updated wastewater master plan and 25-year capital improvement program for all of its wastewater plants and pumping stations.

“It will take a regional watershed approach to mitigating ... the types of impacts we experienced during last week’s storm ... as these types of extreme weather events become the new normal,” Rademaekers wrote in an email. “The City of Philadelphia has jurisdiction over less than 2% of the drainage area that contributes to the Schuylkill River. We were fortunate that upstream flood protection and reservoir management programs significantly helped in managing the rainfall amounts the watershed accumulated during Ida.”

He said it would cost “billions of dollars to plan, design, and construct” a system to manage extreme events, but that still would not be enough, he said.

“However, no investment in hard infrastructure within Philadelphia could have prevented the impacts caused by the record flood levels ... in the watershed upstream of the city,” Rademaekers wrote, noting that the river has a 2,000-square-mile drainage area that the city cannot control.

Robert Traver, the Villanova professor who has researched both Hurricane Katrina and Hurricane Sandy, said buildings need to be planned with bigger storms in mind. Older infrastructure designed for, say, 20-year storm events, now need to handle at least 100-year storms (meaning a storm of certain magnitude has about a 1% chance a year of occurring).

He said Philadelphia, which is surveying residents affected by Ida, is taking a solid step in identifying the impacts on flood-prone areas in order to assess needs.

“The first action we need to take is to understand how the climate is changing and what protection we can expect from our current water infrastructure,” Traver wrote in an August opinion piece for the Hill. “Without this understanding we cannot effectively implement decisions and projects to reduce our exposure, nor meet our obligation to inform our community what their risk is and will be in the future.”

Montalto, of Drexel, agrees that cities will need a neighborhood-by-neighborhood approach. Cities need to be “thinking differently ... in a localized and customized way,” he said.

Philadelphia can’t just rip up all of its pipes and replace them with bigger ones, Montalto said, as that would just send more water during storms into already swollen waterways. The city’s system simply wasn’t designed to handle the “massive amounts of precipitation” climate change is bringing, he said.

Rather, he said, mitigation efforts could be aimed at using landscapes that allow for flooding, but without extensive damage to homes or businesses. Parks and playgrounds, for example, could double as flood control areas by holding or absorbing floodwater. He cited Copenhagen as an example, where streets are designed to convey water during storms to catchment areas.

He’s been working with a neighborhood group in the Eastwick section near Philadelphia International Airport that Hurricane Isaias inundated in August 2020. But the heaviest rains of Ida hit a bit too far east to have an impact on Darby and Cobbs creeks, which border Eastwick.

“The frequency of extreme weather events like the recent storms Ida and Henri continues to highlight the impacts of climate change on all aspects of the water cycle,” said Steve Tambini, executive director of the Delaware River Basin Commission, a federal-interstate agency that oversees water quality and use for the river. “Ida is a teachable moment for water resource managers and water stakeholders.”