The Philly area’s most flood-prone waterways, mapped

Late on a stormy September night, Katie and Anthony Young were watching a horror show. In this instance, it was also a reality show.

A surveillance camera showed 8-foot floodwaters drowning the generator in the rear of their restaurant, Hank’s Place. The water crashed into the dining and kitchen areas, tossing around furniture and emptying the contents of refrigerators.

The remnants of Hurricane Ida in 2021 had overwhelmed the Brandywine Creek at Chadds Ford, one of the region’s most picturesque locales, made famous by a frequent Hank’s customer, artist Andrew Wyeth.

“I don’t think either one of us was anticipating it being that catastrophic,” Katie Young said.

In a region where flooding is a perennial threat, an Inquirer analysis of the area’s most flood-prone waterways found the Brandywine ranks among the elite, based on available U.S. Geological Survey data.

A total of 61 major and moderate floods have occurred since 2005 at Chadds Ford and three other gages on the main stem of the Brandywine and its branches. The Brandywine East Branch near Downingtown has registered more major and moderate floods, with 33 combined, than any gage point in the region.

No. 2 on the list was the Delaware River at Burlington, with seven major and 19 moderate floods, although that is not quite the same as stream flooding. Technically, flooding measured on the five Delaware River gages south of Trenton, including the one at Washington Avenue in South Philly, is “tidal,” since it is influenced by the behavior of the Atlantic Ocean and the Delaware Bay.

The chaotic behavior of the atmosphere may forever be elusive; however, more flooding along the Brandywine, the Delaware, and the rest of the region’s waterways is an absolute certainty.

Various studies have documented increases in extreme precipitation events with the warming of the planet. But humans are affecting the flood calculus immeasurably by hard-topping rain-absorbent vegetation.

A comprehensive analysis of the Brandywine watershed published in April reported that impervious surfaces increased by 15% along the Great Valley’s Route 30 corridor from 2001 to 2020.

Those increases are “significant,” said Gerald Kauffman, director of the University of Delaware’s Water Resources Center, which coauthored the study along with the Brandywine Conservancy and the Chester County Water Resources Authority.

Municipalities welcome tax-generating development. Conversely, Kauffman said: “You get more value if you build next to a greenway. It’s the eternal debate.”

From 2001 to 2020, the population in the Brandywine watershed grew nearly 25%, to 265,000 people, with 150,000 more expected by the end of the century. That’s a lot of rooftops and driveways feeding water into the stream, which empties into the Delaware River.

Along the Delaware, the rising water levels of the Atlantic Ocean and the Delaware Bay are forecast to generate more flooding. The National Oceanic and Atmospheric Administration estimates that sea levels have been rising about an inch every five years.

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While the oceanic salt line — the boundary between ocean and freshwater — usually stays well south of Philly, the tidal pulses contribute to flooding in the city and areas to the north, where the channel narrows, said Amy Shallcross, water resources operations manager at the Delaware River Basin Commission.

The five tidal gages along the Delaware from Newbold Island to Marcus Hook have registered more than 90 significant floods since 2005, according to the Inquirer analysis.

The study was limited to the 33 USGS gages that had a period of record of at least 20 years and list designated flood stages.

Along with the Delaware River sites, other stream gages that appeared in the top 10 list for major and moderate flooding were those on the Perkiomen, Chester, Neshaminy, and Frankford Creeks.

The gage network cannot capture all the episodic flooding from the likes of thunderstorm downpours.

Flood frequency is not the only consideration for siting a gage, said Tyler Madsen, a hydrologist with NOAA’s Middle Atlantic River Forecast Center, in State College. For example, the gages have to be located in areas free of unnatural barriers, such as bridge abutments.

Plus, a major consideration is funding. They are costly to monitor and operate, serving multiple purposes such as measuring water quality and streamflow.

They rely on a variety of funding sources, including state and local governments, that are not available everywhere.

The Brandywine study’s suggested remedies included adding flood-control structures and beefing up warning systems all along the watershed, but warned: “Even with unlimited financial and technological resources, it would be impossible to eliminate all flood risks.”

The Youngs are prepared to live with those risks, come storms or high water.

Said Katie Young: “Hank’s belongs here.”