After Ida’s 800-year flood, the battle to protect the Brandywine watershed is about to escalate

Flowing through the picturesque settings that captured the artistic sensibilities of the Wyeths, the Brandywine Creek and its lushly foliated banks define one of the most aesthetically attractive landscapes in the region.

But the Brandywine has a not-so-idyllic distinction: Among waterways that have official gauges, it is one of the region’s most flood-prone, says Raymond Kruzdlo, the senior hydrologist at the National Weather Service Office in Mount Holly.

In 101 years of record-keeping, it has experienced 146 floods, according to data from the Middle Atlantic River Forecast Center. And the tide isn’t turning: Of those, 55 occurred in the first two decades of the 21st century.

» READ MORE: Ida was unlike anything experienced in the period of record along the Brandywine

Then came the storm named Ida on Sept. 1, 2021, the worst of them all. Initial calculations affirmed that the force of the rushing water was unprecedented. But further analysis showed that the ferocity had been understated, that the force actually was of an “eye-popping magnitude,” said Gerald Kauffman Jr., director of the University of Delaware Water Resources Center.

Ida turned out to be a battle cry for the historic watershed that has been weathering extreme rains and increasing development.

The Delaware center is taking part in a $500,000, nine-month study, announced Aug. 22, that will be part-forensics — as in, what is with all this flooding — and part what can be done about it, says the Brandywine Conservancy, which is leading it.

The potential fixes — the likes of culverts, property buyouts, new signage, dam removals — won’t be cheap. “Obviously, it’s going to be a lot of money,” maybe more than $20 million, to mitigate the watershed’s relentless flood hazards, said Seung Ah Byun, executive director of Chester County Water Resources Authority, which is also involved, along with Delaware County and the State of Delaware, which have pieces of the Brandywine.

But it’s not just about the Brandywine, said the conservancy’s Grant DeCosta: “What we do on the Brandywine, we think, is going to be transferable to other watersheds.”

Among all the verifiable record crests incited by the remnants of Ida in the Philly region, at 21.04 feet the Brandywine at Chadds Ford evidently was No. 1 in terms of exceeding the previous record, the 17.15 feet recorded during Tropical Storm Floyd in 1999.

» READ MORE: A timeline of Ida's destruction in the Philadelphia region

Floyd’s crest was a foot higher than that of Agnes, which occurred in June 1972, almost exactly a year after the Brandywine Museum opened in a 19th-century mill building. Agnes was one of five floods that year, but neither Agnes nor Floyd approached Ida.

Initially, it was determined that water rushed past the gauge, located near the museum, at an unprecedented 33,000 cubic feet per second. But that turned out to be an undercount. Kauffman said Wednesday that an analysis of drone footage and modeling showed that the flow actually was 44,000 cubic feet per second, a level not reached in at least 200 years.

The trigger was a once-in-800-year downpour 18 miles upstream of the gauge, said Kauffman, which sent water cascading downhill from 1,000 feet above sea level. A fateful engineering decision decades earlier to narrow the channel not far from the gauge gave an extra jolt to the rising water levels.

The deluges terrorized Modena, which recorded more than 7 inches of rain in six hours; Coatesville; Downingtown; and other communities. Hours later the waters smashed into the Brandywine River Museum — home of some of the region’s most significant artworks, including Andrew Wyeth paintings that famously spoke to the valley’s tranquility. Ida caused an estimated $10 million in damage to the museum complex, which remained closed for nearly three months.

Fortunately, no one was injured and all the Wyeth paintings and other artworks were spared.

» READ MORE: Ida's flood shut down the Brandywine Museum for more than 10 weeks

The East and West Branches of the Brandywine have their sources in western Chester County, forming a “V” before they join the main stem of the Brandywine Creek at Lenape, eventually emptying into the Cristiana River, in Delaware.

But the boundaries and even the identities of various affiliated waterways aren’t always clear. Chester County’s Byun points out that some of the capillaries feeding the creek don’t even have names. In fact, the U.S. Geological Survey estimates that more than 90% of all bodies of water in the country are nameless. Plus, some of the flooding is storm water-related.

The conservancy says one of the study’s first tasks will be to identify the trouble spots.

The Brandywine watershed covers close to two-fifths of Chester County. Much of the land is off-limits to builders, and as of 2015, less than 10% of the watershed — about 13 square miles — were paved over or built upon, Kauffman said.

But the pace of development jumped significantly between 2011 and 2015, he said, as the nation pulled out of a recession, and runoff has been a significant contributor to flooding.

Byun points out that Chester County is the fastest-growing county in the state, projected to add more than 100,000 people by 2045, increasing the population to 650,000.

After diagnosing the issues, county and federal money willing, the plan would address them on an “à la carte” basis, said the conservancy’s DeCosta.

Remedies might range from the labor-intensive and expensive, such as widening that downstream channel that was narrowed sometime in the 19th century to accommodate a bridge abutment, to the simpler, such as adding signage, said Kauffman. One might read, “This is the high-water mark of Ida,” he said.

» READ MORE: NOAA sees Philly at a high risk of extreme rains.

Another possibility would be to organize a “flood response day,” he said, summoning emergency managers for a “dry run” (pun accidental, he says) to go over what they would do in the event of an Ida 2.0.

“There’s a lot of ready-made solutions,” he added. He said he expected the study to be completed by May because “we’re not starting from scratch.”