Founded in 1772 as a mill town, Ellicott City has for the past several decades catered to tourists with its historic charm and antique shops, said Shawn Gladden, the executive director of the Howard County Historical Society.

The town’s location in the Patapsco River Valley has made it susceptible to flooding. On Sunday, some of its roughly 65,000 residents were already drawing comparisons to the two biggest floods in its recorded history, the flood of 1868 and flooding caused by Hurricane Agnes in 1972. “They call them the 100-year floods,” Mr. Gladden said. “And this one may rival that.”

The National Weather Service had issued warnings about potential floods on Saturday, and at one point sent a sharply worded bulletin: “This is a particularly dangerous situation. Seek higher ground now!”

Despite the warnings, the ferocity of the waters came as a surprise to many workers and residents in downtown Ellicott City, trapping them where they were. Inside Bean Hollow, the water rose swiftly to about three feet. With no time to flee, the coffee shop’s workers scrambled to an apartment on the second floor.

“This was a true flash flood,” said Jim Bolton, who has owned Bean Hollow with his wife for about 14 years. “By the time people realized there was a potential hazard, it was far too late for anyone to do anything.”

Mr. Bolton said he expected the rebuilding of Ellicott City to be a long road. “It’s decimated right now,” he said. “But it’s certainly had its share of floods and fires and all sorts of madness over the years. It will rise again.”