“You can have a rainy day where you get less than a tenth of an inch per hour, but today’s rainfall was more than 10 times that amount,” he said.
And in a paved-over city with absorption problems, that makes all the difference.
Fall in the Northeast, when hurricane remnants and nor’easters increasingly come through, is prone to continuous, heavy rainfall, said Upmanu Lall, an engineering professor and the director of the Columbia Water Center at Columbia University. “If we just had a cloudburst during the summer, nothing much happens, because it’s possible to drain out,” he said.
But with climate change, sustained rainfall is now happening in the summer as well, if the recent downpours in July, and subsequent catastrophic flooding that struck parts of Vermont and the Hudson Valley, are any example.
And just as no two storms are alike, flooding also can vary, depending on whether it comes from the coast or the sky, Mr. Kruczkiewicz said.
“In New York City, when we think of coastal flooding, there are areas we know that are high risk,” he said. “But flash flooding has nothing to do with tides,” he said. “It’s coming from the sky and it’s driven by intense precipitation,” so flash floods can pop up anywhere there is poor drainage infrastructure.
“The water from flash floods tends to rise faster than any kind of flood,” Mr. Kruczkiewicz said. And when you add ailing infrastructure or poor drainage into the mix, he added, all bets are off.