The epic flash flooding that hit New York City and Long Island last week, reaching a level that was technically considered “mild”, was a moment where a changing climate collided with a simple but stubborn problem with local infrastructure—when too much rain falls in too short a time and a city’s storm sewer network gets overwhelmed, “it backs up”.
The storm dumped more than five inches (12.5 centimetres) of rain on parts of the area last Friday, “flooding subway stations, roads, and a terminal at LaGuardia Airport.,” NBC News reported. The inundation prompted an emergency message from New York State Governor Kathy Hochul to help residents through a “life-threatening” storm.
“If people decide to venture out in a vehicle, they do so at their own peril,” Hochul warned. “Because even six inches of rain, one foot of rain, it may look pretty innocuous, it’s safe, but that is a condition where your vehicle can be swept away.”
She urged residents to get an emergency plan in place before they need it. “Don’t wait until it’s up to your knees or higher,” she advised. “By then, it could be a barrier to have access to safely get out the door.”
On one level, the severity of the storm was a matter of simple math. “All drainage systems have their limitations and New York City’s is 1.75 inches of rainfall per hour,” the New York Times explained. “Unfortunately for many New Yorkers, the storm that deluged the region on Friday dropped more than two inches between 8 AM and 9 AM—and then kept on coming.”
It wasn’t the first and won’t be the last time the city’s drains, pipes, and water treatment plants received more volume than they could handle.
Climate Changes Faster than Infrastructure
“This changing weather pattern is the result of climate change, and the sad reality is our climate is changing faster than our infrastructure can respond,” said Rohit Aggarwala, commissioner of the New York City Department of Environmental Protection.
“We’re in this new territory where we’re seeing higher intensity rainfalls like this,” added Daniel Zarrilli, a former climate policy advisor to the New York mayor who now advises Columbia University on climate and sustainability. “Once you’ve exceeded the capacity of the sewers themselves, that’s what causes these backups. When the pipes can’t handle it, it backs up.”
Another problem for New York: about 60% of the city’s 7,400-mile drainage system carries stormwater and sewage in the same pipes. When the flow of water is more than twice the system’s design capacity, a mix of rain and untreated sewage flows into local basements and waterways.
“When it gets inundated to this degree, it backflows,” Dave Balkan of Balkan Sewer & Water Main Service told the Times. “That’s when regular people start having sewer water bursting out of their drains or their basement toilets.”
And despite the “tons of calls” he was receiving Friday from disgusted and distraught homeowners, he said there was nothing to be done until the system cleared and began pulling the water back. Bigger-picture solutions will call for “a lot of investment in infrastructure and a lot of creativity,” Zarrilli said.
Apart from the sheer size of New York City, there wasn’t that much difference between last week’s flash flooding and the record rainstorm in mid-August that sent 300 million litres of untreated wastewater into the Ottawa River. Ottawa’s C$232-million Combined Sewer Storage Tunnel hit its limit in that storm, just three years after it opened, and a “mayhem” of flooded basements and health risks was the results, local media reported at the time.
“It was always known that if there were very large storms the city would be faced with a choice of either flooding sewage into people’s basements or into the river,” Toronto city councillor Dianne Saxe said at the time. Knowing that the storms “are going to get worse and worse,” she added, communities can and must do better with a problem that she summed up with one choice adjective: disgusting.
“You’re talking about human poop washing up on the beaches, contaminating the fish, contaminating the sediment, everything that lives in the water, everyone who might want to swim in the water or sail on the water,” said Saxe, who previously served as Ontario environment commissioner. “It’s filth. And it’s dangerous filth that spreads disease.”
Stoking Bigger Downpours
The other takeaway from last week’s flooding in New York is that otherwise unremarkable rainstorms are doing more damage and becoming more frequent.
“It has been raining a lot in New York, which hasn’t seen a September this wet in over a century,” the Times wrote. “Climate change is very likely stoking more ominous and lengthy downpours because as the atmosphere heats up, it can hold more moisture.”
But in contrast to a single, big, fierce, storm, Friday’s flooding was caused by “smaller-scale features, like bands of heavy rainfall and scattered thunderstorms,” said Greg Carbin, chief of forecast operations at the U.S. National Weather Service’s Weather Prediction Center. “Low-pressure systems like nor’easters now have greater amounts of water vapour available to them,” and “with a warmer Atlantic Ocean combining with warmer air, the atmosphere is primed to produce more rainfall.”
Those events are still smaller than a typical, severe storm, making them “difficult to predict with any significant lead time,” he added. But if they drop enough water, in a heavily-paved city limited absorptive capacity, “that makes all the difference,” the Times wrote.
Storms Meet Sea Level Rise
And yet, to scientists, Friday’s rain still only counted as “mild”, since coastal flooding was just two feet above high tide. “The numbers on the rainfall are a piece of it, but you would expect coastal flooding of this sort every year,” Kelly Van Baalen, project manager of Climate Central’s sea level rise team, told the HEATED climate newsletter.
“The coastal flooding aspect of Friday’s story is important to understand,” HEATED writes, citing Van Baalen, “as it paints a more accurate picture of the danger New York City is truly facing as the climate continues to destabilize due to fossil fuels, deforestation, industrial agriculture, and mass consumption.” Climate change is shifting the city’s rainfall patterns, writes correspondent Arielle Samuelson, but it’s also slowly raising sea levels, making storms and high tides more likely to reach city surfaces.
“If we’re only looking at the rainfall piece, we’re only seeing half of the picture—and blinding ourselves to the dire need for solutions,” she warns.
On Tuesday, after absorbing its own latest round of heavy rainfall earlier in the day, Montreal announced plans for 30 new sponge parks and 400 sponge sidewalks by 2025, adding to the seven sponge parks and 800 sponge sidewalks it has installed since 2022, CBC reports.
“The impact of heavy rainfall can be reduced by redirecting water to the river, retaining it until the sewer system is available, or gradually releasing it through the ground,” the city said in a release. “Unlike underground developments, these surface developments are an effective and more economical way to reduce the impacts of heavy rains.”