In late September 2023, Tropical Storm Ophelia marched up the Atlantic coast of the U.S., stalling over New York City for 12 hours. Parts of the metro area were deluged with more than 8 inches of rain, with Brooklyn experiencing more than 3 inches per hour at the peak of the storm.
Flooding was ubiquitous in all boroughs, with water gushing into subways, railways, and basement apartments and businesses. Even street-level buildings were not immune; the Russian Orthodox cathedral in Brooklyn’s Williamsburg neighborhood reported floodwaters came in from the street, spilling in through the doors.
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There are numerous infrastructure-based methods to combat flooding in such scenarios. One that Brooklyn has embraced is porous pavement.
Metropolitan areas like New York City are ripe for flooding during these lingering storm systems. Endless hard surfaces and pavement and aging systems of stormwater infrastructure mean that cities are prone to flooding during storms. And climate change is exacerbating the issue.
“Climate change is bringing with it rainstorms that can overwhelm our sewers and cause flooding across the five boroughs, which is why we are investing in tools that will divert rainwater away from the sewer system, such as porous pavement,” said New York City Department of Environmental Protection Commissioner Rohit T. Aggarwala in a press release.
To help alleviate flooding, the DEP partnered with the Department of Design and Construction to install 7 miles of porous pavement in Brooklyn. The porous pavement was installed in strategic locations along the curbs of streets. It will intercept flowing stormwater to help alleviate stress on the sewer system.
The need to move water
Pavement, by design, is impervious to liquids – water runs along the surface instead of soaking into the ground. In highly paved cities, this effect is amplified by the sheer number of paved surfaces.
“Every time we put asphalt or concrete down, we create runoff – all this water needs to get collected and moved out,” said Somayeh Nassiri, Ph.D., a pavement researcher and associate professor of civil and environmental engineering at the University of California, Davis.
The climate is changing, and many places around the world are experiencing more storms with heavier rainfall with atmospheric rivers and slow-moving storm systems. “We got 3 1/2 inches of rain here in Davis yesterday, and today we’re going to get another inch,” Nassiri said on a recent fall day. While California could use the precipitation, it would be better if it was spread out over a longer time period. Many stormwater systems were designed long ago, Nassiri explained, and do not have the capacity to handle today’s storms.
Replacing impervious pavement with something porous is one way a city can move and store water from city streets. Unlike traditional pavement, porous pavements are designed to have big open pore spaces. “They look like Rice Krispies Treats,” Nassiri said.
The open pores allow water to quickly infiltrate and collect under the paved surfaces. Depending on the site conditions, the water can either soak into the ground below the pavement or be connected to the sewer system. Sometimes, engineers will install gravel layers a few feet below the pavement, to add water storage to an area.
“It’s basically added water storage,” Nassiri explained. The storage gives a city a longer lead time to deal with the incoming rainfall. For example, during a storm a city might reach its peak ability to flush out stormwater two hours in. But adding porous pavement could extend that limit to three hours, giving the stormwater system extra time to flush out the runoff.
Many factors to consider
In Brooklyn, the DEP and DDC placed porous pavement along curbs that added up to 7 miles of pavement. Engineers took soil samples beneath the roadways to determine the ability of the ground to absorb water. In the areas where infiltration was good, the team removed the roadway and underlying material, creating troughs that were 24 inches deep. Drainage cells and stones were added under the pavement to increase storage and drainage and provide structural support.
Nassiri notes that porous pavement isn’t a good fit everywhere, as the installation can have a number of challenges in terms of life-cycle costs and connecting to existing infrastructure. Weighing the costs of removing existing pavements and subsurfaces, adding crushed stone, and considering the type of traffic an area can handle are all factors in placing porous pavements. She added that for projects like these, collaboration between stakeholders is important. City engineers, planners, developers, and private owners should come together to see if porous pavement is the right choice for an area.
In their press release, stakeholders from New York state, the New York City Council, and community boards agreed that the project would be a helpful tool in fighting flooding in Brooklyn and other New York City boroughs.
“I am thrilled to see the installation of 7 miles of porous pavement in Brooklyn. As a resident and representative of this vibrant borough, I know firsthand how critical it is to address flooding and protect the health of New York Harbor,” state Sen. Iwen Chu said in the press release. “This project is not just a significant step forward in our efforts to combat these issues, it is also an important investment in the safety and well-being of our communities.”
