Summer often conjures thoughts of beach vacations, ice cream cones, and backyard barbecues. But many also associate the season with heavy rains and flooding – thanks to higher temperatures helping build up extra pressure and moisture in the atmosphere, resulting in severe storms.
Urban flooding, in particular, has become a growing threat, with the 2017 National Climate Assessment estimating that heavy downpours have not only increased over the last few decades, but will continue to rise across all U.S. regions in the future.
Further reading:
- How civil engineers must adapt to the new risks of flooding
- Analyzing urban flood processes at a smaller scale
- Climate tools map hazards into the future
“This isn’t just happening in the summer anymore. And it’s not (just) our coastal cities that are at risk,” said Katerina Boukin, a structural engineer by training, who is now pursuing a doctorate in civil and environmental engineering at the Massachusetts Institute of Technology. “Flash floods, rain-driven flooding is the largest hazard for 92% of the world. And the costs that result from these floods are incredibly high.”
Many of us remember New York City’s 28th Street subway station filled with water in 2021 or the streets of Houston in 2017 looking more like a lake than a downtown metro area after Hurricane Harvey, which brought Houston’s third 500-year flood in three years. Recent estimates peg direct annual flooding damage to roads and railways worldwide at $3.1 billion to $22 billion – but, as those salient memories remind us, flooding affects much more than transportation infrastructure. The University of Maryland’s Center for Disaster Resilience reports that urban flooding is a significant and growing source of economic loss, usually hitting lower-income neighborhoods hardest.
“Predicting flooding’s impact, unfortunately, is quite complicated – as is trying to come up with the right solutions to mitigate it,” said Jacob Napieralski, Ph.D., a professor of geology and director of the Environmental Interpretive Center at the University of Michigan–Dearborn. “The environment is changing, and (urban flooding) – not just in summer but in the winter months – has become a collective problem that is going to require creative solutions and interdisciplinary thinking to address.”
Where will the water go?
In May, the White House announced $41 million in grants, through the Environmental Protection Agency, to upgrade sewer and stormwater infrastructure to better manage future flooding. While this is welcome news, it is not always clear to local governments and municipalities where to make improvements – and how to best assess flooding risk to make informed decisions.
“We have so many textures in our cities – different areas were built in different times and in different stages, and it can make it very hard to predict which parts of the city will be most affected by flooding,” Boukin said.
As part of her doctoral research, Boukin is developing new models to understand the impact of flood intensity on urban areas. She said that conventional models look at the capacity of stormwater drainage systems – what should be a simple equation involving time and the volume of water. But, she said, this is a one-dimensional approach to a three-dimensional problem. New construction or other changes to the landscape can alter the flow of the water.
“Some of the water will never get to a drainage point because it’s stuck between two buildings or at some other local point,” she said. “The way we designed drainage before doesn’t work anymore. While they were correct on calculating the capacity of a pipe, they are not correct on whether the water ever gets to that pipe. Newer methods that can simulate the topography, the textures of cities, can better tell us what areas are most at risk – and whether floodwaters will ever get to the expected drainage point.”
This, she said, can not only tell engineers where they might make improvements to divert water to help mitigate the effects of urban flooding but also help update emergency management planning. Given that flooding isn’t just “spatial but temporal” – meaning some areas can flood quite quickly – knowing which areas are at highest risk for flash floods benefits evacuation and rescue efforts.
“In the Northeast, the buildings are heavy with basements. People live in basements, and if they do not know this dynamic, extremely fast-paced flooding is coming, they cannot get out in time. These basements can flood in minutes,” Boukin said. “But if we know the buildings in these areas are at the highest risk, we can warn them. They can evacuate.”
Valeriy Ivanov, Ph.D., a professor of civil and environmental engineering at the University of Michigan in Ann Arbor, agreed. The ability to build more complex simulation models, with varied sources of data, could even help civil engineers reassess their design criteria for stormwater systems and floodways in the future.
“I’m talking about data about buildings, bridges, roads, stormwater infrastructure, weather – the more data we can bring into these models, the better,” he said. “When we can start adding in the structure of the physics, we can really look at how water is moving through the landscape and understand the dynamics of that movement. Without proper inputs, we won’t produce accurate outputs. We need the right data so we can have the kind of design criteria that can better manage the variability we are seeing in precipitation events.”
Mitigation in the meantime
Unfortunately, one of the only certainties about urban flooding, Napieralski said, is uncertainty. The very nature of storms means that we can never know when they will hit, how severe they will be, or where they will cause the most damage. Yet, he said, cities can and should take a hard look at the ratio between impervious material, such as concrete rooftops, and tree canopy as they look to protect their infrastructure and citizens from flood damage.
“It’s human nature for us to think we can out-engineer Mother Nature – but I think we have to go back to Mother Nature and put some nature-based solutions back into the urban environment to help with this,” he said. “We know that parks and green spaces can act as a sort of sponge with excess water.”
Boukin, for her part, said that green spaces can make a difference – but added that here, too, data can play a vital role in determining where such solutions can do the most good.
“Mitigation and adaptation strategies should be created with a strong knowledge base,” she said. “There’s a lot of talk about nature-based solutions. Certainly, green spaces can help because they can infiltrate and store water. But if the floodwater is not flowing toward your green space, you’ve just spent millions on something that won’t help you. The key to dealing with this problem starts with understanding your risk because that makes you better aware, able to strategize, and able to design better.”
As civil engineers and infrastructure experts work toward flood mitigation, however, Napieralski, Boukin, and Ivanov agree that any solutions should be multidisciplinary – as it is a problem that is going to require many voices.
“It’s really hard to unbuild built environments,” Napieralski said. “But if we can bring together engineers, scientists, and other stakeholders, we can get creative and find ways to design infrastructure that can hold up even though environmental conditions are going to continue changing.”
