As urban populations grow and temperatures rise, city dwellers will rely on cool, green, shady places as never before.
Green infrastructure—such as green roofs, rain gardens, urban forests—offer sustainable solutions to the water management and heat dissipation challenges unique to cities. Yet for these engineered systems to succeed, they must account for a critical and unpredictable factor: human behavior.
Patricia J. Culligan, the Matthew H. McCloskey Dean of Engineering and professor of civil and environmental engineering and earth sciences at the University of Notre Dame, is an expert in geo-environmental engineering. She uses advanced technologies to improve the management of water, energy and environmental resources, while prioritizing human needs and well-being.
In this conversation, Culligan discusses the ways in which green urban infrastructure can complement traditional “gray” infrastructure, such as water treatment plants and underground storage systems, and offer a human-centric approach to the uncertainties of climate change.
How can green infrastructure make cities more climate resilient?
Green infrastructure absorbs stormwater, reducing the amount that enters sewer systems and decreasing the volume conveyed by massive, piped network systems to wastewater facilities. Street trees and other urban greening strategies also cool and filter air and block damaging winds.
Green infrastructure is flexible. It’s made up of many small components—pocket parks, street trees, rain gardens—that can quickly be added or modified to meet new needs. If a city begins to experience more rain, it can relatively inexpensively—at least, when compared with alternative hard infrastructure solutions—increase the green infrastructure. Less effective components can be quickly replaced and new technologies incorporated.
Surveys show that people value the social and cultural benefits of green infrastructure. They have a particularly deep connection with trees. Strategies for increasing climate resilience that enhance people’s well-being through green space are more likely to be embraced by the community and gain momentum than those that don’t.
“Towards the intentional multifunctionality of urban green infrastructure: a paradox of choice?” Lauren M. Cook, Kelly D. Good, Marco Moretti, Peleg Kremer, Bridget Wadzuk, Robert Traver & Virginia Smith. npj Urban Sustainability volume 4, Article number: 12 (2024) Creative Commons License
What are the challenges of green infrastructure?
Finding a cost-effective way to monitor these systems is a challenge. Sometimes, you can use sensors to do that. I’ve worked with electrical engineers on monitoring soil moisture at the base of trees, using sensors connected to wireless networks with long-distance capability.
Developing a sensor that could gather multiple measurements and relay them to a cloud-based database would allow us to study not just one or two rain gardens, green roofs, or street trees, but tens of thousands.
Such a database that recorded how different green infrastructure elements perform in different geographical areas over time could lead to more effective management of those systems nationally and even globally.
Some of your latest research, conducted with colleagues at Barnard College, Columbia University and the Icahn School of Medicine at Mount Sinai, suggests a connection between green infrastructure and social justice issues. Could you elaborate on that connection?
Low-income neighborhoods have significantly less green space and tree canopy cover than wealthier neighborhoods. This inequality won’t be solved by simply planting more trees in vulnerable neighborhoods. We need to plant younger trees that grow faster, and make data-informed decisions about which tree species to plant.
Your research also highlights the significance of stewardship to the success of green infrastructure. How can stewardship be encouraged?
Compromise is important. I worked with plant ecologists who had the admirable goal of restoring some of New York City’s native grasses by planting them in rain gardens. But, as one of my collaborators pointed out, these native grasses ‘woke up’ at mid-day, moved some water around, and then went back to sleep. In terms of water management, they weren’t doing their job.
These native grasses weren’t the people’s choice either. Flowers or fruits like tomato plants, for instance, might have been far more popular and perhaps better at retaining and releasing water. We could think of a rain garden as a mini community garden.
When people look after their neighborhood trees, or rain and local community gardens, they feel invested in what grows there, and that’s enormously beneficial to their success.
How can engineers be better trained to deal with these challenges?
Most engineers aren’t trained to think about the ways in which the engineered world intersects with the natural and human.
If you design a pipe, you can come back in a few years and expect that pipe to function as you intended. You can’t have similar expectations when people or nature are involved. Human beings sometimes rip up rain garden plants. Birds drop seeds in these gardens, and unwanted plants take over. Heatwaves and hurricanes can take down carefully selected and planted trees.
As engineers, we optimize our solutions for things like stormwater management, energy efficiency, cost-effectiveness alone. But I’ve come to realize that that approach doesn’t yield optimal solutions at all. Optimal solutions take into consideration the ways in which engineered systems interact with the natural world, and how they affect the health and well-being of people.
— Karla Cruise, Notre Dame Engineering; Photo of Dean Culligan by Wes Evard, Notre Dame Engineering
Hero photo shows Patricia J. Culligan with two of her students on a New York City green roof. Photo courtesy of Jane Nisselson.