How Cities Are Using Nature-Based Solutions to Tackle Floods

Catastrophic floods across Europe have caused death and destruction in recent months, including in Austria, the Czech Republic, Poland and Romania in September and the Spanish region of Valencia in October, where they claimed more than 220 lives.

Experts say floods like these could have been less disastrous if the region counted on more nature-based solutions such as floodable parks or green roofs to absorb and slow down the large masses of water. “Instead of fighting nature, we need to start working with it,” said Iñaki Alday, dean of the Tulane School of Architecture in New Orleans.

In Valencia, instead of protecting citizens, an artificial waterway built alongside the Poyo River quickly became a deadly funnel for the water rushing downstream and hitting cities such as Paiporta—dubbed the ground-zero of the floods—in the form of a nearly 10-foot-tall wall of water. Hard engineering solutions, made from materials like concrete, are designed to get rid of water as fast as possible, says Alday, “which means you are creating a cannon.”

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Climate change is already making extreme flooding more frequent and intense and by 2050 100-year flood events are projected to occur at least twice as frequently as today across 40% of the planet. Urban planners are increasingly exploring nature-based solutions such as temporary lagoons, which can store excess water during heavy rains, and green roofs that help to absorb rain and cool down buildings.

According to the National Aeronautics and Space Administration, the proportion of people living in flood-prone areas has risen by 20% to 24% since 2000. Surface water flooding is especially dangerous for cities with a high proportion of impervious soil coverage and hard surfaces—such as roads and parking lots—that prevent rainwater from being absorbed, increasing the likelihood and impact of flooding. According to one study, China and the U.S.have the most impervious areas, along with India, Russia and Brazil.

“The earth needs to breathe, it’s a living organism,” says Rosa Pardo Marín, the former director general of territorial policy and landscape with the regional government of Valencia. While the region of Valencia has urbanized and industrialized rapidly since the 1960s, "the infrastructure is the same, and it has to fill the same function even if the density of the population has changed a lot," she says.

Nature-based solutions can create more sponge-like conditions to help absorb run-off. Yet, cities have long depended on so-called gray solutions—engineered infrastructure made of materials including concrete—or stormwater management, including large pipes and underground water tanks. But nature-based solutions are better at reducing risk because they don’t have the limitations of the dimensions of the hard engineering, explains Alday. “Once the water reaches the limit, there’s nowhere for it to go, there’s no plan B.” In Valencia, the 27-mile-long watercourse Rambla del Poyo, built to hold a maximum of 1,200 cubic meters, reached 1,800 cubic meters during the Oct. 29 floods, with disastrous consequences.

According to Alday, nature-based solutions like floodable parks can also help cities deal with drought by storing water that can be used later. Instead of trying to get rid of water—one of the Earth’s most precious and increasingly scarce resources—as fast as possible, we must make the most of each drop, he explained.

Cities from Europe to South East Asia are increasingly turning to nature-based solutions. Half of Europe’s larger cities have a climate adaptation plan, 91% of which include nature-based solutions, according to the European Environment Agency. Last year, Bangkok experienced two 100-year flood events. Yet, much of the city’s gray infrastructure meant to channel excess water during storms is only built for 5-year rain events, says landscape architect Kotchakorn Voraakhom. She designed the 12-acre Chulalongkorn University Centenary Park, which opened in 2017 and was the city’s first new park in 30 years.

The park is tilted to ensure water can slowly trickle down to a 480,000-gallon retention pond at the bottom, providing extra storage space to absorb heavy rains. When it’s not flooded, it can be used as an amphitheater for concerts and events. The park also counts on the biggest green roof in Thailand, three underground rainwater tanks, and a constructed wetland featuring local plants. The park has a total water capacity of 1 million gallons, or the equivalent of a 50-year rain event. “When it comes to extreme climate, we are on the front row, so either we adapt or die,” says Voraakhom.

Back in the 1990s, Copenhagen used to rely on opening the gates to its sewage system and letting it flow into the harbor to avoid flooding. It was a quick fix, but meant that citizens couldn’t swim in the harbor for decades. After adopting a plan to improve water quality, locals were finally able to enjoy a swim in 2002. But the city soon realized that climate change could endanger the hard-won gains and has since implemented solutions such as green roads and the floodable park Enghaveparken.

“Most of the time there’s no water in them so in the meantime they have to have some recreational value, contribute to biodiversity, and make the city more attractive,” says Jan Rasmussen, project director at the city’s Center for Climate Adaptation.

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Despite the many additional benefits they generate beyond protecting citizens from floods—including improved mental well-being —the difficulty of demonstrating their cost-benefit might hold some cities back from implementing more nature-based solutions. “What value do you give to having a more relaxed time in the park or increased biodiversity? That’s hard to calculate, but we still see them as benefits,” says Rasmussen. It is possible to calculate their cost-benefit, it’s just more complex and less codified by Excel sheets, he adds.

When co-benefits are taken into account, nature-based solutions have been found to be on average 42% cheaper and create 36% more value than fully gray infrastructure solutions. Another analysis showed that surface solutions reduced mitigation costs by over $200 million, compared to conventional piping. 

Yet, these solutions also come with trade-offs, says Georgia Destouni, a professor of hydrology at Stockholm University. If you reconstruct a lot of wetlands, you may create more breeding grounds for mosquitos that can spread diseases such as malaria. Installing flood parks could also push up housing prices unless properly regulated, she notes. “You cannot solve the whole problem of really massive flooding just with a nature-based solution,” she says. “But it can be a great contribution combined with other types of measures, and it also brings co-benefits that the other measures really can't.”

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Finding the space to implement nature-based solutions is always going to be an issue in a big city, and may cause inconveniences like the removal of parking spaces to make room, says Alday. But there’s no one silver bullet to address the complex issues facing cities, he adds. It also requires more long-term thinking, Pardo notes. Many politicians only plan for their four-year mandate, “but they need to put on the high beam headlights.”