CDangerous 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 killed more than 220 people.
Experts say floods like this would have been less catastrophic if the region had relied on more environmentally-based solutions such as floodplain parks or green roofs to absorb and reduce the volume 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 the citizens, the artificial aqueduct built along the Poyo River quickly became a deadly funnel of water flowing down the river and hitting cities like Paiporta—called ground-zero of the flood—almost a 10-foot wall of water. Solid-engineering solutions, made from materials like concrete, are designed to drain water very quickly, Alday said, “which means you’re creating a cannon.”
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Climate change is already making extreme floods more frequent and intense and by 2050 100-year flood events are expected to occur at least twice as often as today across 40% of the planet. Urban planners are increasingly exploring eco-based solutions such as temporary lagoons, which can store excess water during heavy rains, and green roofs that help absorb rain and cool buildings.
According to the National Aeronautics and Space Administration, the number of people living in flood-prone areas has increased by 20 to 24 percent since 2000. Surface water flooding is especially dangerous in cities with a high proportion of impervious soil cover and hard surfaces—such as roads and parking lots—that prevent rainwater from being absorbed, increasing the likelihood and impact of flooding. According to some studies, China and the US have the weakest areas, as well as India, Russia and Brazil.
“The earth needs to breathe, it’s a living thing,” said Rosa Pardo Marín, former director general of spatial policy and landscape in the Valencia regional government. While the Valencia region has urbanized and developed rapidly since the 1960s, “the infrastructure is the same, and it should fulfill the same function even if the population density has changed a lot,” he said.
Nature-based solutions can create sponge-like conditions to help absorb spills. However, cities have long relied on so-called gray solutions—improved infrastructure made of materials such as concrete—or stormwater management, including mains and underground water tanks. But nature-based solutions are better at reducing risk because they don’t have the size limits of a hard engineer, Alday explained. “Once the water reaches the limit, there is nowhere to go, there is no plan B.” In Valencia, the 27-kilometer-long Rambla del Poyo river, built to hold a maximum of 1,200 cubic meters, reached 1,800 cubic meters during the October 29 floods, with disastrous results.
According to Alday, nature-based solutions like flood parks can 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 scarce resources—as quickly as possible, we should make the most of every drop, he explained.
Cities from Europe to South East Asia are increasingly turning to eco-based solutions. Half of Europe’s largest cities have a climate adaptation plan, 91% of which include environmentally-friendly solutions, according to the European Environment Agency. Last year, Bangkok experienced two 100-year floods. However, the city’s gray infrastructure meant to channel excess water during storms is only designed for 5-year rainfall events, said architect Kotchakorn Voraakhom. He 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 sloped to ensure that the water can slowly drain to the 480,000 gallon storage pool below, providing additional storage space to absorb heavy rains. When it is not flooded, it can be used as a theater for concerts and events. The park also features the largest green roof in Thailand, three underground rainwater tanks, and a constructed wetland with local plants. The park has a water capacity of 1 million, or the equivalent of a 50-year rainfall event. Voraakhom says: “When it comes to extreme weather, we are at the forefront, so we adapt or we die.”
Back in the 1990s, Copenhagen used to rely on opening the gates of its sewage system and letting it flow into the harbor to avoid flooding. It was a quick fix, but it meant residents couldn’t swim in the bay for decades. After accepting a water quality improvement plan, residents were finally able to enjoy swimming in 2002. But the city quickly realized that climate change could jeopardize its hard-earned gains and has since implemented solutions such as green streets and the Enghaveparken water park.
“Most of the time there is no water in it so now they have to have some leisure time, contribute to biodiversity, and make the city more attractive,” said Jan Rasmussen, program director at the city’s Center for Climate Adaptation.
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Despite the many additional benefits they provide beyond protecting residents from flooding—including improved mental well-being—the difficulty of demonstrating their cost benefits may hold back some cities from implementing more environmentally-friendly solutions. “What benefit do you offer to have more time in the park or increased biodiversity? It’s hard to quantify that, but we still see it as a benefit,” said Rasmussen. It is possible to calculate their cost-benefit, it is more complicated and subject to planning with Excel sheets, he adds.
Considering the joint benefits, eco-based solutions were found to be 42% cheaper and create 36% more value than completely gray infrastructure solutions. Some analyzes have shown that surface solutions have reduced mitigation costs by more than $200 million, compared to conventional pipelines.
However, these solutions also come with trade-offs, says Georgia Destouni, professor of hydrology at Stockholm University. If you rebuild more wetlands, you can create more breeding grounds for mosquitoes that can spread diseases like malaria. Adding floodplains can also increase real estate prices unless managed properly, he notes. “You can’t solve every major flood problem with an environmentally-based solution,” he said. “But it can be a great contribution in combination with other types of measures, and it brings mutual benefits that other measures really can’t.”
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Finding space to implement environmentally-friendly solutions is always a problem in a big city, and it can cause disruptions such as the removal of parking lots to make room, Alday said. But there is no silver bullet for dealing with the complex problems facing cities, he adds. It also requires more long-term thinking, Pardo notes. Many politicians only plan their mandate for four years, “but they need to put the lights on.”
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