SCIENCE
When a tsunami slams into a coastal area, parks with rolling hills could provide about as much protection as towering seawalls, according to a new paper published in the journal Proceedings of the National Academy of Sciences.
These tsunami mitigation parks, as they’re called, are designed to combine the protection of an engineered landscape with the benefits of a more natural setting. It’s especially appealing to less-affluent countries looking for alternatives to pricey seawalls that also preserve coastal economies and ways of life.
“You can build a wall against anything. The wall doesn’t require any understanding of the phenomena you’re actually trying to protect against,” said Jenny Suckale, a senior author of the study and an assistant professor of geophysics at Stanford University. “But these coastal mitigation parks, they do; they actually target the main problem.”
And that problem, Suckale says, is not the water itself but the enormous amount of energy the it carries in a tsunami. Earthquakes, volcanic eruptions, and other movements along the seafloor can trigger tsunamis, releasing huge amounts of energy. That’s why a tsunami with waves just 30cm high can cause tremendous damage, sweeping people off their feet and homes off their foundations.
“The water hits you with full force,” she said. “The reason you die is because you’re falling, and the water is very fast and throwing all this stuff at you.” Seawalls can block some of that energy, but they can also block access to the water for people whose livelihoods depend on it. And when a tsunami hits, the walls can break apart into debris that smashes through communities along with the waves.
Tsunami mitigation parks are being developed in Chile, Indonesia, and Japan and researchers want to learn how well they might hold up against a wall of water if and when a tsunami hits.
Suckale and her colleagues used computer models to determine what happens when a tsunami crashes against a row of hills to understand how these mitigation parks might work and what can be done to improve their design. They found the hills partially deflect the waves and can reduce the amount of kinetic energy the water brings onshore from waves that aren’t significantly taller than the hills themselves. In doing so, the hills can provide a similar level of protection as a seawall. That combination of walls and hills has recently been adopted in Constitución, Chile, and Miyagi Prefecture, Japan — both of which suffered devastating losses from tsunamis in the past decade.
The study also found that most of the defence offered by mitigation parks comes from the land itself, not from vegetation planted in the hope of dissipating the force of the waves. The vegetation can, however, prevent waves from eating away at the hills.
Using staggered rows of hills that grow smaller further inland, with a buffer zone positioned immediately behind them, might help diminish the risk posed by concentrated flows of water. No matter what communities choose, customising the design of the park to each coastline is key, Suckale adds. She argues mitigation parks could effectively push communities to settle a little further away from shore where they’re much safer — while still allowing them to reach the water to make a living or to enjoy the shore. They also offer a place for residents to seek shelter atop the hills during a tsunami since it’s best to evacuate to higher ground when there isn’t enough time to flee far enough inland, she notes.
Seawalls are still the conventional way many coastal communities seek to protect themselves from a tsunami. Japan has spent more than US$12 billion on almost 400km of seawalls since the 2011 Fukushima earthquake and tsunami. But the walls have drawn criticism from some professional fishers. “It feels like we’re in jail, even though we haven’t done anything bad,” oyster fisher Atsushi Fujita told Reuters in 2018.
Experts in countries such as Indonesia, which suffered the deadliest tsunami ever recorded in 2004 and several tsunamis since, are hopeful mitigation parks could provide its residents with greater protection and some peace of mind.
ABOVE Adding extra vegetation cover along the shoreline in Constitución, Chile