Researchers Propose Alternative to Seawalls for Tsunami Defense

A concrete wall on the coast of Japan. (Image credit: iStock)
A concrete wall on the coast of Japan. (Image credit: iStock)

Published May 4, 2020 6:41 PM by The Maritime Executive

Stanford University researchers have proposed that rows of green hills strategically arranged along coastlines can help prevent the destruction caused by tsunamis and may offer a better option than towering seawalls.

Giant seawalls are the conventional approach to mitigating tsunami risk. Japan, for example, has built hundreds of miles of concrete walls, taller than 40 feet in some places, at a cost of more than $12 billion since tsunami waves crashed through seawalls and flattened coastal communities throughout eastern Japan in March 2011.

But seawalls tend to be expensive to build, tough on local tourism and fishing industries, disruptive to coastal communities and environmentally destructive. Failures can be catastrophic, said senior study author Jenny Suckale, an assistant professor of geophysics in the School of Earth, Energy & Environmental Sciences (Stanford Earth). Seawalls can not only create a false sense of security that can discourage swift evacuations, she says, they can also end up breaking apart into blocks of rubble that tsunami waves then toss throughout a city.

Suckale and her team, including scientists from the Naval Postgraduate School, the New Jersey Institute of Technology, MIT and Indonesia’s Ministry of Marine Affairs and Fisheries, attempted to quantify how tsunami waves of different heights interact with mounds of various sizes and shapes arranged near the water’s edge. 

Coastal forests can help put the brakes on tsunami flow speeds in nearby towns and villages. These and other nature-based solutions are increasingly important in plans for coastal risk management, the researchers write. Yet it takes decades for trees to grow sturdy enough to provide meaningful protection.

And, according to the new study, vegetation has little effect on an incoming wave’s energy. Plants may still play an important role in fighting erosion, however, thereby helping to maintain the shape, height and spacing of hills that make them effective.

Until now, designs for tsunami mitigation parks have been informed more by aesthetics than science. “Right now, our designs are not strategic enough,” Suckale said. By numerically modeling what happens to a tsunami wave when it slams into a single row of hills, the researchers show mounds can reflect and dampen a tsunami wave’s destructive power about as well as a typical seawall can. And in the event of a huge, one-in-a-thousand-years tsunami, they will fail no worse than even the most imposing walls. As a result, the study finds, there’s little extra value to be gained from combining walls and hills – a common approach in designs from Constitución, Chile, to Morino, Japan.

Customizing the shape of the hills based on the shape of the coastline, the direction that tsunamis are likely to approach from, and other site-specific factors can help to maximize the amount of energy reflected back. This is key, Suckale said, because if a tsunami inundates an area at full throttle with even one foot of water, it will leave few survivors. “It just slams everything. You can’t stay on your feet, and once you fall, it’s very dangerous. It throws cars at buildings. You’ll easily be knocked over by things carried in the water.”

The study points to the need for homes and infrastructure to be set back behind a broad buffer zone, because hills can speed up flows and increase damage in the area immediately surrounding the park. To avoid this unintended consequence, the researchers suggest considering designs with multiple staggered rows of hills that are larger toward the shore and smaller inland.

“Our study shows that design matters. There’s a wrong and a right spacing; there’s a wrong and a right shape,” Suckale said. “You should not use aesthetic criteria to design this.”

The research was published in the journal Proceedings of the National Academy of Sciences.