Study makes a case for forested hills for tsunami defense
Seawalls have become a key defensive tool for coastal communities looking to protect themselves against the devastation of a tsunami, though alternatives do exist. Known as tsunami mitigation parks, hilly stretches of coastline covered in plant-life can also stem the flow of a giant wave, and a new study has highlighted how carefully engineered examples can not just offer comparable safety, but better preserve key industries like fishing and tourism at the same time.
Japan is one example of a place where seawalls are an important pillar of coastline defense, with the country spending more than US$12 billion on structures that stretch along hundreds of miles of beachfront since the 2011 earthquake and tsunami. But not all tsunami-prone countries have the resources of Japan, and for them, nature-based solutions may shape as a more viable proposition.
Forested coastal areas have been a part of tsunami mitigation measures around the world for a long time, and are often combined with some form of engineered barrier. But in the eyes of the international research team behind the new study, these are more often guided by aesthetics rather than science.
“Right now, our designs are not strategic enough,” says senior study author Jenny Suckale, an assistant professor of geophysics at Stanford University. “This paper is a starting point for understanding how to design these parks to derive maximum risk mitigation benefits from them.”
The scientists used numerical modeling to analyze how a single row of hills can suck much of the energy out of tsunami wave. The team found that mounds of a certain size can temper the destructive power of a tsunami about as effectively as a standard seawall. Even more so when the shape of the hills is tailored to the shape of the coastline and the likely direction of an oncoming tsunami.
“These hills reflect a surprising amount of wave energy for small and intermediate tsunamis,” says Suckale.
According to the team’s analysis, it is important to establish a large buffer zone, as the hills can cause the water to pick up momentum and destroy surrounding infrastructure. For this reason, the team says that an optimal design could include staggered rows of hills with larger mounds positioned at the shore and smaller ones inland of that.
“Our study shows that design matters,” says Suckale. “There’s a wrong and a right spacing; there’s a wrong and a right shape. You should not use aesthetic criteria to design this.”
Another interesting takeaway from the study is that vegetation appears to play a secondary role in the tsunami-slowing effect. It is predominantly the hills themselves that sap the energy of an incoming wave, though vegetation is vital in preventing erosion and maintaining the shape of the landscape.
While this type of tsunami mitigation may well be cheaper than constructing seawalls, a more profound benefit may come from the increased access to the shoreline that they offer. By not shutting out those that rely on ready access to the sea, such as people in the fishing and tourism industries, they shape as a far less disruptive solution, and perhaps without compromising on safety.
“Most coastal communities want to maximize their well-being, not minimize their risk at the expense of everything else,” says Suckale. “Do you really want to live behind a huge concrete wall because there is a small chance that a big tsunami will hit you? Let’s put more options on the table and have an informed debate.”
The research was published in the Proceedings of the National Academy of Sciences.
Source: Stanford University