Green infrastructure provides a viable alternative to seawalls for storm surge risk
Parks, green spaces, and plant-covered hills are an effective defence against storm surges and tsunamis, according to a Stanford University study. The research concludes that carefully engineered green infrastructure can offer similar levels of protection as large seawalls, while also benefiting for marine and coastal biodiversity, the aesthetic environment, and reducing costs.
The study, published on 4 May 2020 in the journal Proceedings of the National Academy of Sciences, quantified how tsunami waves of different heights interact with structures of various sizes and shapes at the coast. The research calls into question the wisdom of conventional approaches to coastal storm management, which are dominated by hard infrastructure development such as construction of large seawalls.
However, seawalls have many disadvantages. They are expensive and inflexible, so they are hard to adapt if, for example, sea levels rise by more than expected. They can also damage marine ecosystems and local economies in some sectors such as fishing or tourism.
“If the wall collapses, the consequences are life shattering,” said senior study author Jenny Suckale, an assistant professor of geophysics in the School of Earth, Energy & Environmental Sciences. “Seawalls can not only create a false sense of security that can discourage swift evacuations. They can also end up breaking apart into blocks of rubble that tsunami waves then toss throughout a city.”
“It’s sort of intuitive that the moment you see it as a threat, you build a wall,” Suckale said. But while it’s true that seawalls can address some tsunami risks, the factors that make a place livable can be far more complicated. Most coastal communities want to maximize their well-being, not minimize their risk at the expense of everything else,” she said. “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.”
Will is a climate risk and resilience communications expert working for climate adaptation advisory firm Acclimatise. He is currently working with the UCCRTF on knowledge sharing and communications.
Three examples of tsunami mitigation parks along the ring of fire that are currently being planned and/or constructed in South Java, Indonesia (image courtesy of A.M.); Miyagi Prefecture, Japan and Constitución, Chile. Miyagi prefecture image and Constitución image credit: Morino Project and Felipe Diaz Contardo (photographer).
Green infrastructure must be carefully designed
When considering alternatives to sea walls, the study found that green infrastructure solutions needed to be carefully designed and well built in order to deliver the desired levels of protection. The study notes that while coastal forests offer protection against storm surges, it takes decades for trees to grow large enough to offer robust protection, and they are not viable in some areas where protection is most needed – such as to protect vulnerable towns and cities.
The study noted that as much attention needs to be paid to the design and engineering of green infrastructure as to conventional infrastructure development. According to the study, vegetation alone, has little effect on an incoming wave’s energy. However, plants play an important role in fighting erosion, thereby helping to maintain the shape, height and spacing of hills and mounds, which do offer significant protection.
Suckale says that to date, green infrastructure has been designed more for aesthetics than for performance. “Our study shows that design matters. 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. Right now, our designs are not strategic enough,” she said. “This paper is a starting point for understanding how to design these parks to derive maximum risk mitigation benefits from them.”
To test the efficacy of hills and mounds in providing coastal defences, the researchers modelled what happens when a tsunami wave hits a single row of hills. They found that mounds reflect and dampen a tsunami wave’s energy about as well as a seawall. Hills were also found to perform equally well in the case of a very extreme event – a one-in-a-thousand-year tsunami. As a result, the study concluded that there is little extra value in combining hills with seawalls.
Along with improving the design green infrastructure, the study also recommends that more space should be given between urban development and the water’s edge. The researchers note that homes and infrastructure should be set back with a broad buffer zone between them and multiple staggered rows of hills that are larger toward the shore and smaller inland.