Seawalls, forests show mixed effectiveness at reducing deaths, damage from tsunami

The Japanese government has a 31.5 trillion yen, 10-year reconstruction program which includes the building of tsunami seawalls along the entirety of Tohoku's Pacific coast. But not everyone agrees with this plan.

Many critics claim that seawalls are detrimental because they offer a false sense of security, and prevent residents from being able to see the approaching danger first-hand. Moreover, they also say that the presence of seawalls tends to encourage residents to build homes in vulnerable areas rather than in safer places further inland or uphill.

Similar viewpoints exist with respect to coastal "tsunami control" forests. Supporters claim that the forests help to reduce the force of the tsunami, while detractors argue that during a tsunami, trees often get uprooted and become dangerous projectiles.

To put an end to the debate over the pros and cons of seawalls and forests, Purdue University's Roshanak Nateghi and her research collaborators accessed historical government documents to compile a comprehensive database of damage and death rates resulting from every destructive tsunami that has hit the coasts of Miyagi and Iwate Prefectures since 1896.

The team then applied this data to several statistical pattern recognition methods, to determine the most accurate predictive algorithm for death and damage rates as a function of maximum tsunami run-up elevation, maximum and minimum seawall elevations, and coastal forest area in each municipality. The best fit was achieved with a machine learning model known as Random Forest.

According to the results, small seawalls (5m or less in elevation) actually tended to exacerbate the destruction rates. "This is because the presence of the seawalls encourages residents to build homes in vulnerable areas but provide insufficient protection for these homes when a tsunami hits," explained co-author Seth Guikema of the University of Michigan.

Conversely, large seawalls, despite having failed in some locations due to overtopping and scouring, actually reduced destruction rates, compared to cases where no seawalls existed.

As for tsunami control forests, the trend is simple: larger forests correspond to lower home destruction rates.

"This suggests that either the forests were effective at reducing the impact of these tsunamis, or that the presence of the forests discouraged building developments that otherwise would have occurred along the coast" said co-author Akane Bessho.

In analyzing data for both property destruction and loss of life, the researchers found that there was a slight discrepancy.

While small seawalls were proved to be counterproductive to protecting property from destruction, researchers found that the presence of seawalls of any size did in fact lower the death rate.

"What this means is that while seawalls may cause complacency in some cases, leading to some people building in high risk areas, it is unlikely that they directly hampered evacuation efforts or caused more casualties," said co-author Jeremy Bricker of Tohoku University. "However, the fact that small seawalls do exacerbate property damage is important, and must be considered in evaluating the benefits of building them."