Analysis of tsunamis along the Pacific coast of Japan’s Tohoku region suggests that both seawalls higher than 5 meters and coastal forests reduce damage and death.

The Japanese have embarked on a 10-year reconstruction project costing about 31.5 trillion yen ($315 billion), which includes the construction of tsunami seawalls along Tohoku’s Pacific coast. Critics of the program have voiced skepticism about the effectiveness of seawalls.

Both both seawalls higher than 5 meters and coastal forests reduce damage and death.“Some have suggested that seawalls cause a sense of complacency in residents, leading to lower evacuation rates and the tendency to develop residences in hazardous low-lying areas,” says Seth Guikema, associate professor in the Department of Industrial and Operations Engineering at the University of Michigan. “However, our results cast serious doubt on this theory. The presence or size of seawalls and coastal forests does not strongly influence residents’ decisions on whether or not to evacuate.”

Researchers, led by Roshanak Nateghi, assistant professor in Purdue’s School of Industrial Engineering, used a modeling method called Random Forest, which harnesses numerous “decision trees” to capture complex non-linear relationships of data. The researchers studied data from tsunamis that hit Japan in 1896, 1933, 1960 and 2011.

Seawalls more than 5 meters high were shown to reduce damage, and more specifically, walls of 10 meters in height were associated with a 5% to 6% decrease in destruction rate. However, seawalls less than 5 meters high appear to have encouraged development in vulnerable areas and exacerbated damage. The presence of coastal forests, meanwhile, was found to reduce death and destruction rates by displacing development that would otherwise have been damaged.

The researchers say their findings also showed that the extent of flooding is a critical factor in death and building damage rates, suggesting that additional measures, such as multiple lines of defense and elevating topography, could contribute to reducing the impacts of tsunamis.