Relationship between Coastal Hazard Countermeasures and Community Resilience in the Tōhoku Region of Japan Following the 2011 Tsunami

Many coastal communities around the world that are vulnerable to inundation from tsunamis or storm surges are investing significant resources into countermeasures such as seawalls, levees, and breakwaters to limit damage from such events. A potential framework for studying the relationship between coastal hazard countermeasures and their impact on community resilience is proposed in this investigation and is applied to 27 coastal cities in the Tōhoku region of Japan following the 2011 tsunami. Community resilience was quantified using a modified version of the resilience inference measurement (RIM) method in which parameters of intensity, exposure, damage, and recovery were utilized in a $k$-means clustering analysis to categorize respective cities into four different resilience groups. As a supplemental analysis to the modified RIM method, historical data representing the exposure and damage parameters from the 1896, 1933, and 2011 Tōhoku tsunamis were utilized to establish limit states (LSs) for fragility curves to demonstrate the level of response for a specific damage condition. Three limit states were selected for 2%, 20%, and 40% of dwellings in a city being destroyed for a given tsunami event. An ordinal logistic regression was performed to determine whether there was a relationship between the resilience group of each city and the maximum countermeasure height. No statistically significant correlation was found between the maximum height of a coastal hazard countermeasure and the resilience group of each city, which indicates that investing in such infrastructure does not necessarily impact overall community resilience for an extreme event such as the 2011 Tōhoku tsunami. For this scenario, the probability of damage was assessed by developing fragility curves that demonstrated larger probability of exceedance discrepancies between LSs at lower maximum tsunami runup values. Coastal hazard countermeasures therefore may still be effective in limiting damage and thereby improve community resilience for less extreme tsunami events. The results of this investigation will assist government officials and engineers in selecting coastal resilience policies and strategies to limit future impacts from coastal hazards.

Coastal hazard countermeasures such as seawalls, levees, and breakwaters are being implemented by coastal communities across the globe to protect people and infrastructure from damage caused by tsunamis and storm surges. Built structures such as these have significant economic, environmental, and social impacts. A potential framework for studying the relationship between coastal hazard countermeasures and their influence on community resilience is proposed in this investigation. The framework was applied to the 2011 tsunami event that devasted coastal cities in the Tōhoku region of Japan. Most of these cities had constructed seawalls prior to this disaster. The application of this event to the proposed framework did not result in a statistically significant correlation between the maximum height of seawalls and the resilience behavior of a city, which indicates that investing in such infrastructure does not necessarily improve overall community resilience for an extreme event such as the 2011 Tōhoku tsunami. Coastal hazard countermeasures may still be effective in limiting damage for less extreme tsunami events and thereby improve community resilience for such events. The framework proposed in this report will assist government officials and engineers in selecting coastal resilience policies and strategies to limit future impacts from coastal hazards.