Method for Quick Assessment of Caisson Breakwater Failure due to Tsunamis: Retrospective Analysis of Data from the 2011 Great East Japan Earthquake and Tsunami

Abstract

Given the importance of ports to immediate disaster relief operations after a major tsunami, breakwaters are necessary to sustain a minimum level of functionality required to carry out port operations. However, most breakwaters that protect fishery or industrial ports are not designed to withstand a tsunami. For this reason, many of these breakwaters were destroyed by the 2011 Great East Japan Earthquake and Tsunami. However, some breakwaters withstood the tsunami, which facilitated the rapid resumption of port operations. In this study, 67 ports were investigated by using satellite images to identify the percentage of breakwaters that suffered damage due to the 2011 tsunami and how the tsunami height and breakwater width influenced the degree of breakwater damage. Because tsunami heights were not measured at the breakwaters, they were estimated by interpolating the data observed at nearby locations with a Gaussian weighting function at a reasonable accuracy. Possible drawbacks of this simplified method are also discussed because a certain degree of estimation error is inevitable. Breakwaters less than 8 m wide were quantitatively demonstrated to inevitably experience extensive damage when the tsunami height exceeded 14 m. In contrast, breakwaters that were more than 14 m wide, exposed to tsunami heights of less than 6 m, or both were not greatly damaged. An examination of the effectiveness of armor blocks also revealed that their placement could substantially reduce breakwater damage due to a tsunami. The results suggest a simplified method, particularly for assisting local fishery administrators or other authorities concerned, to practically estimate the risk of a port suffering damage because of future tsunamis. The proposed method should contribute to prioritizing the protection of ports in order of their likelihood of failure.

Keywords:

• Tsunami
• 2011 Great East Japan Earthquake and Tsunami
• caisson breakwater
• breakwater damage ratio
• tsunami height
• breakwater width
• armor blocks