The challenge
Towns on Japan's northeast coast faced repeated deadly tsunamis.
The case
After deadly tsunamis in 1896 and 1933, towns on Japan's Tohoku coast built large sea defences. Taro began two 10-metre seawalls in 1934, designed for waves far lower than those of 2011. Kamaishi finished a bay-mouth breakwater in the late 2000s after three decades of work and nearly $1.6 billion; it was once listed by Guinness as the world's deepest. On 11 March 2011 the tsunami overtopped Taro's walls and largely broke the Kamaishi breakwater, mainly through foundation failure. Studies found the damaged breakwater still lowered tsunami heights and flooded area on land, but thousands died along the coast, and some residents may have felt falsely safe behind the walls.
Why the results were mixed
Hard defences can buy time and reduce flooding, but they fail against events bigger than their design; they work best alongside evacuation drills and land-use rules, not as a replacement.
Sources
- Onishi, N. (2011). Japan revives a sea barrier that failed to hold. The New York Times, via Honolulu Star-Advertiser.
- Kyoto University Research Information Repository. Study of breakwater effects during the 2011 Great East Japan tsunami.
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