At least 510 dams and irrigation ponds for agricultural use have poor quake resistance strength, according to a nationwide survey by local governments.
At least 510 dams and irrigation ponds for agricultural use have poor quake resistance strength, according to a nationwide survey by local governments.
The continuing general survey began after a dam in Fukushima Prefecture collapsed during the March 2011 Great East Japan Earthquake, resulting in a number of fatalities.
The number of dams with insufficient quake resistance will likely increase as thousands of other locations have yet to be surveyed.
The agriculture ministry has requested that municipalities strengthen irrigation reservoirs and prepare seismic hazard maps.
Since fiscal 2013, the ministry has been requesting that municipalities conduct visual surveys of large-scale irrigation reservoirs that could affect local households in the event of a dam collapse.
At least 70,000 locations have been surveyed by August, The Asahi Shimbun confirmed with the prefectures. The ministry plans to increase that number to 110,000 sites across the country in two years.
Although the government standard requires that each reservoir or dam have resistance against a moderate-sized earthquake, which occurs about once every 100 years, 6,000 locations in 42 prefectures are suspected of not meeting the safety standard and were subjects of the quake resistance survey.
Of the 1,400 locations in 13 prefectures that have been surveyed by this month, about 510 were found to have poor earthquake resistance strength.
In 2011, the Great East Japan Earthquake caused the earth-filled Fujinuma dam in Sukagawa, Fukushima Prefecture, to collapse, releasing 1.5 million tons of water downstream that killed seven people, left one missing and destroyed 22 houses.
According to Fukushima Prefecture, the earth became liquefied due to earthquake tremors.
“The collapse of dams due to earthquakes is virtually unheard of in the world," said University of Tokyo professor emeritus Tadatsugu Tanaka, who studied the seismic stability of dams and reservoirs. "But because many of the dams and reservoirs in Japan are too old for us to adequately understand how they were constructed, we cannot say for certain that a failure will not occur again."
(This article was written by Eiji Zakota and Junichi Kamiyama.)