Sea walls, that the Japanese government relied on to protect people and property from a tsunami, failed catastrophically. The failure of sea walls to protect the back up diesel generators at the Fukushima nuclear power power plants led to the loss of coolant accident and the partial fuel melt at the Daiichi reactors.
The risks of dependence on seawalls were most evident in the crisis at the Daiichi and Daini nuclear power plants, both located along the coast close to the earthquake zone. The tsunami that followed the quake washed over walls that were supposed to protect the plants, disabling the diesel generators crucial to maintaining power for the reactors’ cooling systems during shutdown.
Cooling system malfunctions caused overheating and partial fuel meltdowns at two reactors at the Daiichi plant, becoming Japan’s worst nuclear accident.
Peter Yanev, one of the world’s best-known consultants on designing nuclear plants to withstand earthquakes, said the seawalls at the Japanese plants probably could not handle tsunami waves of the height that struck them. And the diesel generators were situated in a low spot on the assumption that the walls were high enough to protect against any likely tsunami.
Sea walls failed catastrophically to protect people and property up and down the tsunami-affected coast of Japan. Because a major tsunami can have a period of 10 minutes or more and a wavelength of 100 miles or more, it may pile up and wash over seawalls like flood waters rushing over a dam. Tsunamis slow down, shorten wavelength and grow higher as they move into shallower water.
Image source: S. Nelson, Tulane
Sea walls that appeared to provide protection hid the danger of the incoming tsunami from complacent citizens. Large numbers of people were trapped and died when the tsunami waves overwhelmed the sea walls.
Kamaishi City built the world's largest sea wall at a cost of 1.5 billion dollars. Much of Kamaishi City is now destroyed.
While the world waits, watches, and worries about the crippled nuclear power plants, rescuers search city streets hoping to save survivors in the wreckage left behind failed sea walls. Japan's buildings, designed to Japan's strong building codes, fared well in the earthquake. A small fraction of the deaths and injuries were caused directly by the earthquake. The tsunami was responsible for the vast majority of injuries and deaths.
The lesson to be learned is that sea walls and engineered structures can protect key facilities if they are built high enough and strongly enough, but they cannot be depended on to protect large areas in the largest tsunamis or the strongest hurricanes. Water will simply pass by well-designed structures that protect key facilities, leaving them undamaged. However, large tsunamis and hurricane storm surges can pile up water in front of large sea walls then rush over them like water over a dam. In general, hardened coastlines damage the coastal environment, contribute to loss of sand and provide a false sense of security to property owners and local residents. When they are needed the most, sea walls fail.
Update:
Seawalls did not fail structurally. They failed to do the job they were built to do, protect people and property. The failure of the world's largest seawall, which cost $1.5 billion, to protect a port city shows that building larger and stronger sea walls to protect large areas would have been too costly to be effective. In the case of the ongoing crisis at the nuclear power plants, higher and stronger sea walls should have been built if power plants were to be built at that site. However, the Japanese engineers made a fundamental design error. They built walls for a far smaller design basis earthquake than actually occurred. They built the reactor and sea walls for a maximum of M8.2 when this earthquake was M8.9.
When the reactors were constructed the underestimate of the magnitude of the design basis earthquake may have been consistent with the state of the art of knowledge on subduction zone earthquakes. However, since the great tsunami in the Indian ocean after the M9.3 Sumatran earthquake, the safety of every reactor in tsunami-prone subduction zone environment should have been reassessed.
When I worked on high level waste management research at the U.S. Nuclear Regulatory Commission, one of the hardest parts of my job was convincing engineers to design adequately for geologic hazards. At Yucca Mountain, Nevada, the Department of Energy consistently used assessment methods that minimized the probability and size of earthquakes and volcanoes that might affect the proposed waste repository.
The “No one could have foreseen the magnitude of this earthquake” argument is not tenable. The the Kamchatka earthquake of 1952, the Aleutian Islands earthquake of 1957, the 1960 Chilean earthquake, the 1964 Alaskan earthquake were all M9.0 or larger. They were all on subduction zones similar to the one underlying northern Japan.
So, in response to the comment by kakumeiji maru:
“Please remember how catastrophic this was”
“I am not discounting the failure of the government to build better sea walls. Clearly, they could have; but they probably didn't think that they needed to. The Fukushima reactor was rated as earthquake safe up to 8.2, when in fact the actual earthquake was somewhere between 8.9 and 9.1, seven to nine times more powerful than what it was built for. This was bigger than anyone expected. 8.2 was supposed to be the "once in five hundred years" catastrophic maximum.”
I'm sorry. The geologists, engineers and regulatory authorities involved in designing, building and regulating Japan's nuclear power plants made an enormous mistake. An earthquake of this magnitude was foreseeable.
Analysis of the Honshu earthquake by USGS seismologists (USGS Honshu earthquake poster) shows that the earthquake did not rupture the full fault length from the triple point with with the Philippine sea plate to the bend in the subduction zone at the north end of of the island. A larger earthquake than this one was possible.
