Tsunami, or “Harbour Wave,” in Japanese, is also referred to (more descriptively), as a seismic sea wave. Tsunami defense is an ongoing battle for the Japanese, so let’s take a peek at Japan’s engineering solutions to future tsunamis.
Japan’s location on the ring of fire, a geologically riotous area of the world, led to a long history of volcanism, earthquakes and tsunamis. Such is the destruction nature has inflicted, and Japan has had to cope in innovative and imaginative ways.
So what are Tsunamis?
Tsunamis are large ocean waves caused by underwater earthquakes, subaquatic or terrestrial landslides or violent volcanic explosions.
Generated by the large, often rapid, massive displacement of oceanic water mass occurs. Most commonly this occurs from subaquatic uplift of the sea floor associated with earthquakes. They have also historically been generated from meteorite impacts, a good example being the mass extinction event at the end of the Cretaceous period.
[Image source: Pixabay]
Why are they so destructive?
Tsunamis often cause more destruction and rack up higher death tolls than the generating source event. Tsunamis are a series of extremely long wavelength waves, and they travel for kilometers with little loss of energy. Once these waves meet shallow waters they become compressed and force water levels much higher, and with much greater energy, than would normally be expected from tidal waves.
Unfortunately, water hugging areas are some of the preferred sites for habitation, understandably. The wave heights and momentum are so great, that their transported energy and high density of buildings and population combined are a “recipe for disaster”. Once they hit landfall the power is dissipated with great devastation. Once the waves begin to swap the surrounding coastal areas they also start to “pick up” large objects such as boats, bits of buildings and trees that compound the destructive tendency of the waves.
In the past decade 16 lethal tsunamis, mainly in the Pacific, resulting in more than 6,000 deaths. This is comparable to the death toll of all earthquakes globally for the same period. The 2004 Indian Ocean tsunami claimed more than 230,000 lives and is one of the worst natural disasters in history.
So how is Japan preparing for future inevitable events? Let’s take a look.
Early bird catches the worm
Japan’s experience led to the development of a strategy of countermeasure, defensive structures and relocation of population centers
Many strategies revolve around moving population centers away from coastal areas or providing as much advanced warning as possible to evacuate. Where this is not possible physical barriers are the only solution, but as we have seen these are not always reliable.
International warning systems have been deployed to attempt to detect tsunami triggering events. This is usually coupled or supplemented with radio and television broadcast and loudspeaker networks to warn the public.
These measures are often useless in the event that the triggering event occurs close to the coast. This may only allow five to ten munites to evacuate. Clearly, for large population centers, this is less than ideal.
Japan has a highly developed public information campaign that informs as many citizens as possible.
Japan has tried to employ earthquake engineering to reduce damage onshore. Japan first began researching response measures to Tsunamis in 1896. It has produced ever-more elaborate countermeasures and response plans.
A large part of the Japan’s engineering solutions revolves around massive sea walls up to 12 meters tall. This is a response to attempt to protect populated coastal areas. Other solutions include massive floodgates up to 15.5 meters tall with channels to redirect or tamper the incoming waves. Japan has also tested vertical structures, essentially tall platforms for people to stand on above the tsunami maximum height.
Nuclear facilities, in particular, have in recent years tended to be sited far from the coast, with “safe modes” as soon as an earthquake is detected. Evacuation is not an easy option on many low-lying islands. Where structural engineering can be implemented, it can offer options to minimise the death toll after tsunami impact.
[Image source: Pixabay]
Reach for the stars
Researchers have found, using a wave tank, that allowing the water through without obstruction was the best course of action.
“Instead of trying to obstruct the wave, you allow the wave to pass through the structure while causing minimum damage. Large doorways and windows offer a clear path to go through,” said Tiziana Rossetto, a reader in earthquake engineering at University College London.
The objective here was to allow for easier reconstruction rather than prevention of damage. It also allowed for occupants to flee to higher ground. Fenestrations are easier to replace than lives after all. Japan’s tendency to build in timber also assists where design cannot building design cannot be altered.
“Internal doorways are aligned rather than staggered. If you have staggered doors, the wave gets ‘locked’ into the house,” Rossetto said.
Research has found that houses on piles could also be an answer, but this is not always practicable in all regions.
Japan has drawn inspiration from other Tsunami impacted areas such as Hawaii. Here the traditionally built concrete buildings that leave the ground floor clear, perhaps as a parking area. This effectively produces a building on stilts.
As like most tsunami prone areas, Japan has developed a mixed strategy which primarily relies on evacuation rather than defense. As seismic detection and preemptive warnings improve, death tolls can and will be reduced.
Infrastructure damage can be repaired rapidly if the building design accommodates strategic designs to either limit or allow easy repair. Defensive structures, such as sea walls, have proved to be ineffective but do provide a psychological boost to the local populace.
Given the need for improvement in detection systems and ineffectiveness of defensive structures, Japan, amongst other affected nations, has tended to place its concentration on detection to save lives. After all, a building can be rebuilt. Lives cannot.