Important first correction. The wave moves at that speed. The water itself does not. It mostly bobs up and down slightly.
Important second correction. The height of the tsunami when it hits shore is not the height of the tsunami when it is out at sea. All waves, tsunamis included, rise up when the bottom of the water starts to compress the bottom of the wave, compressing its energy into a smaller volume.
With these corrections, a tsunami out at sea is a wave a few cm high moving at 800 km/h with the water barely moving, but with the whole water column moving. It is therefore entirely conceivable that a well-built dike could withstand such a wave.
Obviously such dikes were not in place this time, but the feasibility of building them has been investigated. By Japanese scientists no less. See http://www.pwri.go.jp/eng/ujnr/joint/37/paper/13kato.pdf for one paper on the topic.
There actually were such dikes in some places, you can see them overflowing in some of the videos.
The thing is, you have to decide how high a dike you're willing to pay for, which is a cost vs. remaining risk tradeoff. The risk of the 5th strongest quake ever recorded occuring right before your piece of the coast was apparently considered acceptable - Hindsight always wins.
Thanks - I was trying to get across that this is a block of water the mass of a small mountain dropping on you.
You can earthquake proof a building but you can't do much against this.
Even a regular storm surge in the north sea needs some serious mega-project engineering to protect a single river/city. Protecting the entire coastline of Japan against a Tsunami would be tricky
Important second correction. The height of the tsunami when it hits shore is not the height of the tsunami when it is out at sea. All waves, tsunamis included, rise up when the bottom of the water starts to compress the bottom of the wave, compressing its energy into a smaller volume.
With these corrections, a tsunami out at sea is a wave a few cm high moving at 800 km/h with the water barely moving, but with the whole water column moving. It is therefore entirely conceivable that a well-built dike could withstand such a wave.
Obviously such dikes were not in place this time, but the feasibility of building them has been investigated. By Japanese scientists no less. See http://www.pwri.go.jp/eng/ujnr/joint/37/paper/13kato.pdf for one paper on the topic.