Tsunami is the name associated with waves of huge wavelength, that are caused by earthquakes or volcanic explosions under an ocean. They are not tidal waves as have nothing to do with tidal movement.
When a Tsunami is created in the deep ocean,
they are able to have a wavelength of several thousand meters, but only be around 1 meter in height above the surface. The gradient of them is so shallow that they will noteven be noticed by any deep-sea boats that they pass.
In deep ocean, the reality is that all but the tip of the huge wave is rolling along under the water. The trouble for mankind starts when these waves are thrust into shallow waters and then land.
The Tsunami travels across the deep ocean at 700 km/hour, hits the continental shelf and is slowed to
160km/hour. In shallower inshore waters it is forced to slow again.
Each decrease in speed pushes the wave higher and it is now of the shape and form in which it will hit its next obstacle..... land.
What comes ashore is a 15 to 30 km/hour, 2 to 4meter high breaking wave front that stretches back around 500 meters and takes 30 to 60 seconds to relentlessly poor in from the ocean.
This simple 2D animation shows how a Tsunami is formed in the deep ocean due to a massive collapse of part of a deep ocean mountain due to an earthquake.