Perhaps you can explain to me how these 7' waves can be more dangerous than any other 7' waves? If it was 7' on the open ocean, then yeah that would be a big problem, but if it's 7' after it has compressed in shallow water, it shouldn't be any worse than any other 7' wave. The tsunami waves they're showing on the news look no different than normal waves (obviously I'm talking about the ones in hawaii. the video from japan is showing a much bigger, more destructive wave).
The difference is primarily in the wavelength. Wind waves and ground swell, which are just wind waves that were generated a long distance away, have wavelengths on the order of tens or hundreds of meters. In contrast, tsunamis generate waves with wavelengths that are tens of kilometers. That means two things: first, tsunamis travel extremely fast (hundreds of km/hr); and, second, there is a tremendous volume of water in each tsunami wave. Volume times velocity results in total wave energy.
When a wind wave (or groundswell) hits the beach the volume of water that gets thrown up onto the shore is fairly small and washes back into the ocean right away. When a tsunami hits the beach the volume of water keeps flowing landward for quite some time. Often, the land hasn't completely drained before the next wave in the sequence hit shore, compounding the effect. If subsequent waves in a tsunami sequence hit the water receding from previous waves, and/or of the topography is just "right", the waves will start to break, generating crests (and water velocities) much larger than the initial waves.
Fortunately, unlike wind-generated waves, tsunamis (because to their tremendous wavelengths) interact with the sea floor all the way across the ocean. This makes it possible for sensors to detect them long before they reach the coast, AND it means that they lose much more energy as they travel.
Never forget them. Do something to prevent it from happening again.
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