In the collective imagination, the tsunami they are often represented as gigantic waves that above bridges and skyscrapers. In some Hollywood films are portrayed as imposing walls waterhigh beyond 100 meterswhich are violently fledged on the US metropolises. In reality, beyond a few exceptions, tsunami waves reach heights generally lower than 10 foot Near the coast, even lower at the height of the common marine waves. These dimensions may not seem much, but just think that even a wave of tsunami high alone 1 meter it could be potentially lethal for the resident population in coastal areas. But then, if it is not the height of the wave, what makes tsunami so dangerous?
The tsunami is not just a very high wave
Both tsunami and marine waves are, in fact, of wave And as such, they share the same basic anatomy. Both types have one crestthat is, the highest part of the wave, and a bellythat is, the lowest portion. The distance between two subsequent ridges (or two winds) takes the name of wavelengthwhile thewave height It is defined by the vertical distance between the ridge and the belly. However, the tsunami behave and propagate in a very different way than in marine waves, they are these waves “good weather“Or“Storm waves“.
Marine waves, produced by windpropagate in the portion more superficial of the water column with a speed between 8 and 100 km/h and have a wavelength generally not exceeding 180 meters. The periodthat is, the time between two subsequent waves is normally less than minute. On the contrary, the tsunami involve a significantly greater volume of water and spread along theentire water columnfrom the surface to the oceanic bottom. Their wavelength can exceed 500-1000 kmwhile the period of the wave can vary from 5 minutes to over 2 hours. In general, the greater the wavelength, the greater the volume of water involved and, consequently, the energy transported.
The tsunami, therefore, move at great speed. Their speed depends mainly on depth of the seabed: the more deep the seabed, the greater the speed. In the open sea, one tsunami can move beyond 800 km/h and cross an entire ocean in less than a day. However, due to the huge wavelength and low height, the tsunami are almost imperceptible in sea open where, paradoxically, a wave of high storm even beyond 9 meters It appears much more threatening.
Tsunami transformation along the coast
The waves of tsunami evolve drastically near the coast where the reduced depth of the seabed causes a progressive one slowdownup to speeds often lower than 50 km/h. Slowing, the wavelength is shortened, so that the tail of the tsunami (still in rapid movement) reaches the ridge. At this point, thekinetic energy transported up to that moment by the tsunami is transformed into potential energymaking the wave grows in height. The maximum height reached by a tsunami along the coast is defined runup. In most cases recorded, the runup does not exceed i three meterswith the exception of some tsunami and mega tsunami in which the waves have passed i 15 meters. For example, the tsunami that in 2004 He hit the coasts of theIndonesia reached a maximum height of 14 meters.
Unlike common marine waves, which, due to their relatively short wavelength, rapidly raise when they meet low seabed, sitting in the shore in the classic curved shape – theSurf waveto be clear – by dissipating their energy just as quickly, the tsunami present themselves as Very strong tides, floods or real walls of waterand only rarely turn into imposing waves. However, since their energy is not spent immediately along the coast, the wave can penetrate deeply in the hinterland, pouring its energy into homes, infrastructures and people. Thus, a tsunami only one meter is more than enough to damage buildings, overwhelm structures and cause victims. In fact, the real danger of tsunami does not reside in the height of the wave, as in the presence of strong currents, in the high speeds and in the large quantities of transported debris. If you are affected by a wave of tsunami tall even until the knee remained standing is almost impossible, and the risk of being overwhelmed or affected by drifting objects is extremely high, with very high chances of fatal consequences.
