These days we received the unfortunate news that a tsunami has left nearly 20,000 dead in Southeast Asia. Once again, nature becomes a whip is there to remind us that our planet is a dynamic planet.
again turns to see the human drama of the disaster.
From here I will try to give some brief hints about natural phenomenon, that of the tsunamis.
WAVE SPEED
Although treatment of the waves in a specific coastal area is complicated and involves the use of statistics, it is possible some simple mathematical formalization of the phenomenon of waves.
The waves can be treated as a wave, like all waves, can be characterized by a series of parameters: wavelength, which is the distance between two crests of the wave amplitude, which is the height of the wave, and wave speed.
There are certain mathematical expressions that allow us to estimate the speed of a wave if we know the amplitude and wavelength.
It can be shown that, in regard to the speed of a wave there are 2 cases:
- In the case of waves that meet the wavelength is much smaller than the depth of the water column (which is called short waves), as when a wave travels in deep water, the speed is given by the expression:
V = [(GXL) / 2pi] exp 1 / 2, where g is the value of the acceleration of gravity, l the wavelength and pi pi.
Our interest in this expression is that when the wave travels in deep water, its speed depends only on its wavelength.
- Where the waves where the wavelength is much greater than the depth of water (so-called long waves) and waves propagating in shallow water near shore, is satisfied that:
V = (gxh) exp ½, where "h" is the height of the wave.
As in the previous case, it is interesting to note that when a wave moves near the coast, its velocity only depends on the depth, so that the smaller the depth, the lower its speed.
NOTE: For the waves "intermediate" in that its wavelength is of the same order of magnitude as the depth, the expression of its velocity is more complicated and depends on the depth and the wavelength.
This and another idea, let us understand a phenomenon we've all seen: the surf.
That other idea is pervasive in science, the principle of conservation of energy: it says that any process in which no external forces act, the energy is conserved, not lost or created, it becomes ..
Well, a wave will have a certain energy, this energy will be of two types: the "kinetic energy", associated and dependent on his speed, and another called "potential energy" which depends on the height wave.
waves arise, usually due to the wind stress exerted on the water surface. Normally form at some distance from the beach.
At that time, the wave will have a certain kinetic energy, related to its velocity as the wave gets closer to the coastline, will Sience shrinking the depth of the water column have lower until it reaches a time when the wavelength is greater than the depth. At that time, if we remember what we said above, is satisfied that its speed will be decreased by less than the depth. This will mean that as the wave approaches the beach, will have less kinetic energy.
But we said that if no external forces act (and if we handle simplified wave can be assumed, although there is some friction because friction with the air surface water and bottom), the energy is to be retained.
If the kinetic energy decreases, the only possibility for that energy is conserved, is that kinetic energy is "lost" becomes potential energy. We say that the potential energy depends on the height. Therefore, if the potential energy is increasing, that means you are increasing the height of the wave.
is, when a wave is near the beach, as you proceed, following the principle of conservation of energy, will increase its height. This will cause the wave becomes unstable and reaches a time when their height is greater than the depth: at the time the wave "clash" with the bottom and break.
WHAT HAPPENS TO A TSUNAMI?
A tsunami, a wave or wave train produced a body of water when violently displaced by a force of great magnitude, as can be an undersea earthquake, a volcanic colpaso, a landslide or ( is espcula with) the fall of a meteorite.
not be confused with tidal waves tsunami or the called sea confused, generated by storms, high winds or hurricanes, because those are generated by horizontal movements of surface water caused by wind, rarely spread beyond 100 meters coast line and its energy decreases from the area where they occur. Tsunamis, by contrast, originate from sea by vertical movements, and retain their power from the beginning to the end
As we have seen, tsunamis are caused by any reason that causes the mass of ocean water moves vertically abruptly, causing that water is removed from its normal balance. At that point, the water will attempt to recover its equilibrium position and result in the wave or wave train of unique characteristics that call tsunami.
Today I heard on television that this process is analogous to a whip, and it really is a very good example, because the movement makes the water surface is very similar to a whip. Tsunamis are
to be waves with a wavelength of magnitude equal to the radius of the ocean where they are generated, hence, this wave lonitud will never be less than 100-200 km. His height, at the point where the open sea where it is generated, is only half a meter to a meter.
If we consider that the depth of the ocean has an average of about 5 km, this implies that the wavelength of the wave is much greater than the depth of the water column.
If we remember what we saw in the previous section, this is the characteristic of waves are called long waves. And in these waves, speed is proportional to the depth. For this reason, in water, tsunamis have enormous velocities of the order of 700-800 km / h. These high speeds, along with his small height offshore, are responsible for ensuring that ships are sailing at high distances from the coast not notice the wave. These two characteristics mean that these waves only affect the open sea.
The problem arises when tsunamis approach the coast. In this case the situation is similar to the phenomenon of breaking that described above.
Approaching the shelf and coastal environment, the depth will be lower. Consequently, the speed is reduced (remember that the speed tsunami depends on the depth), which determines the kinetic energy is reduced. As the principle of energy conservation "Mandates" that energy is conserved, kinetic energy is being lost is turning it into potential energy. The potential energy depends on the height, which implies that increasing the potential energy increases the height of the wave.
This increase in wave height near the coast, coupled with the fact that since the lower speed makes waves are approaching coast be-determined stacking a large mass of water rushed inland , sometimes leading to affect several miles away (taking advantage sometimes enter rivers and estuaries).
To this we must add two aggravating circumstances: in some coasts, particularly sheltered shores, resonance phenomena occur (the free movement of water itself is combined with the tsunami), which amplifies the wave and its destructive effects, and drag the tsunami due to the effects of erosion, rock fragments, remains of buildings demolished, ... that increase their negative effects. For
tsunamis in 1949 Inamura, proposed a level of destructiveness, analogous to the Richter scale earthquake or hurricane Saffir.
is amazing that with an idea, a principle as abstract and arbitrary as the principle of conservation of energy, science achieves explain a phenomenon as real and complicated as the tsunamis.
A beautiful demonstration of effectiveness of the mathematization of science. Serve as a good replica for those who say that the mathematization of science has been far from reality and the explanation of natural phenomena.
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