how is water in a tsunami destructive?

Answer 1

Often referred to as "tidal waves", a tsunami does not look like the popular impression of "a normal wave, only much bigger". Instead it looks rather like an endlessly onrushing tide which forces its way around and through any obstacle. Most of the damage is caused by the huge mass of water behind the initial wave front, as the height of the sea keeps rising fast and floods powerfully into the coastal area. The sheer weight of water is enough to pulverise objects in its path, often reducing buildings to their foundations and scouring exposed ground to the bedrock. Large objects such as ships and boulders can be carried several miles inland before the tsunami subsides.

Tsunamis act very differently from typical surf swells: they contain immense energy, propagate at high speeds and can travel great trans-oceanic distances with little overall energy loss. A tsunami can cause damage thousands of kilometres from its origin, so there may be several hours between its creation and its impact on a coast, arriving long after the seismic wave generated by the originating event arrives. Although the total or overall loss of energy is small, the total energy is spread over a larger and larger circumference as the wave travels. The energy per linear metre in the wave is proportional to the inverse of the distance from the source. (In other words, it decreases linearly with distance.) This is the two-dimensional equivalent of the inverse square law, which is obeyed by waves which propagate in three dimensions (in a sphere instead of a circle).

A single tsunami event may involve a series of waves of varying heights; so the set of waves is called a train. In open water, tsunamis have extremely long periods (the time for the next wave top to pass a point after the previous one), from minutes to hours, and long wavelengths of up to several hundred kilometres. This is very different from typical wind-generated swells on the ocean, which might have periods of about 10 seconds and wavelengths of 150 metres.

The height of a tsunami wave in open water is often less than one metre, and the height is spread over the wavelength of the tsunami which is multiple kilometres. This is unnoticeable to people on ships in deep water. Because it has such a large wavelength, the energy of a tsunami mobilizes the entire water column down to the sea bed. Typical ocean surface waves in deep water cause water motion to a depth equal to half their wavelength. This means that ocean surface wave motion will only reach down to a depth of a few 100 m or less. Tsunamis, by contrast, behave as 'shallow water waves' in the deep ocean.

Because a tsunami behaves like a 'shallow water wave,' its speed is based on the depth of the water. Typically, a tsunami wave will travel across a deep ocean at an average speed of 400 to 500 mph. As the wave approaches land, the sea shallows and the tsunami wave no longer travels as quickly, so it begins to 'pile-up'; the wave-front becomes steeper and taller, and there is less distance between crests. While a person at the surface of deep water would probably not even notice the tsunami, the wave can increase to a height of six stories or more as it approaches the coastline and compresses. The steepening process is analogous to the cracking of a tapered whip. As a wave goes down the whip from handle to tip, the same energy is carried by less and less material, which as a consequence then moves more violently.

A wave becomes a 'shallow-water wave' when the ratio between the water depth and its wavelength gets very small, and since a tsunami has an extremely large wavelength (hundreds of kilometres), tsunamis act as a shallow-water wave even in deep oceanic water. Shallow-water waves move at a speed that is equal to the square root of the product of the acceleration of gravity (9.8 m/s2) and the water depth. For example, in the Pacific Ocean, where the typical water depth is about 4000 m, a tsunami travels at about 200 m/s (720 km/h or 450 mph) with little energy loss, even over long distances. At a water depth of 40 m, the speed would be 20 m/s (about 72 km/h or 45 mph), which is much slower than the speed in the open ocean but the wave would still be difficult to outrun.

Tsunamis propagate outward from their source, so coasts in the "shadow" of affected land masses are usually fairly safe. However, tsunami waves can diffract around land masses (as shown in this Indian Ocean tsunami animation as the waves reach southern Sri Lanka and India). They are also not necessarily symmetrical; tsunami waves may be much stronger in one direction than another, depending on the nature of the source and the surrounding geography.

Local geographic peculiarities can lead to seiche or standing waves forming, which can amplify the onshore damage. For instance, the tsunami that hit Hawaii on April 1, 1946 had a fifteen-minute interval between wave fronts. The natural resonant period of Hilo Bay is about thirty minutes. That meant that every second wave was in phase with the motion of Hilo Bay, creating a seiche in the bay. As a result, Hilo suffered worse damage than any other place in Hawaii, with the tsunami/seiche reaching a height of 14 m and killing 159 inhabitants.

Tsunamis are big. :)

Answer 2

Any wave becomes destructive when it gets to the shore.
Water molecules of a wave simply oscillate up and down, in the open sea, without damaging floating objects. The amplitude of the oscillation gives the height of the wave.
When they reach the shore or waters shallower than the amplitude of the oscillation, the waves "break", the oscillation becomes a traslation, the water actually moves forward and its impact becomes destructive.
More so depending on the height of the waves, which determines the mass of water that moves forward. Tsunamis can generate very high destructive waves.
The destruction is not limited to the impact of the forward movement. It occurs also when the water returns towards the sea.

Answer 3

One of the biggest and most devistating factors of the tsunami is when it rolls back out to sea. When you look at the waves you see the wave go up the sand and then back down into the ocean. The tsunami has the same effect but on a larger scale. It first does a ton of damage because over time it gets higher and when it approaches the shore it is monsterous and destroys buildings and people. But then when it rolls back out to the ocean it destroys even more property, and breaks down buildings severly weakened by the initiative crashing of the waves.

Answer 4

It comes at you so fast and powerful then because the ocean is pulling the water back out it causes undertows which are very dangerous and can drowned even the most experienced swimmer.