How does tsunami travel and what happens when it approaches shore ?
Tsunami is a series of waves of extremely long wavelength and period generated in a body of water by an impulsive disturbance that displaces the water. The energy of an event mentioned above travels outward in all directions from the source (look at the picure on the left). Tsunami in deep water can have a wavelength greater than 500 kilometers and period (time between two successive waves) of about an hour. Because of their long wavelengths, tsunamis behave as shallow-water waves. The wave can be called shallow-water when the ratio between the water depth and wavelength is very small. 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/s*s) and the water depth. The deeper the water is, the faster tsunami travels. In the Pacific Ocean, where the typical water depth is about 4000 m, a tsunami travels at about 200 m/s, or over 700 km/hr. Because a wave loses energy at a rate inversely related to its wavelength, tsunamis can travel at high speeds for a long period of time and lose very little energy in the process. The height of the wave in deep ocean is vey small - about twelve to twenty-three inches. The high speed and low height makes tsunami very hard to recognize. As tsunami approaches shore, the water depth decreases and the wave slows. The tsunami's energy flux, which is dependent on both its wave speed and wave height, remains nearly constant. Consequently, as the tsunami travels into shallower water, its speed diminishes and its height grows. It can finally reach the height of 10 to 30 meters. When it finally reaches the coast, a tsunami may appear as a rapidly rising or falling tide, a series of breaking waves, or even a bore. Tsunamis begin to lose energy as they rush onshore - part of the wave energy is reflected offshore, while the shoreward-propagating wave energy is dissipated through bottom friction and turbulence. Despite these losses, tsunamis still reach the coast with tremendous amounts of energy. Tsunamis have great erosional potential, stripping beaches of sand that may have taken years to accumulate and undermining trees and other coastal vegetation. Capable of inundating, or flooding, hundreds of meters inland past the typical high-water level, the fast-moving water associated with the inundating tsunami can crush homes and other coastal structures.