Disaster Management in Fisheries

2.1.4 Tsunami

Tsunami is a Japanese word with the English translation, "harbour wave." Represented by two characters, the top character, "tsu," means harbour, while the bottom character, "nami," means "wave." In the past, tsunamis were sometimes referred to as "tidal waves" by the general public, and as "seismic sea waves" by the scientific community. The term "tidal wave" is a misnomer; although a tsunami's impact upon a coastline is dependent upon the tidal level at the time a tsunami strikes, tsunamis are unrelated to the tides. Tides result from the imbalanced, extraterrestrial, gravitational influences of the moon, sun, and planets. The term "seismic sea wave" is also misleading. "Seismic" implies an earthquake-related generation mechanism, but a tsunami can also be caused by a non-seismic event, such as a landslide or meteorite impact.

Tsunami is large, powerful waves caused by sudden displacement of large quantities of water in the sea / ocean. Common causes for the displacement of large quantities of water are: earth quakes, volcanic activity and landslides. They can also be caused by detonation of nuclear devices under water, or by the impact of an asteroid. As the wave of a tsunami moves into shallower water, it gets much higher, so they can be very big and very destructive when they hit coastal areas. Sometimes (but not always) the water will recede dramatically just before a tsunami strikes a coastal region - if you see the sea suddenly pull back, going further out than a normal low tide in a very short time then you really want to get out of there as quickly as possible and seek safety

Tsunamis are unlike wind-generated waves, which many of us may have observed on a local lake or at a coastal beach, in that they are characterised as shallow-water waves, with long periods and wave lengths. The wind-generated swell one sees at a California beach, for example, spawned by a storm out in the Pacific and rhythmically rolling in, one wave after another, might have a period of about 10 seconds and a wave length of 150 m. A tsunami, on the other hand, can have a wavelength in excess of 100 km and period on the order of one hour.

As a result of their long wave lengths, tsunamis behave as shallow-water waves. A wave becomes a shallow-water wave when the ratio between the water depth and its wave length gets 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. Let's see what this implies: 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 the rate at which a wave loses its energy is inversely related to its wave length, tsunamis not only propagate at high speeds, they can also travel great, transoceanic distances with limited energy losses. The earthquake-generated 1960 Chilean tsunami, for instance, travelled across over 17,000 km across the Pacific to hit Japan. The wave crests bend as the tsunami travels —- this is called refraction. Wave refraction is caused by segments of the wave moving at different speeds as the water depth along the crest varies.

As a tsunami approaches shore, we've learned in the "What happens to a tsunami as it approaches land?" section that it begins to slow and grow in height. Just like other water waves, 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. Tsunamis may reach a maximum vertical height onshore above sea level, often called a run-up height, of 10, 20 and even 30 meters.