Tsunamis - Extreme Events: A Physical Reconstruction and Risk Assessment - page 109

5

Tsunamis

Tsunami characteristics and formation

Tsunamis of substantial size are waves of tremendous power and have
brought great devastation to many coastal human populations around the globe.
They have the ability to surge onto dry land beyond that reached by normal (wind
generated) ocean waves. Because they are rare events, coastal populations have
often settled in places that are occasionally, and surprisingly for these people,
inundated by these waves. They have consequently acquired a reputation for the
damage they can do to coastal infrastructure and the deaths to many who are
swept away.
Tsunamis are very different from wind generated ocean waves. Tsunamis are
long-period, shallow-water waves. They have periods (time between successive
peaks or troughs) of many minutes compared to wind- and storm-generated
waves which have periods of seconds. Because of their long period, the distance
between tsunami wave crests can be many kilometres unlike the tens of metres
for wind-generated waves. This results in the tsunami effectively touching bot-
tomorthe sea floor across even the deepest ocean basins. This is why tsunamis
are referred to as shallow water waves. Shallow water waves occur where the
water depth is less than half of the wavelength; the period and wavelength of a
wave in the ocean is related to the velocity or celerity of the wave. The equation
describing the celerity of shallow water waves in the ocean is

C = ( gd ) 0 . 5 (5.1)

where C = celerity, g = gravitational constant (9.8 m s − 2 )and d = water depth.
This equation shows that the deeper the water, the faster the wave will travel.
As storm waves have much smaller periods, their wavelengths are much shorter
than the water depth until they reach the shallow waters of continental shelves

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