The Tsunami Threat for Coastal Areas

Tsunamis are much more than freak ocean waves and there are four ways in which a tsunami can result, each with varying degrees of power. What sets a tsunami apart from normal or even giant surf beach waves is their phenomenal wavelength, which is the linear distance from peak to trough. A standard beach wave may exhibit a wave length ranging from a few feet to yards depending on atmospheric conditions, tides, ocean currents and of course the geology of the sea floor close to land.

A tsunami can exhibit wavelengths of hundreds of miles and this invariably results in devastation once these waves make landfall. This happens when the sea floor starts to shallow as the wave approaches land. The lower part of the wave slows down as the upper surface continues at close to the propagation rate. Water begins to pile up and this is where the wave peaks however with such an incredible wavelength the problem is not simply a crash onto shore then back out to sea all the water piled up behind the wave keeps on pushing it inland and this is why a relatively small tsunami of about 25 feet caused so much destruction on boxing day two years ago.

An undersea earthquake is one of the common causes of tsunamis however these waves are limited in height by approximately 30 feet. If the sea floor suddenly drops by this amount the resultant wave is of the same dimension yet pressure of seawater seems to limit these undersea displacement events. Most times a tsunami will not result however if the displacement is over a wide area and spans hundreds of feet a tsunami of proportional dimensions will usually generate.

Another cause of tsunami events come from beyond the earth’s atmosphere, it is thought this phenomena has occurred many times throughout our planet’s history with resultant tsunamis generated to hundreds or perhaps even thousands of feet in height, and which rush many hundreds of miles across continents. The span of human existence on earth is negligible when compared to the calculated age of this planet and fortunately it seems these events only occur every few hundred thousand years or so. The problem is nobody really knows when the last one occurred or when the next is likely to happen.

In the nineteenth century the Indonesian island of Krakatoa suddenly exploded in one massive event that vaporized over five cubic miles of solid rock. The gaping hole instantly flooded as seawater crashed in to fill the voided magma chamber with resultant tsunamis causing the deaths of thousands living near coastlines all over the world.

Volcanic activity is a fairly common cause of tsunami events and some of these can generate waves of incredible proportions. This was demonstrated on an Island near Alaska shortly after WWII. A scientist and his son were studying an unusual scouring phenomena up to a height of nearly 500 feet along the shoreline of an estuary where trees exhibited signs of some unknown but terrific force and stress. While on site a wall of white water greater than this height crashed through the bay entrance and by the grace of God their boat was lifted then carried across treetops before being dumped back into water on the other side of the bay. This event was reported and upon returning to the original site all vegetation was stripped bare to nearly 600 feet up the sides of hills from this tsunami event.

The cause was soon discovered on an island close by. A minor volcanic rumble had dislodged one side of the island causing billions of tons of rock and debris to crash down in the space of a few seconds. The water displacement was proportional to height of land that slipped into the sea. The resultant tsunami raced off at great speed and straight into where the unsuspecting scientist was working from his boat in the nearby bay.

Tsunami events are relatively rare however there is no coastline in the world that is immune from the experience. It is part of our planetary environment much as super-storms, hurricanes, tornadoes, volcanoes, earth quakes indeed every conceivable natural disaster that defines the unstable and unpredictable planet on which we live. There is little we can do but hope sufficient warning is issued for evacuation to higher ground. This earth is a wondrous place yet fraught with natural disasters that can happen without warning, and will no doubt continue to happen long after the human race fades to extinction.