Tuateawa Tsunami?

The Scream Edvard Munch

In the late summer of 1883 the evening skies of Norway were dramatically changed by the sudden appearance of fiery tongues of light in a blood red sky. It must have seemed like some apocalyptic message to Edvard Munch. It provoked him to produce his painting "The Scream" portraying deep anguish and despair. It has now entered popular culture in the form of Halloween masks and tacky teen horror movies.

On the other side of the world, on the 29th of August 1883 the seas suddenly rose by 1.8m. in both Whitianga and Tairua. The event that connected these two happenings was the eruption of Krakatoa in Indonesia on the 26th of August of that year. Quantities of dust in the air produced memorable sunsets around the world and reduced the planets temperature by over a degree. The sonic boom was so large it was heard in Australia and it's power created sets of sea waves known as wave trains. When these reach land we know them as tsunami or harbour waves.

This tsunami was one of eleven to hit the Coromandel since 1840. Most of the tsunami originate from vast earthquakes of the western shores of S.America, particularly Chile and Peru. Wave trains are created that can have an height of 18m. at source. This conjures up an impression of huge walls of water rushing toward NZ to create death and destruction. The reality is a little different. It takes almost 18hrs for the waves to reach NZ. By this time the individual waves are quite small and present absolutely no threat to vessels out to sea. While the height of the waves are not maintained the distance between them is. This is the most crucial aspect as it is one of two factors that will determine how destructive a tsunami might be.

Young children have an almost intuitive understandingof the mechanics of tsunami. They love to make an impact on their environment and the bath is a great place to do just that! A slight movement of the body makes a wave, another slight movement, at just the right time adds another wave on top of the other. Before very long water is rushing to and fro soon to produce a flooded bathroom and a rather annoyed parent! The very best effects come when there is a very precise time interval between the creation of waves. The longer the bath the longer the time interval must be.

In the Coromandel environment the equivalent of the bath is known as the Whangapoua embayment. It is the sea that is enclosed between Port Charles and Otama and includes the Mercury Is. The precise time interval between waves that will produce tsunamai will vary from location to location. In 2003 Environments Waikato and Bay of Plenty jointly funded a three year study into tsunamai in their regions of responsibility. Part of these studies involved modelling two different possibilities. Both involved simulating wave trains, with the individual waves separated not by seconds as in the bath but by 75 and 90 minutes. The result of this mapping located hotspots where the largest tsunamai might occur. In these sites an increase in wave height by a factor of twenty was possible. So, quite small waves could be amplified into something extremely destructive.

The significant period for our location was 90 minutes and for Whitianga 75 minutes between waves. In 1960 Whitianga did experience a tsunami with a wave interval of around 40 minutes. It was close to optimum. There was some flooding, sea level reached 2.5m. above normal, boats were washed from moorings and live fish were found flapping on the coast road.

Port Charles is our local hotspot. It was hit by tsunami in both 1868 and 1877. Neither event was severe. Logs and milled timber was washed away and some houses were flooded to a depth of 2ft. in 1868. The sea rose by 6ft. in just 5min. In 1877 the sea fluctuated in level by upto 12 ft which doesn't much exceed the change in a normal tidal cycle. Here though the tide rushed in and out every 20 min. for most of the day!

The past is a reasonable predictor of the future. In the normal course of events a "run up" of 2.5m. above normal height is expected to be a worse case scenario for the area. Archeologists have found evidence of run ups over 5m. but the most recent is many hundreds of years in the past. These days there is a Tsunami Warning System which, for tsunami originating at considerable distances, would provide early notice. Tsunami originating closer to home with consequently less warning time are thought to be unlikely. There are some details regarding safety measures in a tsunami to be found in the back of the Yellow Pages.