14/10/2010 | by Alex Dick-Read
As widely reported, early Monday April 2nd a tsunami warning was issued in Australia by the Bureau of Meteorology. An earthquake had occurred just off the coast of the Solomon Islands at 20.55 on April 1st … and this was definitely no joke. The warning, posted by the Pacific Tsunami Warning Centre (NOAA) carried the chilling statistics of:
“ORIGIN TIME – 2040Z 01 APR 2007, COORDINATES – 8.6 SOUTH 157.2 EAST, LOCATION – SOLOMON ISLANDS, MAGNITUDE – 7.8. The warning continued … ”AN EARTHQUAKE OF THIS SIZE HAS THE POTENTIAL TO GENERATE A DESTRUCTIVE TSUNAMI THAT CAN STRIKE COASTLINES IN THE REGION NEAR THE EPICENTRE WITHIN MINUTES TO HOURS. AUTHORITIES IN THE REGION SHOULD TAKE APPROPRIATE ACTION IN RESPONSE TO THIS POSSIBILITY.”
Residents of the Solomon Islands and adjoining land masses soon learned of the destructive power of this event, with tragic consequences and loss of life.
In Australia, media bulletins pumped out the news and everyone was in a state of semi disbelief … the memories of the 2004 Boxing Day tsunami had everyone wary. Councils and Emergency services swung into action, with beaches closed and low lying schools and workplaces on the coast evacuated.
Along with the Bureau of Meteorology and Government agencies, Coastalwatch was inundated with phone calls and emails, requesting urgent information and advice. Most enquiries were concerned about the safety factor of family and friends, while a few wanted to know if the swell was going to get big and could they surf it.
People logged onto the Coastalwatch web site in record numbers to observe if the waterline was going to recede before giant waves hit our coastline. It was the busiest day in the history of Coastalwatch, with 197,385 page views and 46,907 user sessions, which translated into the highest page views on a single day, or double the average traffic on site.
Coastalwatch employed the abilities of its innovative CoastalCOMS system, an automated observation system for coastal monitoring comprised of a shore-mounted video camera networks using sophisticated image processing routines for analysis and display of coastal environment information. CoastalCOMS provided comprehensive real-time and predictive information about beach and surf conditions.
As well as the live vision, the real-time data includes offshore wave height, near shore wave height and wind direction and strength. When combined with data from the Bureau of Meteorology and other sources, the data is stored and used to train a predictive computer-based system for short-term forecasting of beach and surf conditions, providing reference materials for future review.
Coastalwatch Staff monitored CoastalCOMS and the latest updates from the Bureau of Meteorology, with particular focus on Willis Island off the north Queensland coast. The island would be the first place ‘hit’ by the tsunami in Australian waters. There was a feeling of relief after a report came through from the Bureau that Wallis Island had suffered no damage and no significant rise in sea level. The news got even better as reports came in with the same results from North Queensland.
The most common type of Tsunami is also known as a seismic sea wave, these are caused when the sea floor is suddenly displaced by an earthquake. Other causes of a Tsunami can be sudden shifts in the sea floor due to underwater landslides (also often associated with earthquakes), meteorite impacts, volcanic eruptions or even nuclear detonations. Tsunamis differ from regular ocean waves and swell as they have extremely long wavelengths. Whereas waves in a typical ocean swell in deepwater may be spaced around 200 m apart, Tsunami waves can have a wavelength of hundreds of kilometres. Waves are classified as shallow water waves if the wavelength is large compared to the depth, thus Tsunamis are considered as shallow water waves even when travelling in the deep ocean. Tsunamis can travel at speeds of many hundreds of km/h in the open ocean.
Since the distance from the trough to crest is so large, a Tsunami has a very gradual slope in deep water and is hardly noticeable to ships at sea. In the same way that swell waves entering shallow water refract and shoal in response to changes in the underwater landscape, Tsunami waves ‘feel’ the bottom even at great depths. As the Tsunami enters shallower water it slows down and the height increases.
During the shoaling process there are various ways in which the wave energy can be dissipated. Bottom friction, reflection from underwater features such as the continental shelf, turbulence, wave breaking and refraction, the bending of the waves around headlands and islands can all reduce the energy contained within a tsunami as it propagates away from the source.
Resulting impacts for Australia were thankfully minor, with small fluctuations in sea level of around 20-30 cms were detected in New South Wales, and 30-50 cm in Queensland by the Bureau of Meteorology SEAFRAME sea level monitoring network. Despite the excitement, there were no reports of any significant impacts on Australia’s East Coast.
Gold Coast City Council Lifeguards were on high alert but did not observe any significant changes in shoreline behaviour around the expected arrival time. Lifeguards were able to monitor beaches using the Coastalwatch camera network, and beaches were closed in the interest of public safety. Once the threat had passed and the Bureau alerts were downgraded beaches were re-opened.
Later in the day after the warning had been cancelled, a trip to the beach revealed that there had been no damage done and all activities were back in full swing. How lucky the Australians were, and it was definitely better to be safe than sorry. Our thoughts and best wishes go to the people of the Pacific affected by the tsunami.
Australia’s beaches are a precious resource. By providing stakeholders, and the public with comprehensive real-time and predictive information about a range of beach conditions, Coastalwatch makes a significant contribution to safer beach use, and coastal management.