World Oceans Day : 8th June

As we celebrate World Oceans Day on 8th June, it is a perfect opportunity to learn more about waves.

Recently, the Bureau of Meteorology (BOM) has replaced its two existing wave buoys, one approximately 10km off the west coast of Tasmania and the other nearly 8km from Kangaroo Island in South Australia, with TRIAXYS wave buoys. These buoys record the waves experienced over a 20 minute period, typically at the start of each hour – this is called the record. The summary of this data is then sent via satellite and displayed in near real-time every 20 minutes on the BOM website. But what do the observations mean?

Easiest to appreciate is Hmax, or the maximum height of the waves, which is the biggest wave measured in the record. The wave is measured from the trough to the peak of the wave and is reported in metres.

Significant Wave Height, or Hs. is more representative of the conditions and is the average height of the highest one third of the waves in the record. Care should be taken as the likely maximum wave height can be up to twice the significant wave height.

Also reported is the average time (Tz) which is the average time (in seconds) between all waves in the record while Tp is the period (again in seconds) of the waves with highest energy in the record.

Each buoy is moored with a combination of chain, rope and a rubber bungy cord which is used to attach the buoy to a 500kg weight. Although the site off Tasmania is moored in water depth of 100m, the mooring has to be much longer than this to make sure that it is not dragged under water. It also has to be able to withstand the tremendous force of the Southern Ocean. In 2012, a wave of 20m was measured at this location and waves of over 10m are frequently recorded.

As well as the sensors inside the buoy to measure the waves, the buoy has a water temperature sensor, 4 large batteries, solar panels to charge them, GPS, satellite modems and a processor to make it all run. Alarms trigger if water enters the buoy or if it moves off location and the sensors can be remotely monitored via satellite. They have the option to be upgraded in the future with the addition of current profilers should they be required by BOM.

Although the buoys can use and recharge their batteries for over 3 years, the buoys are serviced annually to remove marine growth and to replace the mooring. The buoys were sold by Hobart and Cape Town based company Metocean Services International, who themselves have deployed buoys in over 50 countries.

The wave buoy off Cape Sorrell was named Captain Fathom by ABC listeners in 2015 to celebrate the centenary of BOM.

As well as being referred to on ABC Radio, data is quality controlled and made freely available by BOM at http://www.bom.gov.au/products/IDT65014.shtml (Tas)

and http://www.bom.gov.au/products/IDS65030.shtml (SA).

Cape Sorell 1
Cape Sorell Buoy (Tasmania) showing Sig. Wave Height of 6m at 0000 30/5/19 but Max Wave Height of nearly 12m at the same time (courtesy BOM website)

 

See also from the web:

“Record-breaking wave off Tasmania’s west coast”  http://media.bom.gov.au/social/blog/32/record-breaking-wave-off-tasmanias-west-coast/

“Monster 23.8m-wave is largest ever recorded in southern hemisphere” https://www.abc.net.au/news/2018-05-11/largest-wave-ever-recorded-in-southern-ocean/9751680

 

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