Extreme surfer's guide to the reef
An on-line Wave Atlas to check the likely size of waves during a cyclone is now available to users and managers of the Great Barrier Reef.
Professor Tom Hardy and his team at the Marine Modelling Unit, in James Cook University's School of Engineering, developed the guide called the Atlas of Tropical Cyclone Waves in the Great Barrier Reef.
"Marine planners are interested in cyclones because although rather infrequent when they come, they can really rip up large sections of reef," said Professor Hardy.
"We want to see how the wave models can predict destruction of reef areas. This has very important implications for their management and protection."
To compile the atlas, the researchers first devised a computer model to generate a set of 6,000 cyclones to represent those that threaten the Coral Sea.
This population of 'synthetic' cyclones was then analysed to provide wave data for more than 150,000 grid points (or every 1.5km) along the Great Barrier Reef.
"Coral reefs are very difficult for modelling because waves travel over thousands of kilometres but are affected by very small objects. So modelling both the extent and the resolution is very difficult," said Professor Hardy.
Nevertheless, the research team is currently expanding the Wave Atlas from cyclone waves to 'normal' waves, which requires additional computer modelling of six years of wind data.
"Biologists don't know how much the wave energy is. What we're hoping to do is give biologists and planners a much greater feel for the kind of forces out there and the occurrence interval of those forces", said Professor Hardy.
The atlas uses the idea of 'encounter probability' to enable data from a range of 'return periods' to be queried.
A return period is the waiting time between an event of equal or greater magnitude. If a wave height of six metres has a return period of 100 years, it is likely data for 1000 years will show wave height to be six metres or greater on roughly 10 occasions.
In its first application, the new Wave Atlas has been used to develop guidelines for planning and designing floating structures such as pontoons popular with tourists who snorkel and make day trips to the reef.
"If you want to site a pontoon, you can check the atlas. Even though a spot looks sheltered during normal conditions, it may have six metre waves during a cyclone," said Professor Hardy.
Approximately 20 large floating pontoon-type facilities are allowed to operate in the Great Barrier Reef Marine Park. These are more than 50 metres long, normally located on outer reef sites up to 60km from the coast, and receive up to 450 visitors a day from fast catamarans.
Yet not much time, money and academic expertise has been applied to their design and operation, unlike other floating structures such as ships and oil platforms, says Professor Hardy.
"Pontoons are catamaran-shaped rather than barge or ship-shaped," he explained. "They're in shallow water with lots of obstructions that grow. And they're at least a thousand times less expensive than ships. There hasn't been any real guidance for the forces on these things and how their mooring systems are going to operate."
Environmental problems have occurred with pontoons and their moorings. A major concern has been the risk of coral damage from a pontoon sinking or breaking free in bad weather or cyclonic conditions.
"It used to be suck-it-and-see, let's stick something in, if it moves too much, let's make it heavier. But things have really improved since the 1980s", said Professor Hardy, adding the guidelines are an opportunity to bring together information and make suggestions.
"They're not prescriptive standards, they're not laws. It would be bad engineering to say this is how these things have to be done, because that would stifle innovation."