The model demonstrates that healthy reefs offer the coast at least twice as much protection as dead reefs. The finding provides the first quantitative confirmation of a widely held theory regarding the value of living coral reefs as a defense against tsunami waves, which are often generated by powerful undersea earthquakes.

Princeton professor Michael Oppenheimer said his team's work will give scientists the ability to quantify how much any given reef will benefit its particular stretch of coast.

"Healthy reefs have rougher surfaces, which provide friction that slows the waves substantially in comparison with smoother, unhealthy ones," said Oppenheimer, the Albert G. Milbank Professor of Geosciences and International Affairs. "Scientists had never before studied this effect by the numbers, nor had they ever analyzed it over a wide variety of coastal shapes. This study provides yet another motivating factor for protecting the planet's coral reefs from degradation."

The team's findings appear in the Dec. 14 edition of the journal, Geophysical Review Letters. In addition to Oppenheimer, other team members include Robert Hallberg, who is head of the Oceans and Climate Group at the National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Lab, and Catherine Kunkel, who is the paper's lead author. Kunkel spearheaded the work during her senior undergraduate year at Princeton, from which she graduated with a physics degree in June.

Though anecdotal observations of reefs' effects on tsunami abounded after the 2004 Indian Ocean strike, Kunkel said, it was difficult to form any real conclusions because so many of these observations came after the fact. The goal of this study, which began as Kunkel's senior thesis project, was to provide a systematic framework by which to examine the assumption that healthy reefs protect shorelines more effectively.

"For our purposes, we assumed that the health of the reef would only be important in terms of the drag it exerted on the wave," said Kunkel, who is currently working as a research assistant at Tsinghua University in China. "If you have a healthy reef, it has lots of live coral branching out, sticking a lot of small obstacles into the water. A dead reef, on the other hand, is not as rough -- it tends to erode and exerts less drag on the wave."

A turbulent mountain of water crashing over a complicated rough surface presented Kunkel with a number of obstacles for her own study -- specifically, how to find a way to express each of these effects with a mathematical formula that a computer could employ to simulate it. Different complex parameters had to be considered one by one: the width and depth of the reef; the roughness of its surface; the size of the lagoon behind it; and the slope of the coast beyond. And the overarching element was the wave itself and its interaction with all these obstacles. Eventually, Kunkel found a set of equations that provided a limited but comprehensive picture of a tsunami strike.

"We had to idealize a number of factors, because we wanted to create a model that could be used for a general shoreline," Kunkel said. "For example, we had to consider a perfectly even ocean floor, because uneven ones can funnel a wave into a certain area."

Despite the limitations of the model, Oppenheimer said it provides a sound basis for the team's conclusions.

"The general conclusion is that a healthy reef might provide twice as much protection as a dead one," he said. "This could translate into sparing large sections of inshore area from destruction."

Because coral reefs are dying from rising ocean temperatures, increasing ocean acidity, and direct human damage, Oppenheimer said the findings offer yet another reason to protect these fragile offshore ecosystems.

"This study shows yet another way that protecting the environment relates to humanity in a very tangible way," he said. "Villages get built behind coral reefs for good reasons, and this is one of them."

Kunkel said that she hoped the study would inspire other scientists to continue the research by obtaining more observational data. Incorporating such data into the team's theoretical model, she said, would then allow scientists to plan better for future tsunami strikes along local coastlines.

"We now have a basic idea of what variables are important, but if you want to quantify the effectiveness of a barrier reef around a particular island, you'd want to model that island directly," Kunkel said.

earlier related report
Brisbane, Australia (SPX) Dec 14 - Large scale coral die-offs are now occurring more frequently than at any time in the last 11 000 years, according to a new study by Australian-based scientists.

Investigations by Associate Professor John Pandolfi, of the ARC Centre of Excellence in Coral Reef Studies and The University of Queensland, of fossilized reefs in Papua New Guinea show how often the reefs were 'wiped out' by disastrous events in past times.

"What we found was in stark contrast to what we see in the modern day," says A/Prof Pandolfi. "The frequency of reef [die-off] events in the fossils is at least an order of magnitude less than it is today," he says.

Studying the fossil reefs of the Huon Peninsula, which are now cliff faces rising up to 25 metres above the beach, A/Prof. Pandolfi and his team were able to see where large scale disturbances had occurred in the history of these reefs which dated back as far as 11 000 years ago.

"The sequence of fossils is like a three dimensional movie of what happened in the reefs' past," says Pandolfi.

Over the 6000 years recorded in the fossil strata the team found 4 devastating events which resulted in the death of most of the reef - indicating such events had occurred about once every 1500 years.

"The cause of some of these events was volcanic, but others may have been due to bleaching, disease, or something else - we just don't know. Regardless, what is clear is that the frequency of die-off was so much lower than it is today," says Pandolfi of his findings.

The results, published in the scientific journal 'Geology', sound a warning bell to modern day reef management practices as today's reefs face more stress than ever.

But while the ancient reefs warn that we are seeing abnormal die-off rates, they also show that the reefs of the past recovered rapidly after these events, taking as little as 100 years to be repopulated by the corals that normally occurred there.

Although they are struck by more high impact events, some of today's reefs are still proving to be able to 'bounce back' to good health - with a little help.

"The recovery of the Great Barrier Reef from the devastating impact of the crown of thorns starfish took less than a few decades, at least in part due to comprehensive reef management," says Pandolfi,"...but this rate of recovery isn't seen in other parts of the world....some reefs still haven't recovered from [events in the last century]," he says.

With the help of the ARC Centre of Excellence in Coral Reef Studies, A/Prof. Pandolfi plans to continue studying fossil coral reefs around the world in order to help build a bigger picture of how the world's reefs have survived devastation in the past - so that managers can help them continue to survive into the future.

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