Healthy reefs may help shield against tsunami damage
By Jan TenBruggencate
Advertiser Science Writer
By Jan TenBruggencate
A living and healthy coral reef provides dramatically more protection from tsunami and storm surf than a degraded or dead reef — an argument for strong protections for reefs against sediment runoff, uncontrolled dredging and climate change, researchers say.
"One would expect a healthy reef to provide a considerable amount of protection against a tsunami," said Michael Oppenheimer, a Princeton University geoscientist and co-author of a study in the journal Geophysical Review Letters that models the protection offered by healthy versus dead reefs during the Christmas season tsunami of 2004 in the Indian Ocean.
The same may be somewhat true for Hawai'i, local coral reef researchers say, although Oppenheimer's study does not accurately reflect the condition on most Hawaiian coastlines.
"With lots of coral, you get lots of drag (on moving water) and lots of protection," said coral reef ecologist Paul Jokiel, of the University of Hawai'i's Hawai'i Institute of Marine Biology.
Reefs develop a structure as they grow that creates friction for water, said researcher Robert Richmond of the University of Hawai'i's Pacific Biosciences Research Center at Kewalo Marine Lab.
"A healthy reef, generally speaking, would provide better protection from tsunami. Coral reefs are incredibly important in protecting coastlines," he said. But he suggested that the Hawaiian reefs may be better at protecting the Islands from storm surf than from tsunami.
Oppenheimer and his team developed a computer model of a tsunami strike on a volcanic island surrounded by reef. Their computer model for a healthy reef is one with a great deal of vertical structure representing coral heads, channels, and the kind of physical complexity an actual reef has. A dead reef was represented by a largely featureless surface, representing a reef that has died because of dredging or pollution and whose corals have been broken down by erosion.
They found that under some circumstances, a healthy reef can provide twice the protection of a dead one. It does that simply by creating friction or drag that breaks up the energy of the tsunami.
"The precise amount of beneficial effect depends on the height of the tsunami and the configuration of the coastline," he said. The team concluded that a coast near the epicenter of a tsunami-generating quake might not have much protection, but a healthy reef on a coastline distant from the site of the quake would.
"As an example, some of the people in Sri Lanka would have been in much better shape if they had had a healthy coral reef," Oppenheimer said. Areas with generally healthy reefs fared better, he said.
Coral reef biologist Jim Maragos, of the U.S. Fish and Wildlife Service, said that Oppenheimer's model reflects a coastline with a broad, shallow reef, which is not the condition around Hawai'i generally.
"Unlike the situation Southeast Asia, most Hawaiian reefs are neither shallow or broad," he said, largely because the water is a little cold for fast coral growth, and because heavy waves from trade winds and storms from the north-northwest inhibit their development.
Most tropical islands have wide, shallow reefs, but in Hawai'i they tend to be steeper, meaning a tsunami would be affected less by them than it would elsewhere, he said.
Richmond said that one of the classic formations of healthy reefs on the slopes approaching the Hawaiian Islands is the "spur and groove" reef. These are complex environments with tall stony live coral ridges cut by deep, often sand-bottom gullies.
"They are extremely effective in breaking the strength of the wave," he said.
Coral reef bio-geochemist Marlin Atkinson said that Hawai'i coral reefs are generally in pretty good shape, although there are significant problems in localized areas — areas where eroded mud settles and smother reefs, where there has been dredging, and so on.
Atkinson also said that reefs are built to handle occasional storms and tsunamis—in fact, some corals depend on these events to break the up, so the pieces can resettle and the corals can spread, he said.
"Large waves can wipe out corals. Our reefs get blasted by high energy. As a result, Hawai'i reefs do not tend to have the expansive, highly branched corals that some areas have. They are relatively flat," he said. But they are still effective, he said. The said the Kane'ohe Bay barrier reef forces waves to break repeatedly as they approach the bay, so that there is very little energy left when the reach the shallows.
Islands with robust offshore reefs protect the shore by removing energy from the waves, he said.
"The water is just rubbing against the bottom, and that slows the water down, forces that wave to break, and in breaking it loses more energy," Atkinson said.
Both Atkinson and Oppenheimer said that one of the biggest threats to coral reefs is the increasing levels of carbon-dioxide in the atmosphere, not because of global warming, but because it tends to make the oceans more acidic.
Atkinson said reef growth drops in more acidic water, meaning less rock and less sand.
"Acidification is a major problem. The reefs don't seem to adjust. They don't seem to adapt," he said.
Oppenheimer agreed: "The ocean is acidifying, and that is bad for calcification (the creation of calcium carbonate, the substance from which coral rock is made). The only solution is reducing the buildup of carbon-dioxide. There is no other alternative.
"There are lots of reasons to protect reefs. Ecosystems like coral reefs provide a lot of value besides aesthetic value," he said.
Not the least of these is that a good complex reef generally has significantly more fish, Jokiel said.
"High roughness — what we call rugosity — is one of the most important predictors of high biomass of fish," he said.
Reach Jan TenBruggencate at firstname.lastname@example.org.