Fernando et al. writing in Eos (86, 301, 304; 2005) demonstrate that in Sri Lanka tsunami damage was mitigated by the presence of intact coral reefs, demonstrating the regulating ecosystem service that coral reefs can provide.

From a press release from ASU states:

Lead author Fernando, an ASU professor of mechanical and aerospace engineering, reports that in the town of Peraliya, a 10-meter (30-foot) high wave swept 1.5 kilometers (one mile) inland, carrying a passenger train about 50 meters (200 feet) off its tracks, with a death toll of 1,700. Yet, a mere three kilometers (two miles) south, in Hikkaduwa, the tsunami measured just 2 to 3 meters (7 to 10 feet) in height, traveled only 50 meters (200 feet) inland, and caused no deaths.

The researchers found that this pattern of patchy inundation to be characteristic of the study area and was not related to such coastline features as headlands, bays and river channels. Rather, the key factor was the presence or absence of coral and rock reefs offshore.

At Hikkaduwa, the hotel strip is fronted by a rock reef and further protected by coral reefs that the local hoteliers protect and nurture, the researchers report. Relatively little damage and few deaths were recorded from there to Dodanduwa, around six kilometers (four miles) to the south.

From Hikkaduwa north to Akuralla, however, damage and loss of life was extensive. Local residents, interviewed by the authors, say that coral reefs in that area had been decimated by illegal mining, especially by use of explosives that result in harvests of both coral and fish.

Some eyewitnesses to the tsunami described a visible reduction in the height of the water wall and its deflection parallel with the shore as it approached the coral reef. The researchers conclude that waves that had been blocked by the reef caused even more inundation and damage where they found low resistance gaps due to removal of coral by humans.

A news report in Nature Tsunami damage was enhanced by coral theft adds:

There is a precedent for this phenomenon. In Nicaragua in 1992, a tsunami surged through a break in the coral reef made to let boats through. “Within this passage, water went one kilometre inland,” says Fernando. “But nearby, where the coral was intact, there were still beach umbrellas standing.”

In Sri Lanka, coral is illegally mined to provide souvenirs for tourists, or to be ground up for use in house paint. Coral harvesters sometimes blow the reefs up with dynamite in order to collect fish at the same time. Often, the reefs in the best shape are those in front of hotels, as the hotel owners maintain them for the tourists. Fernando hopes that his finding will encourage the Sri Lankan government to enforce its laws against coral mining.

Policy may very well change, says M. Sanjayan, an ecologist with the Nature Conservancy, who surveyed the environmental damage after the tsunami. “There has been a groundswell of support in Sri Lanka for better enforcement of laws,” he says. “There is a window of opportunity right now.”