Still Standing: Tsunamis won’t wash away Maldives atolls

Tiny coral-reef islands such as those in the Maldives archipelago may appear fragile, but they aren’t easily swept away, a new study shows. The waves of the Dec. 26, 2004 tsunami were devastating to the islands’ inhabitants, but researchers now find that the waves’ geological impact on the islands themselves was minor and had little effect on their long-term stability.

TOUGH ISLES. The December 2004 Sumatran tsunami battered, but didn’t break, the 1,200 atolls of the Maldives. NASA Earth Observatory

The Maldives includes about 1,200 coral-reef islands, or atolls, in the Indian Ocean. The reefs sit atop the craters of a string of undersea volcanoes south-southwest of India. The low-lying islands are vulnerable to sea level rise. Monsoon winds, which reverse direction from winter to summer, also impose a seasonal effect on the shorelines, redistributing beach sands to alternating coasts.

Many scientists had assumed that the islands would be highly vulnerable to tsunamis, says coastal geomorphologist Paul Kench of the University of Auckland in New Zealand. In the absence of previous data on the impact of tsunamis on the structure of reef islands, Kench and his colleagues set out to assess how the 2004 disaster affected the long-term stability of the Maldives.

The tsunami, generated by an earthquake 2,500 kilometers away off the coast of Sumatra, reached the eastern islands of the Maldives within 4 hours. A series of surges inundated the country, leaving 80 people dead and many islands uninhabitable.

Kench’s group compared current shorelines and sand depths with pre-tsunami data. “We were able to detect geological changes on the islands, but these were not catastrophic,” Kench says.

The tsunami appeared to mimic the effect of a full season’s monsoon winds, eroding 5 to 9 percent of the easternmost islands’ area and 1 to 5 percent of the western islands. It also redistributed sand to the sheltered sides, rather than washing it away, the researchers report in the March Geology.

The way tsunami waves build in height probably prevented severe geological damage, Kench says. As the waves approach most shores, the sea’s depth decreases, and wave height increases. The waves that hit Thailand, for example, were 10 to 15 meters high.

However, the sudden transition from deep ocean to steep reef islands gave the waves no time to build, Kench says. Along the Maldives, the tsunami waves were no more than 3 m tall.

The study presents a “unique opportunity” to directly observe both pre- and post-tsunami effects on these islands, says sedimentologist Gene Rankey of the University of Miami’s Rosenstiel School of Marine and Atmospheric Science. The new tsunami data can provide comparisons for future research.

“They’ll be able to see what is preserved 6 months from now,” he notes, “when the winds change again.”

Carolyn Gramling is the earth & climate writer. She has bachelor’s degrees in geology and European history and a Ph.D. in marine geochemistry from MIT and the Woods Hole Oceanographic Institution.

More Stories from Science News on Earth