PHYSICAL IMPACTS OF THE 2004 SUMATRAN TSUNAMI ON REEF ISLANDS IN THE MALDIVES ARCHIPELAGO

Mid-ocean atoll islands are generally viewed as fragile landforms that are particularly vulnerable to sea-level rise, climate change and extreme events such as hurricanes and tsunami. Presumably, these island systems have been affected by multiple tsunami during their geological history, but to date there is no published work detailing the affect of these potentially catastrophic events on reef island stability or long-term evolution. Detailed observations of the physical changes caused by the 2004 Sumatran tsunami on mid-ocean coral reef islands are presented here, based on pre- and post-tsunami surveys of the planform and topography of 13 islands in the Maldives atoll of Baa. We compare island form in 2002 and 2003 with that surveyed in February 2005, six weeks after the Sumatran tsunami and show measurable, but relatively minor, shoreline erosion and re-deposition of sediment. Changes in island surface area was less than 9% for 12 of the study islands, manifest as localised erosion of pre-existing island scarps. One island, a partly vegetated sand cay, experienced 80% loss of vegetated surface area but was the anomaly. The most common and significant tsunami impact was a shift in the distribution and morphology of sandy beaches fringing the islands. The scale of these changes was sufficient to accentuate regular seasonal changes in shoreline shape, with some beaches showing up to 9 m retreat and exposing the island core as a low scarp. Sand transported by the tsunami was typically re-deposited as cuspate spits on the leeward sides of islands and as washover sheets on the outer edges of the island interior. These sand sheets range in thickness up to 30 cm and cover a maximum of 17% of island area. In no cases did sediment re-deposition extend across an island, due mainly to the presence of thick vegetation. Bioturbation of tsunami deposits was evident at many sites, with crab burrowing contributing to the rapid in-situ turnover of the deposit. We conclude that unmodified (vegetated) mid-ocean atoll islands are physically robust landforms that can withstand extreme inundation events such as tsunami, and that the geological evidence of tsunami has low preservation potential in this setting due to the effects of regular changes in beach morphology driven by monsoonal shifts in wind and wave direction, and bioturbation.