U.S. Geological Survey (USGS) scientist Bruce Richmond returned in early March from a two-week trip to examine the impacts of the December 26 tsunami on the low-lying atolls of the Republic of Maldives in the Indian Ocean. The country consists of 1,190 islands grouped in 26 atolls that stretch more than 800 km from north to south, crossing the equator at their south end. Although the archipelago is about 2,500 km from the epicenter of the earthquake that triggered the tsunami, its average elevation of only 1.5 m left it exposed to the tsunami waves, which swept completely across many of the islands. The Maldives was the only country where the effects of the tsunami were felt across the entire country, rather than in certain parts or regions. Total damage estimates are about $470 million. Approximately 100,000 people, or a third of the population of nearly 300,000, have been severely affected, with approximately 12,000 displaced and 8,500 in temporary shelters. About a quarter of the nearly 200 inhabited islands in the archipelago were severely damaged, and 10 percent were made uninhabitable. At the time of this writing, more than 80 people have been reported dead, and 26 are missing. The tsunami occurred during daylight hours near low tide, two factors that probably helped keep the death toll relatively low.
The recent field study was undertaken at the request of the Maldive government's Ministry of Environment and Construction (MEC) and funded by the U.S. Agency for International Development (USAID)'s Office of Foreign Disaster Assistance. Richmond, a coastal geologist from the USGS Pacific Science Center in Santa Cruz, CA, was part of a United Nations Environment Programme (UNEP) Rapid Assessment Team that included experts in waste management, ground water, soils, biodiversity, sanitation, construction, and agriculture. The outside experts were joined by local counterparts from various Maldive government ministries who assisted with data collection and interpretation. The team visited 17 islands on 7 atolls over a 10-day period.
The islands of the Maldives are Holocene features that began forming 3,000 to 5,500 years ago. They are composed primarily of carbonate sediment derived from coral reefs and deposited by waves and currents. The submerged mountain chain on which the coral reefs are built has been in existence for millions of years, whereas the islands are some of the youngest land surfaces on Earth. Because they are largely unconsolidated, the islands should be considered ephemeral features over geologic time scales, and their low elevation makes them particularly vulnerable to storms and changes in sea level.
Tsunami water levels measured by the assessment team reached a maximum of 3.25 m; most measurements ranged from 2.0 to 2.6 m. The tsunami wave heights typically decreased from east to west as the waves traveled across the islands. Eyewitnesses on many islands reported tsunami waves approaching from the west. This phenomenon is believed to be a result of the tsunami refracting around the ends of the individual islands. The tsunami's extent ranged from complete island overwash to inundation around island margins. Wave effects were most pronounced on eastern shores, but flooding was widespread among the islands.
Alteration of coastal landforms by the tsunami was greatest among islands situated close to the eastern reef rim facing the direction of tsunami approach. Beach erosion was widespread and characterized by the formation of erosional scarps, typically 0.3 to 0.5 m high. Where the eastern beaches consist of mostly gravel-size reef debris, material was deposited as sheets, 10 to 20 m wide and 10 to 20 cm deep, landward of the berm. On islands that were overwashed, prominent scarps formed along western sandy shorelines, and the eroded sediment was deposited westward into adjacent reef-flat and lagoon areas. Changes to island interiors were generally limited to local scour around obstacles, such as buildings, and thin, patchy sediment deposits. Parameters that appear to be controlling factors in local variations in the tsunami's impacts include the following:
Although offshore bathymetry appears to have influenced the characteristics of the tsunami as it approached land, bathymetric information from the Maldives is so scanty that its effects are difficult to quantify.
The tsunami was described by many eyewitnesses as multiple waves (generally three) about 20 seconds to minutes apart. Each succeeding wave was higher than the previous wave, with insufficient time for water to recede before the next wave hit. Tidal records, however, show that the main tsunami waves were about 20 minutes apart. What the eyewitnesses were observing was the evolution of a single wave into multiple bores, called undulatory bores and (or) solitons, as the tsunami passed from deep water to shallow water over the reefs.
There were many reports of freshwater flowing out of wells and from the ground immediately before the tsunami's arrival. In many places, house floors were buckled upward by the pressure. These occurrences suggest high permeability of the subsurface and a direct connection between ground water and the surrounding seawater.
Some eyewitnesses took photographs of water spouts occurring exactly at the same time as the tsunami struck. This phenomenon is unusual because water spouts typically do not occur near the equator.
As in other countries hit hard by the tsunami, human activities made some of the tsunami's impacts more severe. For example, dredging of sand and gravel from reef areasa widespread practice in the Maldives that supplies needed construction materialsappears to have caused long-term erosion at the north end of the Maldive island of K. Guraidhoo. The tsunami accelerated this erosion, resulting in the undermining and collapse of several coastal structures. The assessment team observed at numerous sites that natural shorelines and land surfaces, either on uninhabited islands or in natural areas of inhabited islands, were much less damaged than developed areas.
The impact of the tsunami serves as an indicator of the vulnerability of the Maldives to external forces of nature. The tsunami provides an opportunity to better educate the people, to establish guidelines for human activitiessuch as dredging and constructionnear the shoreline, and to develop plans for future disaster mitigation.
in this issue:
Assessing Tsunami Impacts in the Maldives