Tsunami hazards in Sri Lanka : assessment of exposure levels of the southern coast

Abstract

In Sri Lanka, monsoonal floods, droughts, cyclones and landslides are the common and well-known natural hazards and almost every year the country experiences one or more of these hazards. Tsunamis have not been frequent in the history of Sri Lanka but the Indian Ocean tsunami in 2004, subsequent tsunami alerts issued in the country and historical accounts of tsunami events in the past have clearly highlighted the exposure of Sri Lanka to tsunami hazards. Tsunamis are generated by a variety of causes and undersea earthquakes have been identified as the most common cause for tsunami generation. Considering the location of Sri Lanka and the undersea earthquake prone regions in the world, it is evident that the country is exposed to potential tsunami events generated at the Sunda Trench located to the east and the Makran Fault located to the north-west. The Indian Ocean Tsunami was caused by an earthquake occurred in the Sunda Trench. The Indian Ocean Tsunami caused widespread damages in coastal areas of the country and the exposure of many coastal areas to tsunami hazards became evident form the damages experienced. The southern area of the country was significantly affected by the in 2004 and the 03 main populated/urban areas along the southern coastline-Galle, Matara and Hambantota-were considered for the assessment of the levels of exposure.

Numerical models are widely used to simulate tsunami events in order to assess the exposure of coastal areas to tsunami hazards. The numerical model MOST with the user interface ComMIT, developed by the National Oceanic and Atmospheric Administration (NOAA) of the United States Department of Commerce was used to simulate the selected earthquakes at Sunda Trench and Makran Fault in this study. The tsunami wave height in coastal waters and the tsunami arrival times were obtained by numerical modelling were considered to assess the level of exposure. Sunda Trench was divided into 4 regions, extending from north to south along the trench, for the purpose of this study. The upper (northern) part of the trench was considered as Region 1, and the middle section were divided in to two regions, Region 2 and Region 3. The lower part was considered as Region 4. Based on the results of the study, it can be concluded that the southern coast is at a high risk due to the earthquakes generated in Regions 2 and 3 of the Sunda Trench. A high level of exposure was evident due to earthquakes of magnitudes higher than 9.0 Richter Scale. No significant level of exposure was evident due to tsunamis generated at the Makran Fault. An early warning system would be very effective in mitigating adverse impacts due to tsunamis and such a system can be based on a large database developed from the results of tsunami simulations similar to the ones carried out in the study. Further improvements of the results of such simulations can be made by considering higher resolution bathymetric information obtained by surveys, together with relevant overland topographic data to assess the inundation characteristics of tsunamis in coastal areas.