Modelling ecological and other risk factors influencing the outcome of the 2004 tsunami in Sri Lanka

The 2004 Asian tsunami caused widespread devastation across the Indian Ocean. Damage from the waves was influenced by complex interplay of many factors. Using regression models incorporating different impact indicators (occurrence of inundation, inundation distance, and human deaths), we examined factors that significantly influenced tsunami damage in southern Sri Lanka. Land elevation (which is positively correlated with forest presence), sand dunes and coastal convexity (which deflects waves), were protective factors, in terms of whether the tsunami wave inundated within an arbitrary distance of 15 m inland. Water bodies, saltpans and built-up areas (e.g., roads, development) increased wave inundation distance, while beaches and sand dunes were ameliorative. Built-up areas also increased death toll. Bathymetry, a proxy for wave height and force, was the only significant factor in all three multivariable models. Coastal areas with a steeper seaward bathymetric slope gradient were less prone to inundation and human fatalities, which is consistent other tsunami research. The presence of coral reefs was highly correlated with bathymetric slope gradient, implying a potentially protective role by coral reefs. Many factors showing greatest protective effect against tsunami damage are geographical features that cannot easily be modified. However, some defence factors identified, such as sand dunes, could potentially be protected. Given the wide-ranging ecosystem services, unrelated to tsunami protection, ongoing loss/degradation of natural systems including mangroves (not a significant factor in the multivariable models), could be damaging. This might impede certain future coastal development options in post-tsunami Sri Lanka.