A deterministic analysis of tsunami hazard and risk for the southwest coast of Sri Lanka

• We use both numerical simulations and field observations to assess tsunami hazard and risk.
• A scenario similar to 2004 Indian Ocean tsunami can be considered as the ‘worst-case’ for the study area.
• Tsunami inundation distribution is strongly influenced by topographic variations.
• Mean flow depth is an appropriate parameter to represent the severity of the tsunami flood hazard.
• Risk distribution computed using the standard risk formula gives satisfactory correlation with statistics of past tsunami impact.

This paper describes a multi-scenario, deterministic analysis carried out as a pilot study to evaluate the tsunami hazard and risk distribution in the southwest coast of Sri Lanka. The hazard and risk assessment procedure adopted was also assessed against available field records of the impact of the Indian Ocean tsunami in 2004. An evaluation of numerically simulated nearshore tsunami amplitudes corresponding to ‘maximum-credible’ scenarios from different subduction segments in the Indian Ocean surrounding Sri Lanka suggests that a seismic event similar to that generated the tsunami in 2004 can still be considered as the ‘worst-case’ scenario for the southwest coast. Furthermore, it appears that formation of edge waves trapped by the primary waves diffracting around the southwest significantly influences the nearshore tsunami wave field and is largely responsible for relatively higher tsunami amplitudes in certain stretches of the coastline under study. The extent of inundation from numerical simulations corresponding to the worst-case scenario shows good overall agreement with the points of maximum penetration of inundation from field measurements in the aftermath of the 2004 tsunami. It can also be seen that the inundation distribution is strongly influenced by onshore topography. The present study indicates that the mean depth of inundation could be utilised as a primary parameter to quantify the spatial distribution of the tsunami hazard. The spatial distribution of the risk of the tsunami hazard to the population and residential buildings computed by employing the standard risk formula shows satisfactory correlation with published statistics of the affected population and the damage to residential property during the tsunami in 2004.