Department of Civil Engineering, University of Peradeniya, Peradeniya 20400, Sri Lanka
The megathrust earthquake of moment magnitude 9.1 – 9.3 on December 26, 2004 unleashed a massive tsunami which devastated the coastal belts of Sri Lanka as well as several other countries bordering the Indian Ocean. Extensive field observations carried out in Sri Lanka in the aftermath of the tsunami clearly showed that the spatial variation of the degree of destruction along the coastal belt was highly non-uniform. The coastal geomorphology, for instance, the presence of sand dunes in some parts of the coast, is one primary factor that had contributed to this nonuniformity. Accordingly, the present paper investigates, as a case study, the effect of a nearly 2 km long sand dune, lying along a part of the seafront of a city on the south coast of Sri Lanka, on the characteristics of the spatial distribution of inundation. Numerical simulations based on non-linear shallow water equations were carried out first with the sand dune and then without the dune to obtain the respective spatial distributions of the maximum values of the depth of inundation and the flow velocity as well as their temporal variations. The results appear to indicate that the peak flood flow rates and flow depths are higher in most areas of the city in the case without the dune compared to that with the dune. However, it appears that the tsunami surge backing up against the sand dune and other elevated beachfronts causes a rise in water level of up to 0.5 m in the case with the sand dune compared to that without the dune. Flow velocities in the absence of the dune too appear to be higher in most areas although there are patches of lower velocities at certain low elevation localities with elevated ground on either side.