Use of momentum flux of tsunami overland flow as an indicator of potential damage to buildings

Abstract

The mega-tsunami of December 2004 caused massive loss of life and damage to property in Sri Lanka and in many other countries bordering the Indian Ocean. Although the destruction caused was unprecedented, given the seemingly low probability of occurrence of destructive tsunami events affecting Sri Lanka, all structures in the vulnerable areas of the coastal belt are not expected to be designed or retrofitted to withstand a future tsunami. However, if it is necessary to locate critical infrastructure in the inundation zone, such structures ought to be designed for tsunami impact to ensure their proper functioning in the event of a disaster. The design of load bearing members of such critical structures requires prior information about the expected intensity of tsunami impact and loading. Although most tsunami hazard maps typically present spatial variations of the flow depth and the flow velocity, it is likely that the combined effect of these two parameters is more reflective of the potential damage to structures. Accordingly, this paper examines the effectiveness of momentum flux, which is proportional to the product of water depth and square of velocity, as an indicator of the potential tsunami loading on structures. A numerical model based on non-linear shallow water equations was used to compute the spatial variations of the flow depth, the velocity and the momentum flux as a case study for four cities in Sri Lanka, which were heavily affected by the 2004 tsunami. The computed values of the above three flow parameters are compared with field data relating to the spatial distribution of the number of houses and other buildings damaged due to the 2004 tsunami. The field observations of the spatial distribution of the percentage of houses and buildings damaged correlate better with the momentum flux than with the flow depth or the flow velocity.