Abstract:
A series of ocean waves with exceptionally long wavelengths resulting from an impulsive
disturbance that displaces water is defined as a Tsunami. These catastrophic waves can be a
result of volcanic eruptions, landslides, the explosion of cosmic objects, and most prominently,
seafloor earthquakes. In recent history, the Indian Ocean Tsunami event of December 2004
invoked colossal destruction in coastal regions of Sri Lanka where nearly 35000 lives were
lost. This demonstrated not only the vulnerability of people, infrastructure, processes,
commodities, information, and services, but also the need for an early warning system to limit
the number of fatalities and injuries caused by tsunamis in coastal regions of Sri Lanka.
Generally, far-field tsunami waves provide a time window of a few hours through which, the
authorities could gain clearance to issue early warnings and start evacuation protocols before
waves strike the coastline in case of a tsunami event. Therefore, an accurate projection of the
exposure can highly benefit the process of incorporating this lag-time effectively to issue early
warnings. Galle district suffered with the second highest deaths from 2004 Tsunami and
Unawatuna has a significantly high population density (1200/sq.km) along with extensive
tourist attraction throught the year. However, recent research has not specifically focussed
Unawatuna coastaline, except for the general studies done along whole the Southern coast.
Hence, this research outlines the exposure assessment for the southern coastline in Unawatuna,
Sri Lanka extending from Watering Point (Bonawistawa) up to Dalawella.
Community Modelling Interface for Tsunamis (ComMIT) model was utilized for the modelling
of tsunami waves with moment magnitude (Mw) of 7.6 -9.3, originating along the Sunda arc
near Indonesia and Makran fault off the coast of Pakistan. Bathymetry data retrieved from
DEM Global Mosaic dataset that was developed by National Centres for Environmental
Information (NCEI) was utilised for developing the model. The developed model was run for
2004 December Tsunami event and the results showed significant similarity to both historical
event modelling and existing observation data. Hence, model was then run for the identified
tsunami generation scenarios to obtain maximum wave heights at 50m depth.
The maximum wave heights at 1m depth in 4 selected locations were calculated, combining
the model run results and the Green’s Law to identify the tsunami exposure levels of the coastal
stretch. It was revealed by the study that the coastline between Bonawistawa and Dalawella is
severely exposed to tsunamis generated by earthquakes of 9.2 Mw and 9.3 Mw. It was also
revealed that the coastline is highly exposed at Bonawistawa and Dalawella while moderately
exposed at Yaddehimulla and Unawatuna Central, to earthquakes of 9.1 Mw. There is
negligible exposure to for magnitude 8.4 along the shore, and none for magnitudes 8.2 or
lower. This hierarchy of exposure will be key in the planning of evacuation and early warning
systems.
