26 December Tsunami caught the people from Asia unprepared. It took away large number of lives and properties in no time. This was the largest tsunami so far – in the region that hardly experienced even minor tsunamis earlier. India was also one of the many countries that had an impact of this on its islands of Andaman and Nicobar and also the main lands especially the south east coastline. As has been indicated elsewhere this institute had no priority on tsunami research. However, after the incident, coastline surveys and cruises were undertaken by NIO scientists to find out impact of tsunami.

Earthquakes below the seawater are one of the root causes of the tsunamis. They create waves (tsunami) in the seawater, which then travel to the coastlines. A tsunami carries considerable energy or has a momentum as it arrives at a shoreline, which is hazardous to the properties and population. The data such as bathymetry, tide level, inundation limits, run-up heights along the coastlines was collected as a first hand information besides interviewing the eye witnesses and villagers staying on the coastline.

On the main land, the Tamil Nadu coast was affected most and therefore NIO team concentrated at large on this coastline. The nature and impact of the phenomenon was recorded. Twenty three locations on Tamil Nadu and one along the Andhra Pradesh coastline (Table 1) were surveyed to find out why and to what extent, the tsunami brought in (landward) the seawater – normally called inundation (Figure 1). The team also tried to estimate the height of the waves at the locations of survey.

The water entered to almost a kilometer at Nagore (Figure 2). The inundation limit was noticed to be dependent on the tsunami height and the topography of the coastline. The team noticed flat coastlines allowed entry of water much deeper. However, at some places, the presence of dunes restricted the entry and reduced impact.

On the coastline, the effect of tsunamis is also measured by ‘run-up heights' - a vertical height (from mean sea level) of the tsunami wave that reaches onshore (Figure 2) . While the inundation was noticed to be highest in south coast of Tamil Nadu, the run-up heights were consistently higher on the northern region. This was attributed to the bathymetry of the shallow waters near coastline. The term bathymetry refers the charting the topography of the seafloor and water depth. The concave bathymetry (Figure 3) helps waves to refract and converge. Thus the waves coming from either side collide with each other increasing their height – the run-up height. The team observed traces of these phenomena at the points where the bathymetry was concave on the northern part of Tamil Nadu. Of course, reverse was the case where the bathymetry was convex. At Periyakalapet (Station 10 – Figure 4), the run-up height was highest. The other places farther north like Mahabalipuram and Sadarangapattinam (stations 15, 16 – Figure 4) also experienced a good run-up height but not as high as that of Periyakalapet because of the wider continental shelf – a relatively shallow, submerged portion of the continent.

The eyewitness reports suggest that there were three distinguishable pulses of tsunami and their magnitude and period appear to have been different at different sites. In many places the second wave was biggest.

The study teaches us that the natural barriers such as sand dunes and coastal vegetations – casurinas, palms, coconut, sand dune grass, mangroves, etc. – have played a major role in minimizing damage and therefore need to be protected to save from such hazards. A fact that the unavoidable structures that come up closer to the coastline need to be constructed using the modern engineering practices and understanding the behaviour of nature to have minimal effects in such a situation.

To consult original article, refer :

Jayakumar, S.; Ilangovan, D.; Naik, K.A.; Gowthaman, R.; Tirodkar, G.; Naik, G.N.; Ganeshan, P.; ManiMurali, R.; Michael, G.S.; Ramana, M.V.; Bhattacharya, G.C. Run-up and inundation limits along southeast coast of India during the 26 December 2004 Indian Ocean tsunami. Curr. Sci.: 88(11); 2005; 1741-1743.

Anon. Observations of post tsunami reconnaissance investigations along eastern coastal tract of India following the devastating tsunami of 26^th December 2004. National Institute of Oceanography Technical Report no. NIO/TR-08/2005. 2005; 109pp.