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How
'shadow' areas escaped the fury Prof. Gopalakrishnan, who did his Ph.D. in Wave Hydronomics, Ocean Engineering at the Madras IIT, said the coastline of countries such as Malaysia and Singapore also escaped the destructive force of the tsunami as they were in the shadow regions of the Indonesian islands, even though they were closer to the epicentre. In India, the 'shadow' coastal regions of Tuticorin, Rameswaram, Kanyakumari, Tiruchendur, Thiruvananthapuram, Kollam, Alappuzha and Kochi were relatively less affected. In these areas, the effects were generated by diffracted waves, which are less intense than the direct impact of tsunamis. Prof. Gopalakrishnan, who is the Principal of Sriram Engineering College, said when a wave encounters a large barrier, its motion penetrates the region of the geometric shadow by a process of scattering known as diffraction. When the waves are diffracted by a large leading body, their height gets progressively diminished, resulting in continued reduction in wave energy flux until it meets with another boundary of the trailing body. He said understanding this phenomenon was of considerable practical significance in establishing the wave action behind large breakwaters or offshore structures and around small islands. Prof. Gopalakrishnan said apart from being in the direct line of the tsunami, Nagapattinam was the worst-hit because its coastline was the target of tidal waves gliding off the tangential boundary formed by the Sri Lankan eastern coastline (see map). Generally, the ocean surface experiences random waves of different magnitude in wave heights and wave periods depending on the average rate of transfer of energy (energy flux) by winds blowing on the ocean surface. This energy is transported through wave forms to the shoreline until it is fully dissipated by sea bed friction and wave breaking ('shoaling') as the waves approach shallow waters near the shore. Waves generated under normal wind conditions are harmless except in cyclone storms when higher order waves are generated with severe breaking force nearer the shoreline.However, unlike wind-generated waves, tsunamis are characterised as shallow-water waves with long periods and wave lengths. If a wind-generated wave can have a wave period of 5-10 seconds and a wave length of 100-150 m, tsunamis can have a wave length of 100-500 km and wave period of an hour or more. In the context of the Indian Ocean, where the typical water depth is about 3,000 m, a tsunami travels at a speed of about 600 kmph. Tsunamis not only propagate at high speeds, they also travel great transoceanic distances with limited energy losses, he said. - Hindu (Jan. 13, 2005) |
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