The growing wave of water

Along both coasts, the Indian sub-continent extends into the sea to varying degrees. This extension is known as the continental shelf. In peninsular India, the shelf is a lot narrower than that along the northern parts of the east and west coasts. The shelf is also much wider along Mumbai’s coast and along Bangladesh’s coastal regions. Towards the Bay of Bengal for example, the shelf extends for up to 300 km into the sea, while in parts of peninsular India, it extends for less than 100 km at some points.

As the wave travelled from the undersea quake off Indonesia towards the Indian sub-continent, it did not manifest in the form of a giant wave in the open seas because of the depth of the ocean. In the Bay of Bengal, the December 26 tsunami wave measured only 0.6 m in height. So ships in the open ocean would not have even felt it passed beneath.

However, as Dr Krishna explains, when the wave—travelling at over 750 km/hr—hit the Indian sub-continental shelf, due to the sudden and sharp rise in the level of the ocean floor, the kinetic energy of the wave—essentially a huge column of water—was transformed into potential energy. The column of water, after hitting the shelf-edge, manifested itself in the form of a great wall of water several meters high, which rushed towards the coastline.

‘‘It is here that the shelf width comes into play,’’ explains Dr Krishna. ‘‘If the shelf-width is large, as in the Bay of Bengal, the energy of the tsunami wave is dissipated by the time it hits the coastline. However, in south India, with the shelf-width being much shorter, the wave still has a lot of energy in it as it hits the coastline. This causes the destruction.’’

What this means therefore, is that peninsular India (both the east and west coasts) stands a much higher risk of devastation compared to the northern coastline, in the event of a tsunami similar to the one that hit on December 26.

Solving the mystery of how the tsunami waves affected certain parts of the west peninsular coast, Dr Krishna explains: ‘‘The tsunami effect on the west coast seen in Kerala and Goa were not from the direct waves, but from diffracted (redirected) waves from the Maldives islands.’’

‘‘The waves hit Visakhapatnam at 9.05 a.m. (IST) and Tuticorin at 9.57 a.m. on the east coast. It hit the Maldives at about 9.45 a.m. while it hit Cochin at 11.10 a.m. and Goa at 12.25 a.m. on the west peninsular coast. These timings are indicative of the tsunami touching various places. As we can see, Maldives islands were hit much before the west coast of peninsular India. We can infer that the tsunami seen on the Kerala and Goa coasts was not from direct waves, but from the waves diffracted (redirected) from the Maldives,’’ explains Dr Krishna.