The Indian National Centre for Ocean Information Services (INCOIS) predicted in May 2024, that high sea waves, also known as swell waves, may hit coastal areas in Goa, Maharashtra, West Bengal, Odisha, Tamil Nadu, Kerala, Lakshadweep, Karnataka, Gujarat, and Andaman and Nicobar Islands. Swell waves, simply referred to as swells, are a type of surface gravity waves that occur in the open ocean and seas caused by distant storms like hurricanes, or even long periods of fierce gale winds, moving at a speed of 34-47 knots, instead of local winds. These waves are formed by an ocean swell; hence, the name swell surge. They are also referred to as surface gravity waves. On the other hand, waves are raised by winds blowing locally; the stronger the winds, the higher the waves. They die out, when the wind stops. A common example is the swell observed over coastal waters generated by winds of a distant tropical cyclone. Swells, which travel at a speed faster than the movement of tropical cyclone itself, have historically been considered as a sign for an approaching typhoon.
Kallakkadal
Kallakkadal are the flooding events like the one experienced by Kerala in March 2024 due to swell waves, with Alappuzha, Kollam, and Thiruvananthapuram districts being the most severely affected. Kallakkadal are coastal flash flood events that occur without any significant changes in local winds or other environmental signs. The word ‘Kallakkadal’ is derived from two Malayalam words kallan meaning thief and kadal meaning sea. The words were combined to mean an ocean that arrives as a thief. The term was officially approved by the United Nations Educational, Scientific and Cultural Organization (UNESCO) in 2012. Usually, Kallakkadal occurs in the southern Indian Ocean, which generates an ocean swell that travels north to reach the coast within two to three days. Other local term for such flooding event is Rissaga used in the Balearic islands.
Important characteristics of Kallakkadal includes intense wave-activity that continues for a few days; it inundates the low-lying coasts; the phenomenon is primarily observed during the pre-monsoon and occasionally post-monsoon season; and during high tide run-up, the water level can reach as much as 3–4 metres above the maximum water level (MWL).
Formation of Swell Waves
Swells originate from wind waves, which are created by the wind blowing over the ocean’s surface. The energy transferred from the air to the water generates waves. These waves can grow larger if the wind continues to blow steadily over a long distance and for a sufficient period. As the waves disperse from the wind-influenced area, they begin to form smoother and more uniform waves. The process of dispersion aids in classifying waves based on their speed and wavelength, with larger waves moving faster and surpassing shorter ones. The remaining organised and long-period waves that have travelled out of their generating area are known as swell waves. They have the ability to travel entire ocean basins from their source to distant shores. Swell waves are usually mistaken to be a tsunami.
Tsunami is a series of enormous waves, created by earthquakes below or near the oceans and are around 10 times faster than swell waves. The speed of both tsunami and swell waves slows down near the coast, but the former hits land at 30–50 km/h.
Characteristics of Swell Waves
In comparison to wind waves, swell waves have longer wavelengths, often ranging from 100 to 200 metres, and sometimes even more. They can cross entire ocean basins because of their significant speed. Despite losing some energy during their journey, they can still impact the coastlines far from their origin, causing significant damage to coastal features. They have a smoother appearance with more rounded crests. Since swell-generated waves are mixed with normal sea waves, they can be difficult to detect with the naked eye, particularly away from the shore, especially if they are not significantly larger than the normal waves.
How to Forecast Swell Surges
Forecasts are generated using a multi-model forecasting system based on numerical ocean models, incorporating real-time data from buoys deployed in coastal waters, open ocean, and deep south in the southern Indian ocean. The INCOIS, under the Ministry of Earth Sciences, issues swell surge warnings. INCOIS has forecast wave height of up to 3 m on the east and west coast, including in the islands of Andaman and Nicobar and Lakshadweep.
Impact of Swell Waves
Swell waves originating from the southern Atlantic Ocean, around 10,000 km from the Indian coast, are formed after a low-pressure system moved over the southern Atlantic Ocean, and cause strong winds and swell waves up to 11m in height, which then travel towards the southern Indian ocean.
They have the potential to cause significant damage to coastal structures and displacement of families. Water can flow on to the land and flood areas along the coast. Swell waves, particularly during storms, can significantly contribute to coastal erosion due to their amplified energy.
Conclusion
The understanding of swells is crucial for maritime navigation, ensuring safety and efficiency in shipping and small craft operations. Research should also be conducted on the potential of utilising the consistent nature of swell waves for the production of electricity as a renewable energy source. Their study is also significant in the dynamics of the ocean and have huge impacts on the coastal environment and human activities. The study of swell waves is also important in the fields of marine planning, coastal management, and oceanography. The study of the dynamics of swell waves would greatly help in terms of preparedness, mitigation strategies, and minimisation of damage.
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