Storm surges associated with severe tropical cyclones (TC) constitute the world’s worst marine hazard. An abnormal rise in sea level, a storm surge is caused by a TC moving over a continental shelf. The temporary rise of the sea level along the coast is in excess of the predicted astronomical tidal level. Storm surges are oscillations in the period range of a few minutes to a few days in water levels in a coastal or inland body of water. By this definition wind-generated waves and swell, formations of which range from a few to several seconds, are excluded.
Many a time, the term ‘storm tide’ is used to refer to the phenomenon of rising sea level at the time of a TC crossing the coast. The storm tide is actually the combined water level of the astronomical tide, storm surge and breaking wave set-up (Figure 1). Intense winds, characteristic TCs, blowing over a shallow basin can pile up sea water and lead to sudden inundation and flooding of coastal regions. In addition, flooding of the river deltas occur from the combined effects of tides and surges from the sea penetrating landward. In fact, the destruction associated with storm surges rank them as one of the foremost natural disasters, often surpassing earthquakes.
Storm surge may be divided into three stages —forerunner, main surge, and resurgence. A forerunner is the gradual rise of sea level preceding the TC—even when the TC is far from the coast, broad scale disturbances producing variations in the coastal sea level. Thus, a forerunner is an indicator of the arrival of a TC. The main surge follows after the storm surge rises to its peak. The period of the main surge at a place varies from a few hours to more than two or three days, depending on the speed of the TC. Resurgence follows the main surge. The sea level gradually tends to return to the normal state in this stage. It includes various oscillations due to topographic and other effects. The resurgence may sometimes continue for two to three days. Resurgences are also hazardous, particularly for marine and inland shipping.
Vulnerability of the Indian coasts
Storm surges in the Bay of Bengal: Storm surge is an extremely serious hazard along the eastern coast of India, Bangladesh, Myanmar, and Sri Lanka. Although Sri Lanka is affected only occasionally by storm surges, the TC of November 1964, November 1978 and November 1992 caused extensive loss of life and property in the region. Storm surges affect Myanmar to a much lesser extent in comparison to Bangladesh and India. Of the notable storm surges that affected Myanmar, the most recent has been the 2008 Cyclone Nargis, which generated storm surges in excess of 4 m near Ayeyarwady deltaic region. The entire deltaic coast of Myanmar was flooded with surges ranging from 1.5-4. 5 m.
There are well-documented accounts of devastation caused by storm surges associated with TCs hitting the Indian coasts. The severe cyclone of October 7, 1737 that crossed into West Bengal near the Hooghly River recorded a water level of 13 m which reached 100 km inland. About 3,00,000 people perished in lower Bengal and it was described as one of nature’s greatest catastrophes at the time. The cyclone of October 5, 1864, which crossed near Contai, saw storm waves rise to 10 m, with the maximum attaining 13 m in height. The October 31/ November 1, 1876 Backerganj Cyclone (Backerganj is now in Bangladesh) saw storm surges of between 3 m and 13 m at many places, and classified by many as the fiercest of that century. The destruction due to TCs and associated storm surges was sufficient to prompt the Government of British India to set up the India Meteorological Department (IMD) in 1875 with a mandate to minimise the loss of lives and property.
Although the frequency of TCs in the Bay of Bengal is not as high as that in the northwest Pacific, the coastal regions of India, Bangladesh and Myanmar suffer far more in terms of loss of life and property. The main factors contributing to disastrous surges in the Bay of Bengal may be summarised as: (a) shallow coastal water, (b) convergence of the bay, (c) high astronomical tides (d) thickly populated low-lying islands, (e) favourable TC track and (f) innumerable number of inlets including the world’s largest river system (Ganga-Brahmaputra-Meghna).
Storm surges in the Arabian Sea: Although the frequency of storm surges is less in the Arabian Sea than in the Bay of Bengal, major destructive surges can occur occasionally along the Gujarat coast of India and Pakistan. Historic records show that surges are frequent in the Gujarat coast where astronomical tides are quite high and surge heights reach up to 2-3 m. The Saurashtra-Kutch coast too is vulnerable to storm surges, reaching 3 m in height. Very few storm surges are reported around Mumbai or Honavar. The Kerala and Goa coasts are almost free from storm surges. Kerala, south Karnataka, and Goa coasts are the minimal surge prone zones, recording less than 1.5 m in surge. Storm surges along the Gulf of Oman are also rare. However, there have been instances of destruction caused by storm surges along the Oman coast. The most recent example has been that of super cyclone Gonu which struck Oman in 2007 and caused around 4 billion USD worth of damage.
Surge prediction data and forecasting
In order to achieve greater confidence in surge prediction in the Indian seas, input parameters that include oceanographic and meteorological factors (including storm characteristics), hydrological input, basin characteristics and coastal geometry, wind stress, seabed friction and information about the astronomical tides need to be sound. It has been seen that in many cases these input parameters strongly influence the development of a surge.
In India, P K Das (1972) pioneered the study of numerical storm surge prediction. Subsequently, several others worked on it and developed a real-time storm surge prediction system for the coastal regions of India, Bangladesh, Myanmar, and Sri Lanka. The IIT model used for predicting storm surges can be run on any personal computer in an operational office. Under the auspices of Tropical Cyclone Programme of the World Meteorological Organization (WMO), the technology has been transferred to the National Meteorological and Hydrological Services of the NIO region. Since 2009, the Regional Specialised Meteorological Centre (RSMC), New Delhi is using this model for providing ‘storm surge guidance’ to countries in the region.