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Storm Surge Studies in the North Indian Ocean: a Review

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Storm Surge Studies in the North Indian Ocean: a Review Indian Journal of Geo-Marine Sciences Vol. 43 (2), February 2014, pp. 125-147 Storm surge studies in the North Indian Ocean: A review C. Shaji1*, S.K. Kar2 & T. Vishal1 1Centre for Oceans, Rivers, Atmosphere and Land Sciences (CORAL) Indian Institute of Technology, Kharagpur 721 302, West Bengal, India. 2I. M. Systems Group, Inc., Rockville, Maryland, USA. [E-mail: [email protected] ] Received 28 February 2013; revised 16 August 2013 Warm tropical North Indian Ocean (NIO), like the warm tropical North Atlantic and South and Northeast Pacific oceans, is a breeding ground for tropical cyclones. Tropical cyclones occur, quite often with severe intensity, in the tropical NIO during pre-monsoon and post-monsoon seasons, and are accompanied by very strong winds, torrential rains and storm surges. A storm surge is a meteorologically forced long wave motion, which can generate sustained elevations of the sea surface above the levels caused by the normal astronomical tides. Although storm surges result from the combined action of extreme wind stress and, to a lesser extent, reduced atmospheric pressure on shallow coastal shelf seas, the precise impact of the surge at any particular location is sensitive to certain meteorological, topographic and hydrological parameters, which include i) intensity and path of the cyclone and its spatial and temporal scales, ii) width and slope of the continental shelf, iii) geometry of local coastal and shelf features (bays, headlands, inlets, barrier islands, offshore islands and reefs), and iv) interactions of surge, astronomical tides, wind waves, river discharge and precipitation. The havoc caused by storm surges is found to be extremely severe in many countries situated around the NIO rim. Hence, real time monitoring and prediction of storm surges is of great importance in these regions. Various storm surge studies so far have taken place in the NIO region, particularly those in the Bay of Bengal and Arabian Sea, are dealt with in this article. After providing the details of the equations governing storm surges, an up-to-date review is attempted. Eventually, the authors’ views on what future directions could be taken in order to improve numerical storm surge modeling and predictions in the NIO region are mentioned. [Keywords: Tropical cyclone, Storm surge, Bay of Bengal, Arabian Sea, Models] Introduction usually referred to as a storm tide. Oscillations of water levels in a coastal or inland water body Floods and flood-related destructions in low-lying associated with a storm surge or storm tide can last coastal areas occurring from natural hazards such as from a few minutes to a few days, depending on the tsunamis, intense local precipitation, high river flows intensity of forcing mechanism2. During storm surges, and storm surges have been well known for hundreds water level can either rise or fall by several meters. of years. Among all the natural hazards, storm surges Rising of water level can be referred as “positive remain as the world’s leading ones, especially due to surge”, and lowering as “negative surge”. Positive their frequent occurrences in many coastal regions storm surges, when combined with high tides and and the heavy loss of life and property and damage to wind waves, can cause enormous coastal floods, coastal structures1. Throughout the world, widespread whereas negative surges reduce water depth and can efforts to mitigate coastal flood hazards are already be a threat to navigation. Associated storm surge in place, and are likely to intensify in the future. currents, superimposed on tidal and wave-generated flows, can also contribute to extremes of currents and A storm surge is an abnormal variation of water bottom stresses responsible for coastal erosion. Thus, level generated by a cyclonic storm (tropical or extra- a proper understanding of storm surges, the ability to tropical) over and above the predicted astronomical predict various aspects of them, and measures to tides. Change in water level due to the combined mitigate their destructive effects require special effect of storm surge and an astronomical tide is attention from oceanographers. *Corresponding author 126 INDIAN J MAR SCI VOL. 43 (2), FEBRUARY 2014 It is understood that storm surges are caused by deaths due to storm surges occur in countries the interactions of air, sea and land. Large horizontal bordering Bay of Bengal and Andaman Sea, with atmospheric pressure gradients and consequently very Bangladesh alone showing about 40% of the deaths2. strong surface winds moving cyclonically around the Intensity of storm surges along the Bangladesh coast storm provide the prime driving force for the surge, is so high as a result of many factors that include while the low pressure of the storm has minimal shallowness of the water body, funnel shape of the contribution on surge generation. Sea level continues Bay, high astronomical tides, thickly populated low- to ascend as the storm approaches the shallow water, lying islands, favorable cyclone track, and the and reaches a maximum along the coast when the discharge of one of the world’s major river systems– landfall of the storm occurs. Severity of storm surge Ganges-Brahmaputra-Meghna - in the northern sector related damages depends not only on a number of of the Bay3. Table 1 indicates the number of people storm factors such as intensity, forward speed, size, angle of approach to the coast and central pressure of Table 1—Death toll in Bangladesh due to various storm surges the storm, but also on certain coastal characteristics in history during the period 1822-1991 such as width and slope of the continental shelf. (only those cases in which number is > 5000 are considered) There are many coastal areas in the world that Year Estimated approximate No. of deaths are vulnerable to storm surges. Fig. 1 shows those 1822 40,000 coastal areas of the world that are affected by tropical 1876 1,00,000 1897 1,75,000 1912 40,000 1919 40,000 1960 15,000 1963 11,520 1965 19,279 1970 3,00,000 1985 11,069 1991 1,40,000 Fig. 1—Various ocean areas that are affected by storm surges due to tropical cyclones (TC) and extra-tropical cyclones killed in Bangladesh alone due to various storm surges (ETC)2 that occurred during the period 1822-1991. A detailed list of storm surges in the countries bordering the Bay and extra-tropical storms, and thus are prone to of Bengal is already available in the literature2 and significant surges. Damage from land falling cyclones hence, will not be reproduced in this article. is usually more due to storm surges than from rain and strong winds. About 90% of the damage is due to Main objective of this paper is to provide a inland inundation by the seawater. Moreover, flooding comprehensive review of the various storm surge of river deltas occurs due to the combined effects of studies carried out thus far in the North Indian Ocean tides and surges from the ocean, which intrudes into region, with emphasis given to India’s east coast in the river in spite of excess water from the heavy the Bay of Bengal and west coast in the Arabian Sea. rainfall flowing out of the river into the ocean. The ensuing sections in this paper are organized as Although the frequency of tropical cyclones in the follows. After providing the basic governing North Indian Ocean is not very high, the coastal equations used for surges computation in section 2, regions of India, Bangladesh and Myanmar suffer the advances made so far using various storm surge most in terms of loss of life and property caused by models are discussed in section 3. Finally, the storm surges. It should be noted that about 60% of all concluding remarks are drawn in section 4. 2172-feb SHAJI et al: STROM SURGE STUDIES 127 Governing storm surge computation equations conditions need to be prescribed. Thus, with reference Storm surges are long gravity waves that belong to the origin of the Cartesian co-ordinate system to the same class as of tides and tsunamis. Most of located at the undisturbed level of the ocean surface ç the dynamical theory of tides and storm surges is (i.e. at z = 0), the free surface is expressed as z = (x, y, t) based on the depth-averaged hydro-dynamical and the bottom as z = -D(x, y). equations of momentum and continuity. These Subsequently, the following surface boundary equations are usually referred to as shallow water or conditions are given at the ocean’s free surface at free surface equations. The validity of shallow water z = ç(x, y, t). equations depends on the ratio H/L << 1, where H is the depth scale and L is the length scale. In storm (1) surges the main concern is usually about water level elevation occurring due to surface forcing, = PP (2) disregarding the water level elevation from the direct a influence of the variations of currents with distance (3) below the sea surface. Because of this, the elimination of depth dependence on the vertical co-ordinate during where: the depth-averaging process can in fact give a useful t = time, and better-approximating simplified problem through ,, wvu = ocean velocity fields in the x, y and z the shallow water storm surge equations. directions, Storm surge equations can be formulated either Z = - D (x, y) = undisturbed depth of the in the spherical co-ordinate system or the rectangular ocean, Cartesian co-ordinate system. Since most of the storm Z = ç (x, y, t) = sea surface elevation, surge models developed is meant for regional = x and y components of the frictional applications, formulation of the equations in Cartesian stress, in the upper ocean, co-ordinate system can in effect give better results.
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