Indian Journal of Geo-Marine Sciences Vol. 43 (2), February 2014, pp. 125-147

Storm surge studies in the North : 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 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 , 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: , 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.

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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. = x and y components of wind stress, Thus, herein, we discuss briefly the salient features at the ocean surface, of storm surge equations written in a Cartesian coordinate system, leaving further details to be read P = pressure at the ocean surface, 2&4 from the literature . To formulate storm surge Pa = atmospheric pressure at the ocean equations, several steps need to be followed, which surface. are briefly discussed below. In the above surface boundary conditions, the First, certain valid assumptions are made in the water body. These include treating the ocean as a) condition expressed through equation (1) shows that incompressible (i.e. vertical accelerations can be the tangential atmospheric wind stress components ignored and density variations affect the buoyancy at the ocean surface can provide the horizontal of water only), b) homogeneous (i.e. constant water frictional stress components (Reynolds stresses) to density) and c) having friction arising out of vertical the upper ocean body. The second surface boundary shear much more significant than to the horizontal condition as through equation (2) reveals that the friction. The right-handed Cartesian co-ordinate pressure at the ocean surface is treated as equal to the system (x, y, z) followed is in such a way that the z- atmospheric pressure. The third condition expressed axis orients vertically upward from the mean sea level through equation (3) is the kinematic surface (MSL), x-axis points eastward and y-axis points boundary condition, which otherwise indicates that northward. Then the basic hydrodynamic equations the free surface of the ocean materially follows the fluid. of momentum and continuity are written in the Cartesian co-ordinate system. The boundary condition given at the ocean bottom Second, to close the above system of hydrodynamic at z = -D(x, y) is that all ocean velocity fields vanish equations, appropriate surface and bottom boundary there. Hence at z = -D(x, y), we have,

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wvu === 0 ... (4) f = the Coriolis parameter (= 2ÙsinÖ, where Ù is angular velocity of Earth’s rotation Third, since the main objective of storm surge and Ö is the latitude). equations are the prediction of long waves in shallow The final equations (5)-(7) consist of a set of three coastal waters, as the next step the hydrostatic coupled equations for the unknowns U, V and ç. In approximation is brought in to simplify the vertical the above storm surge equations (5) and (6), the momentum equation. This is done based on the two acceleration of water on the left side is equated to the valid assumptions that 1) the amplitude of surge is forces acting on it on the right side. The small compared to the water depth and 2) the meteorological forces generating storm surges are the horizontal scale of surge is large compared to the wind stress and horizontal gradients of surface water depth. With this, finally the hydrodynamic atmospheric pressure associated with traveling storms equations are reduced to two horizontal momentum (i.e. the inverted barometric effect). Both atmospheric equations, one vertical hydrostatic equation, and a pressure and wind effects are present in all storm continuity equation. surges, but their relative importance varies with Fourth, a simplification is usually introduced by location of the storm. Note that in equations (5) and integrating the governing equations vertically. The (6), wind stress is divided by the total water depth h unknown dependent variables then become a) the (third terms on the right hand side). Thus, it appears water transport or the depth-averaged currents U and that wind stress is less effective in generating surges V and b) the sea surface height ç. This procedure has in deep water where h is large, compared to shallow been commonly used for storm surge computations water where h is small. But this does not apply to the as the water level is of primary concern. Thus, influence of atmospheric pressure variations in the integrating the governing equations comprised of two storms because the relevant terms in equations (5) horizontal momentum and a continuity in the vertical and (6) (second terms on the right hand side) are direction from the ocean bottom at z = -D(x, y) to the independent of depth. This indicates that in deep free surface at z = ç(x, y, t), by means of the surface water, surges are produced mainly by changes in and bottom boundary conditions expressed through atmospheric pressure of the storms, whereas in equations (1)-(4), we arrive at the following depth- shallow water and in particular on continental shelves averaged shallow water or free surface equations of and near to coasts, wind stress forcing associated with motion and continuity that can well be used for storm the storms dominates the surge’s generation. Equation surge computations. (7) represents the continuity equation expressed in terms of the depth-averaged velocities. This shows (5) that for a vertical column in the ocean, changes in surface elevation ç due to tide or surge or both are (6) related to the net fluxes of water in or out across its sides. In most of the numerical storm surge models, on the basis of scale analysis, it is possible to avoid (7) the non-linear advection terms from equations (5) and (6). This is justifiable when the characteristic where, all variables except the following are amplitude of the surge is smaller than the already explained earlier: characteristic depth of the basin. But in very shallow r water regions such as the head of the Bay of Bengal, ,VU = components of depth-averaged current U , the non-linear advection terms are especially important and hence must be retained. h = the total water depth (= D + ç, where D is Fifth, to get a closure of the storm surge problem, undisturbed depth), appropriate parameterizations need to be given to the = the density of the ocean, assumed uniform, bottom as well as surface wind stresses. The quadratic friction law is usually used to express bottom stresses g = the acceleration due to gravity, in terms of the depth mean current. That is,

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(8), (disturbances) outwards from the model domain in the form of simple propagating waves, can be given. where k is a non-dimensional bottom friction coefficient that is typically 2.6 x 10-3. Finally, the depth-averaged 2-D partial The surface wind stress associated with the storm is usually parameterized in terms of the surface wind velocity and for that again the quadratic friction law is used. That is, (9) ñ where a is the air density and C10 is a non- dimensional wind stress drag coefficient. Though the parameter C10 is wind-dependent, providing a value of about 3.0 x 10-3 is usually considered as a very good approximation. It should be noted that storm surge models consider wind stresses and atmospheric pressure field as input. Devastating storm surges are normally caused by strong winds with an onshore component. When storms move over the ocean, energy is transferred to the ocean through tangential and normal Fig. 2—An example showing the illustration of the staggered stresses which are generated by pressure gradients. finite difference computational grids used for storm surge This input of energy is partly lost by tangential stresses modeling along the east coast of India6 at the ocean bottom. Most of the energy supply is differential equations describing the storm surge initially kinetic and several processes take place phenomena can be solved numerically by any of the before surges (potential energy) are generated and methods, viz., finite-difference, finite-element, or maintained. Hence, the computation of realistic finite-volume. surface winds and wind stresses associated with a moving storm to a certain degree of accuracy is an essential prerequisite for the realistic simulation of storm surges along the coasts. Any error in the estimation of cyclone winds can lead to substantial error in the wind stress computations, which ultimately can cause erroneous surge computations also. Accurate surge computations, thus, demands the usage of good cyclone wind models that can give not only accurate estimates of cyclone wind fields but also information such as an accurate track and landfall location of the storm. Sixth, to get a complete system of storm surge equations, appropriate lateral boundary conditions also need to be provided, in addition to the above discussed bottom and surface parameterizations. Usually the boundary condition specified at the coastal land boundaries is that normal transport vanishes at the coastal boundaries. At the open ocean Fig. 3—An example showing the illustration of the finite element computational grids used for storm surge modeling lateral boundaries, a radiation type of condition, along the east coast of USA, Gulf of Mexico and Caribbean which allows for the propagation of energy Sea7

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Though here we discussed the salient features of 2-D storm surge models, there have also been increasing usages of 3-D models for storm surge computations (for example, see a previous work5). The main advantage of 3-D models is that they provide information on the vertical structure of currents in the ocean. Finite element methods with even greater flexibility in resolution have also been used for storm surge computations in recent years. Figure 2 is an example of a staggered finite difference grid in which there are three distinct computational points6, while Fig. 3 is that of finite element grids7. Although finite Fig. 5—Average speeds (in knots) of storm movement based element methods and 3-D models have been on the study of Crutcher and Quayle8. The figure is prepared developed and are extensively used for research, most using annual data (1 knot = 1.852 km) of the operational storm surge models are still based surge studies that took place in these tropical oceans on 2-D depth-averaged finite-difference formulations. will be dealt with. Previous storm surge studies in the North Indian In the Bay of Bengal and Arabian Sea, storm surge Ocean generating tropical cyclones normally occur during As a part of the numerical modeling of storm pre-monsoon (March-May) and post-monsoon surges, it is essential to model the tides also as the (October-November) seasons. This is because for combined effect of surges and tides can be disastrous tropical cyclones to form, besides having sufficiently in many circumstances. Based on earlier studies, it large oceanic area with SST greater than 26°C, was recognized that M2, S2, K1 and O1 tidal vertical shear of the horizontal wind between the constituents are significant in the NIO region2. lower and upper troposphere should be very weak. Storm surges occur on India’s east and west Vertical wind shear in the troposphere of the North coasts, although the frequency and severity of surges Indian Ocean is weak during pre-monsoon and post- are found to be higher on the east coast. In this section, monsoon seasons, whereas strong shear exists during we discuss some of the important storm surge studies southwest and northeast monsoon periods. Cyclones previously conducted in the North Indian Ocean, in the North Indian Ocean are usually less frequent especially those in the Bay of Bengal and Arabian and less intense as compared to the hurricanes of the Sea areas, with emphasis given to India’s east and Atlantic Ocean and the typhoons of the Pacific 8 west coasts. In the next section, the most recent storm Ocean . Figure 4 illustrates the preferred tropical cyclone tracks in the global oceans, while Fig. 5 exhibits the contours of speed of movement of tropical storms in various ocean basins8. It is obvious from Fig. 4 that the tracks of storms in the Pacific and Indian Oceans reveal more fine structures than those in the Atlantic Ocean. From the speed of movement of storms (Fig. 5), we can understand that storms attain high speed in the Western North Pacific with maximum values of 35 knots (64.8 km h-1), whereas in the Atlantic with maximum values of 30 knots (55.6 km h-1), in the eastern edge of South Indian Ocean with maximum values of 27.5 knots (50.9 km h-1), in the Bay of Bengal with maximum values of 12.5 knots Fig. 4—Preferred annual tropical cyclone paths based on the (23.2 km h-1) and in the Arabian Sea with maximum study of Crutcher and Quayle8. The arrow widths are values of 15 knots (27.8 km h-1). Besides the fact that proportional to the storm frequencies along indicated paths. It the speed of movement of cyclones is less in the North is clear that frequencies are less in the North Indian Ocean Indian Ocean, cyclones here also bear shorter life span compared to other northern oceans and South Indian Ocean (i.e. ~ 2-3 days) compared to the life span of storms

2172-feb SHAJI et al: STROM SURGE STUDIES 131 in other oceans (i.e. ~ 6 days or more). This observed shorter life span (even for re-curving storms) can be attributed to the relatively short tracks the storms have over the Indian Ocean compared with the other oceans. But this does not mean that the destructive effect of cyclones and accompanying storm surge events is less in the Indian Ocean. To understand the magnitude of devastation, two important cases are worth mentioning. First, in one of the worst storm surges recorded in history, the November 1970 cyclone that hit the Bangladesh coast produced a death toll of about 300,000. Second, the November 1977 Andhra cyclone caused enormous damage to the east coast of India, killing about 20,000 people. In the following sub-sections, we review the previous cardinal storm surge studies took place in Fig. 7—Tracks of a pair of cyclones seen in the Bay of Bengal (during December 7-15 1965) and Arabian Sea (during the Bay of Bengal and Arabian Sea regions. December 7-12 1965)10 Bay of Bengal certain storms take northerly movement and can hit The natural losses due to storm surges are more the Orissa coast. Sometimes, re-curving storms can severe along the east coast of India and coasts of hit the coasts of West Bengal, Bangladesh and Bangladesh, Myanmar and Sri Lanka. Though the Myanmar. Each individual storm is different in many entire coast of Bay of Bengal all the way from Sri respects. To understand this further, it is worthwhile Lanka to Thailand is vulnerable to storm surges, it is to look into the tracks of certain unique and interesting found that on certain stretches storm surges very rarely cyclones that formed in the Bay of Bengal basin. One of the most severe storms that occurred in the North Indian Ocean was the Rameswaram Cyclone of December 19649. This storm (see Fig. 6) in fact

Fig. 6—Track of the severe cyclone, the “Rameswaram Cyclone”, 9 of December 1964 that passed over northern Sri Lanka Fig. 8—Tracks of cyclone pairs observed in the Bay of Bengal 10 occur. For example, along Sri Lanka and Myanmar and the Arabian Sea during the period 1891-1960 coasts storm surges are not so frequent. In the Bay of possessed several unique features. It intensified at a Bengal, tropical cyclones mainly form on the very low latitude of 6.5°N, was very small in size, its southeastern part of the Bay and the Andaman Sea estimated maximum wind speed of 324 km h-1 is during pre-monsoon and post-monsoon seasons. Most perhaps the highest ever in India and Sri Lanka, and of the storms developing in the Andaman Sea usually it might be the only severe cyclone that ever crossed travel northwest and strike the coasts of Tamilnadu the Palk Strait. December 1965 cyclone that formed and Andhra Pradesh rather than traveling towards the in the Bay of Bengal was also so special due to its west and striking the coast of Sri Lanka. Very rarely, many conspicuous characteristics. Not only did this

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Table 2—Details of cyclone pairs seen in the the Bay of Bengal and the Arabian Sea during the period 1891-196010. Date indicated is the original date of formation of the storm. In each pair, the top row represents the Bay of Bengal (BB) storm and bottom row represents the Arabian Sea (AS) storm O rigin Date Latitude Longitude (º N) (º E) Nov. 1, 1891 (BB) 8.0 98.5 Nov. 1, 1891 (AS) 9.5 74.5 Oct. 25, 1912 (BB) 15.0 89.0 Oct. 28, 1912 (AS) 8.5 72.5 Apr. 18, 1922 (BB) 9.0 93.0 Apr. 19, 1922 (AS) 9.5 68.5 May 31, 1927 (BB) 11.5 91.5 May 31, 1927 (AS) 17.5 71.0 Nov. 3, 1936 (BB) 9.5 88.0 Nov. 4, 1936 (AS) 9.0 75.0

and Arabian Sea10. For instance, during the 70 years period 1891-1960, only five such cases were noticed (see Table 2 and the corresponding tracks in Fig. 8). The year 1977 was special with the formation of several cyclones in the Bay of Bengal basin11-13. Figure 9(a-b) shows tracks of three cyclones that occurred in the Bay of Bengal during 1977. In terms of magnitude of destruction, the November 19 cyclone that hit the Andhra Pradesh coast was very large (track A in Fig. 9a). This storm made a landfall near the mouth of Krishna River with maximum wind speed of 193 km h-1, and generated a storm surge amplitude of 5 m that penetrated at least 16 km inland with a -1 Fig. 9—Tracks of various cyclones that occurred during speed of about 16 km h over a coastal stretch of 56 1977-78. a) Track A: November 1977 cyclone that caused km. High water due to the surge remained for about enormous damage to the coastal Andhra Pradesh and Track 10 hrs and other casualties included the damage of B: November 1977 cyclone that caused moderate damage to an area of about 195,000 km2 and the death of about the west coast of India, b) Track C: Cyclone that originated in 20,000 people and another 2 million left homeless or the Bay of Bengal on October 28, 1977 crossed the southern with property loss15. Between November 21 and 22, Indian peninsula, then rejuvenated in the Arabian Sea and another Bay of Bengal storm crossed the southern eventually hit the coast of Arabia, c) Track D: November 1978 cyclone that caused huge damage on the east coast of India and subsequently hit the west coast of India Sri Lanka and Track E: November 1978 cyclone that (track B in Fig. 9a), causing enormous damages. In originated in the Arabian Sea eventually struck the 1977, a storm originated in the Bay of Bengal on coast11-14 October 28 and subsequently crossed southern India and moved west in the Arabian Sea before making storm10 show an unusual track (Fig. 7), but the landfall at the coast of Arabia between November 4 duration was also very long (about 9 days) compared and 5 (track C in Figure 9b). A storm generated near with the average duration in the Bay (about 2-3 days). the Nicobar Islands on November 21, 1978 intensified Eventually, that storm hit Bangladesh and Myanmar, further on November 23 with maximum wind speed which also usually does not happen. As can be seen of 204 km h-1 and then hit the east coast of Sri Lanka in Fig. 7, another storm was simultaneously present (track D in Figure 9c). The resulting storm surge in the Arabian Sea. Only on very rare occasions do together with the cyclone effect took a death toll of cyclones exist simultaneously in the Bay of Bengal 373 people and property loss of about 80,000 houses.

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Table 3—Maximum possible storm surge amplitudes and total water level elevations (surge + wind waves) at selected locations along the east coast of India9. The hypothetical storm considered in this study has a wind speed of 40 m s -1. Type A: total water level < 2 m, Type B: 2-5 m and Type C: > 5 m

Table 4—Relationship between type of coastline and occurrence of storm surges along the coasts of India. Data are mainly for the period 1949-669

Interestingly, this storm thereafter moved to India over Previous studies the Gulf of Mannar where it further killed another 10 In a much earlier study9, empirical relations were people14. used to calculate the total water level elevation along

2172-feb 134 INDIAN J MAR SCI VOL. 43 (2), FEBRUARY 2014 the east and west coasts of India (and the coasts of § Coastal belt around the head Bay north of about Bangladesh, Myanmar, and Pakistan) due to the 20°N. Here the cyclone frequency is high and the combined effects of storm surge, wind waves, and storm tracks are usually favorable for generating coastal bathymetry. One breakthrough of that study maximum surges, especially in the Sunderbans. was that the coasts were classified mainly into three § South Coromandal coast around the Palk Bay. categories, viz., type A, type B and type C, based on Although frequency of storms striking this region the amplitude of water level elevations. This is somewhat less than that of the above mentioned classification is given through Table 3 and Fig. 10. head Bay belt, the major storms that strike this Here, the values mentioned under “storm surge coast usually generate major surges. amplitude” correspond to a hypothetical storm with wind speed up to 40 m s-1 and the values under “total water level” is due to the combined effects of peak surge and wind waves. It can be noted that for a type A coastline, the maximum total water level is less than or equal to 2 m during storm surge, while for type B the amplitude is between 2 and 5 m and for type C it is greater than 5 m. Further, that study9 considered the calculated storm surge as the piling up of water due to wind stress only, by neglecting the inverse barometer effect of the storm by justifying that this effect does not exceed 0.5 m anywhere on the coast. All these three classifications were also verified to a certain extent by comparing with actual

Fig. 11—Nomograms of peak storm surge as a function of pressure drop and radius of maximum winds for the A) northern and B) southern regions of the east coast of India16

§ A small area of type C belt near Nizampatnam Bay. The November 1977 Andhra cyclone produced major surges in this region and killed around 20,000 people. The east coast of India between 14°N and 16.5°N and the Coromandal coast between Point Calimere and Karikal fall under the type B belt, whereas the type A belt is not so severe and is usually confined to Fig. 10—Classification of the Bay of Bengal and the Arabian two regions along the east coast - approximately Sea coasts of India based on total water level elevation9. The between 16.5°N and 20°N and between 10°N and hypothetical storm considered in this study has a wind speed 14°N. of 40 m s-1. Regions with total water level < 2 m come under Type A, while that with total water level between 2 and 5 m The most common alternative to empirical are Type B and > 5 m are Type C methods, viz., the one mentioned above9, is to use numerical models for the computation of surges. Numerical model results can be employed in different data (see Table 4). Among the three types, type C ways to predict surges. In one approach, the model is belts are the most vulnerable ones. In the Bay of used to generate surge scenarios for different possible Bengal, type C belts can be seen in the following three combinations of cyclone parameters, tracks, and locations. bathymetry. Results are then plotted in the form of

2172-feb SHAJI et al: STROM SURGE STUDIES 135 nomograms, which relate surge height at the coast to various input parameters. Once constructed, the nomograms can be used directly to estimate the surge without further reference to the numerical model. For example, in an earlier attempt, Ghosh16 used the Jelesnianski’s Special Program to List Amplitudes of Surges from Hurricanes (SPLASH) model17 to prepare Fig. 13—Storm surge amplitude (m) as a function of storm nomograms for estimating peak surges generated by intensity (mb) and speed of storm movement for the (a) the tropical cyclones impinging on the east coast of northeast track (I), (b) northward track (II) and (c) northwest India. Nomograms accommodated fixed values for track (III) of Figure 1219 pressure drop, radius of maximum wind, vector Though the nomograms approach gives sudden surge motions of storms and offshore bathymetry. Separate estimates, the latter method of numerical prediction nomograms were prepared for the northern region is more general and promising than the nomograms where the slope of the shelf is small and for the approach. remaining part of the coast where the slope is relatively large. Two such typical nomograms from In the beginning, numerical storm surge modeling that study are given in Fig. 11. That study, however, in the Bay of Bengal was concentrated either on the northern shelf region or the east coast of India. The first numerical storm surge model in the Bay of Bengal was developed by Das18 for the east coast of India and Bangladesh. A devastating storm struck the northeastern coast of Bay of Bengal near Chittagong (20°N, 91.5°E) on November 12-13, 1970. Das18 conducted numerical experiments to compute the surge generated by that storm. Computations gave a maximum surge of 3.2 m at a distance 40 km southwest of Chittagong, which is slightly higher than the observed tide gauge value of 1.5 m. Das et al.19 extended the model of Das18 to simulate surges generated by the November 1970 cyclone by

Fig. 12—(a) Computational grids19 meant for storms moving northeast (I) and north (II). The contours represent water depth (m). (b) Computational grids19 meant for storms moving northwest (III) did not deal with a curved coastline or with storms Fig. 14—The track of Andhra cyclone, which occurred in the that move alongshore, recurve, remain stationary, Bay of Bengal during 14-20 November 19776 accelerate, or with storms whose size varied with time. In the second approach of using numerical storm considering three coastal regions in the Bay of Bengal. models, the surge models are effectively adapted to In addition to the atmospheric forcing, the model also compute surges for each individual storm separately. incorporated tides as boundary forcing. Surges were

2172-feb 136 INDIAN J MAR SCI VOL. 43 (2), FEBRUARY 2014 computed for three storm tracks: I) storms moving it also incorporated river systems adjoining the coasts northeastward towards Bangladesh, II) storms moving and showed that the surge might penetrate deep inland north towards the deltaic regions of the Indian coast leading to floods in the inland waterways of and III) storms moving northwest and striking the Bangladesh. From the numerical experiments it was Orissa coast. They used coarse grids for computation clear that the surge response depended critically on of surges from cases I) and II) and nested finer grids the track and diameters of the forcing cyclone. The for computation of surges from case III). Both of these effect of barometric forcing was not included in that grids are given in Fig. 12. Based on these three tracks, model and only wind stress forcing was considered. the authors prepared nomograms for storm surges as Johns et al.6 used three different non-linear numerical a function of storm intensity and speed of movement models to simulate the surge generated by the 1977 of the storm, and the same is given in Fig. 13. They Andhra cyclone (see Fig. 14). One of the models concluded that the linear superposition of surge and (designated as model M3) considered conventional tide can overestimate the water level by about 0.8 to step-wise coastal boundary treatment, whereas the 1.1 m. Das20 further extended the computational other two (designated as models M1 and M2) were based on curvilinear treatment of lateral coastal boundaries. Each of the three models produced a qualitatively similar surge response with peak surge in the range of 4-5 m, while the slight quantitative differences were explained in terms of the different boundary treatments. Fig. 15 illustrates that the sea surface elevation occurred at different time at different points on the east coast of India based on model M1, which used the co-ordinate transformation and model M3, which used the conventional rectangular co- ordinates. Authors argued that the differences as seen in Figs. 15 a and b are due to the over-reflection of the wave response in M3 due to the straight line step- wise coastal boundary usage. It should also be noted that authors could not compare any of their three Fig. 15—The numerically computed6 development with respect model results with actual tide gauge records as to time of the surge elevations (m) at various locations on the realistic observations were not available then. Using east coast of India during the 1977 Andhra cyclone. The the M1 model, a further investigation by Dube et al.22 computations were done using numerical models of the whole revealed that winds in the central region of the storm Bay of Bengal in (a) transformed coordinates (M1) and (b) rectangular coordinates (M3) do not contribute significantly to the surge; rather, the winds at greater distances from the cyclone centre contribute greatly to the surge. From the modeling domain of the Das et al.19 model to include the coasts efforts of Johns and Ali21 , Johns et al.6 and Dube et of West Bengal and Orissa, and incorporated non- al. 22-23, it can be confirmed that: a) the surge linear advection terms and realistic tide-surge development depends critically on the diameter and interactions. In that study, the water levels as well as track of the cyclone, b) the tide-surge interaction is time of occurrence of peak surge was found to be cardinal in shallow water regions, emphasizing the more realistic as to the simulations of Das18 and Das necessity of using a non-linear model for the head et al.19. Bay region and c) the surge can penetrate deep inland After the initiation by Das and co-workers, Johns causing a flood hazard along the river systems. and co-workers developed several numerical storm Usually, the lateral boundaries in storm surge models surge modeling techniques for the Bay of Bengal. are taken to be vertical sidewalls through which no Salient features of some of those studies are worth flux of water is allowed. However, in reality, water mentioning here. First, Johns and Ali21 developed a will usually move continuously inland and the use of non-linear storm surge model for the Bay of Bengal. idealized vertical sidewalls might lead to Model allowed for surge and tide interactions. Further, misrepresentation of the surge development. Realizing

2172-feb SHAJI et al: STROM SURGE STUDIES 137 this shortcoming of storm surge models, Johns et al.24 developed a model which used a continuously deforming lateral fluid boundary instead of using the conventional solid wall boundary at the coast. Numerical results showed that the moving boundary model gave a reduced surge response at the initial coastline position than that in the fixed boundary model. Generally, for storm surge computations the dynamical process is modeled by depth- averaged shallow water equations for the horizontal motions. However, during storm surge the momentum is transferred across the sea surface through the atmospheric wind stresses and is communicated to greater depths by vertical turbulent mixing. Therefore, presuming that a 3D ocean structure would be better in modeling storm surges, Johns et al.5 developed a Fig. 16—Distribution of surge elevations (m) at various fully 3D numerical model for the simulation of storm locations on the east coast of India during the 1977 Andhra cyclone27 surges generated by tropical cyclones off the east coast of India. Model was fairly sophisticated with the shallow water model is good enough for the realistic parameterization of turbulent energy. Numerical numerical investigation of storm surge phenomena. experiments were conducted using wind stress forcing Johns et al.25 investigated how the model resolution based on the 1977 Andhra cyclone and the results and near coastal bathymetry influence the computed obtained were compared with that of the depth- surge. Proper resolution of the near shore bathymetry averaged 2D model of Johns et al.6. It was found that was found to be crucial in determining the storm- even with more refined treatment of frictional induced sea surface elevation. In that study, a processes, there was no substantial difference between northward propagating component of the computed the simulations performed with 2D and 3D models. surge response having the characteristics of a coastally Hence authors concluded that a depth-averaged 2D trapped wave was identified. Thus, the total surge

Table 5—Angles of cyclone approach (with respect to the coastline) to be modeled for the individual impact points27

2172-feb 138 INDIAN J MAR SCI VOL. 43 (2), FEBRUARY 2014 response was due to the contribution from direct wind advection terms, tide-surge-river interactions, and stress together with the northward propagating shape of coastal bathymetry and performance of 2D component. Sensitivity of such trapped waves to the against 3D models. Those initial efforts in fact gave bathymetry further suggests the need to use realistic very useful insights that a researcher should be known bathymetry for accurate surge calculations along the before attempting a real-time surge prediction system. east coast of India. Yet in another study, a numerical Thus, being stimulated by those initial efforts, Dube model was developed by Johns et al.26 for the simulation of tide-surge interactions in the Bay of Bengal and investigated two cyclone cases - the June 1982 Orissa cyclone and the November 1977 Andhra cyclone. In both simulations, authors identified the above mentioned northeasterly propagation of the coastally trapped wave. Besides, the tide-surge interaction was found to be small along the Andhra coast. Jarrel et al.27 did a comprehensive storm surge study in the Bay of Bengal. They developed five models for the Sri Lanka, India, and Bangladesh coastlines, two models for the Myanmar and Thailand coastlines and one for the Andaman region. Based on population centers, a total of 16 tropical cyclone impact points were chosen (see Table 5). The Fig. 18—Contours of a number of tropical cyclones including depressions passing through 2.5° x 2.5° latitude - longitude squares during a 98 year (1877-1974) period in the Arabian Sea30 et al.28 first described a storm surge prediction system for the east coast of India. Arabian Sea Arabian Sea is under the influence of tropical cyclones, but the marginal seas, viz., the Arabian Gulf and Red Sea are mostly affected by extra-tropical cyclones. Extra-tropical cyclones very rarely travel as far south as the Arabian Sea. Tropical cyclones, on the other hand, either generated locally in the Arabian Sea or crossing from the Bay of Bengal, can cause significant surges in the Arabian Sea. Storms of the Arabian Sea are less frequent and less intense and, thus, the accompanying storm surges are also Fig. 17—The common tracks of Arabian Sea cyclones29 less destructive compared to those in the Bay of Bengal. Southeastern part of the Arabian Sea is one directions from which the cyclones can really of the tropical cyclone genesis areas of the globe29. approach the impact points can also be seen in Table In the Arabian Sea, tropical storms occur 5. Computed surge elevations at various locations in predominantly either during pre-monsoon or post- the east coast for the 1977 Andhra cyclone are shown monsoon seasons, but seldom during the monsoon in Fig. 16. In the Bay of Bengal, initial numerical season. An important difference between cyclones of modeling efforts by Das and co-workers and Johns the Arabian Sea and those of the Bay of Bengal is and co-workers were mainly confined to very useful that whereas in the Bay of Bengal more cyclones sensitivity studies, viz., importance of non-linear occur during the post-monsoon season (September–

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Table 6—Maximum possible storm surge amplitudes and total water level elevations (surge + wind waves) at selected locations along the west coast of India in the Arabian Sea9. The hypothetical storm considered in this study has a wind speed of 40 m s -1. Type A: total water level < 2 m, Type B: 2-5 m and Type C: > 5 m

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December) than during the pre-monsoon season (April –May), in the Arabian Sea they are about equally distributed between the two cyclone seasons (May– June and October–November). Although most of the Arabian Sea cyclones are locally generated, occasionally tropical cyclones from the Bay of Bengal cross peninsular India and re-intensify over the Arabian Sea (for example, see track B in Fig. 9a and track D in Fig. 9c). The general tracks of Arabian Sea cyclones are given in Figure 17. It shows that the Arabian Sea cyclones move either west to northwest or due north and then east because of recurvature. Some of the re-curving cyclones make landfall on the west coast of India or on the Pakistan coast, whereas others travel towards west-northwest and strike the south coast of the . It is remarkable (b) The maximum sea surface elevation and time of its that only infrequently do cyclones in the Arabian Sea occurrence along the Gujarat coast: -, peak surge height; —, travel towards the Gulf of and the Arabian Gulf. time of occurrence of peak surge; *, observed storm surge; O, In a previous study30, contours of the number of observed time of occurrence of peak surge32 tropical cyclones including depressions passing through 2.5° × 2.5° latitude - longitude squares over of Bengal storms mature into severe storms, whereas the course of 98 years (1877-1974) was made and the in the Arabian Sea it is about 40 percent. same is given through Fig. 18. From this figure it is Previous studies apparent that a maximum of up to 400 cyclones and Storm surge studies are relatively fewer in the depressions occurred near the coasts of West Bengal Arabian Sea compared to the Bay of Bengal. Rao9, and Bangladesh in the Bay of Bengal basin, whereas using empirical relations, conducted the first storm in the Arabian Sea basin the maximum number is only surge studies on the west coast of India. Similar to 40 with primary occurrence near the - the east coast, he classified the west coast also into Gujarat coast of India. Over a given period of time, type A, B, and C categories (see Fig. 10 and Table 6). the number of cyclones occurring in the Bay of Bengal Earlier, we could see that there are two vulnerable is about four times higher than that in the Arabian zones of type C along the east coast of India. Similarly, Sea. Nevertheless, only about 25 percent of the Bay along the west coast of India there also exist two type C zones. The first one extends from the Konkan coast to the north of 18°N to the coast around the Gulf of

Fig. 20—The maximum sea surface elevation and time of its Fig. 19—(a) Path of the cyclone32. occurrence along the coast33

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Cambay. In this belt, the frequency of storms striking Recent storm surge studies in the North Indian the coast is less. Tidal range here is quite large, for Ocean instance, 8 m at and 11 m at Cambay. Unless On a path similar to that of the model developed peak surge happens close to the time of high tide, no by Dube et al.28, several location specific storm surge major water level oscillations are expected in this belt. models evolved recently in the Bay of Bengal and The second vulnerable zone of type C goes from Arabian Sea regions. The salient features of such Dwarka in India to in Pakistan. It covers the location specific storm surge modeling system are the extensive marshy and mostly uninhabited areas, viz., following: the “Rann of Kutch”. In this belt also, the frequency of storms is less and the tracks are also quite • The storm surge model is fully non-linear and is unfavorable for major storm surge development. forced by surface wind stress and quadratic However, when they occur, storm surges with several bottom friction. The surface wind stress is meters in amplitude are expected. In the west coast, calculated based on a bulk quadratic law26, for type B belts are also seen in two regions. First one which the cyclone wind speed and direction are consists of the entire west coast of India south of required as inputs. The storm surge is generated 18°N. In this region the frequency of storms is also by the cyclone moving in the analysis area. Due less and in general the storm tracks are unfavorable to the strong associated cyclone winds and for major surges. The second type B belt falls around consequent high surface wind stresses, the forcing the Kathiawar Peninsula between Diu and Dwarka. from the inverted barometric effect can be Major surge amplitudes that can occur here are about avoided in the storm surge calculation. Thus, the 1.5 m and are about half the tidal range here. In this surface wind field associated with the tropical region, even though the frequency of storms is quite cyclone remains as the prime requirement for high, they are usually not very intense. The type A storm surge modeling. The cyclone wind field at belt is seldom seen along the west coast of India. the sea surface is derived by using a dynamic cyclone wind model developed by Jelesnianski Numerical storm surge modeling studies are very and Taylor34. This cyclone model uses the radius 31 few in the west coast of India. Ghosh et al. ran the of maximum winds and the pressure drop as the 17 SPLASH model of Jelesnianski to compute storm inputs. Main component of the cyclone model is surges of the November 1982 cyclone. Simulated a trajectory model and a wind speed profile peak surge agreed very well with available approximation scheme. Trajectory model 32 observations. In another investigation, Dube et al. represents a balance among pressure gradient, used a fully non-linear, 2D coastal zone model to centrifugal, Coriolis and surface frictional forces compute the storm surges due to the 1975 Porbandar for a stationary storm. Variable pressure deficit, cyclone (see Fig. 19a). Distribution of simulated forward speed and radius of maximum winds are maximum sea surface elevations, observed surge, and used to compute the wind stresses at various the time of occurrence along the Gujarat coast are model grid points to drive the storm surge model. given in Fig. 19b. From this figure it is apparent that Storm strength is substantially reduced after the the computed peak surge of 2.2 m at Porbandar was cyclone crosses the coast. in good agreement with the observed sea surface • elevation of 2.7 m. Later on, Sinha et al.33 also used Bottom stress is computed from the depth the model of Dube et al.32 to simulate the surge integrated current using quadratic friction law induced by the November 1982 Gujarat cyclone. with a constant coefficient of 0.0026. This Model computed maximum surge height was found coefficient value has been selected after several 5 to be in good agreement with the observed values at previous numerical experiments . the locations of , Diu and Jafrabad (see Fig. • Coastal boundaries of the surge model are 20). But at Mangral, which is located about 80 km to prescribed as vertical sidewalls across which the the left of the landfall, the simulated surge was much normal transport vanishes. Though the model has lower than the observed surge, for which the authors the option to use continuously deforming could not give any satisfactory explanation. shoreline, this facility has often not been used

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especially due to the unavailability of detailed For the 1977 cyclone, the extent of inland intrusion onshore topographic data. The open sea computed by the model was more than 7 km along boundaries of the model uses a radiation type the coastal belt between Divi and Machlipatnam, condition35, which allows for the propagation of while a post-storm survey report38 showed the extent energy outwards from the model domain in the of inland intrusion as 12 km. For the 1990 cyclone, form of simple progressive waves. the surge water intruded up to 11.3 km inland between • One of the important features of the model is that Divi and Machlipatnam, which also agreed fairly with it uses more accurate and detailed bathymetry for the available reports. Along the Orissa coast, location the offshore waters. Besides, the model contains specific storm surge model was used to compute a simple drying scheme to avoid the exposure of surges associated with two tropical cyclones – the land near the coast due to strong negative surges. Paradip cyclones of October 1971 and June 1982. A cyclonic storm hit near Paradip coast on 30 October, • The numerical solution of the model equations 1971. The model simulated a peak surge of 3.9 m are obtained by means of a conditionally stable near False Point and a maximum inland inundation semi-explicit finite difference scheme with a of surge water up to about 10 km. About 60 km of staggered horizontal grids arrangement, which coastal stretch to the right of the landfall point was essentially consists of three distinct types of affected by the surge of more than 2 m. According to computational grids upon which the sea surface post-storm survey reports39, the surges varied from 2 elevation and the zonal and meridional depth m to 6 m as one proceeded northward from Paradip averaged currents are computed. Following to Balasore. In the second instance, a tropical cyclone Sielecki36, computational stability is achieved by with wind speed of about 60 ms-1 hit near the Paradip satisfying the Courant-Friedrich-Lewy (CFL) coast on 3 June, 1982. The model simulated a criterion. Location specific storm surge model maximum surge of 4.2 m at False Point and inland configured for any region does not require any inundation of about 10.3 km. This agreed well with large computing facility and can easily be run on the available post-storm survey reports40. a personal computer. In the Indian state of Orissa, a severe cyclone Bay of Bengal having maximum associated winds of around 250 km It is worthwhile to mention a few applications of h-1 hit the Paradip coast on the morning of 29 October, the location specific models used recently along the 2009. Numerical experiments conducted by Dube east coast of India. To compute storm surges and the et al.41 produced a peak surge of about 7.8 m close to extent of coastal inundation, Dube et al.37 used coastal the landfall point. A post storm survey report42 also surge and inundation models along the coasts of showed that the surge was more than 6 m near Paradip. Andhra and Orissa. In those models, the coastal Apart from this, the coastal regions between Konark boundaries were treated in two ways – as vertical and Chandbali showed a simulated surge of more than sidewalls and as continuously moving coastal 5 m. Yet in another study, Chittibabu et al.43 boundaries. The purpose of the latter treatment was formulated a high resolution model for the coastal to estimate the extent of inland inundation due to regions of Tamil Nadu and Sri Lanka to simulate storm surges. Inter-comparison of surges obtained surges associated with the cyclones which struck these from both the coastal treatments revealed that the coastal areas during the years 1964, 1978, 1992, 1993 continuously moving coastal boundary treatment and 1994. A severe cyclone, the 1964 Rameswaram underestimated the surges as compared to the coastal cyclone, crossed the Sri Lankan coast on 22 boundary treatment as vertical solid sidewall. This December, 1964 and on 23 December struck the Tamil can be attributed to the inland intrusion of surge water Nadu coast which is located slightly south of Tondi. in the moving coastal boundaries. The location Maximum wind associated with the storm was about specific storm surge model meant for the Andhra coast 60 ms-1. Near Tondi, the simulated surge was 5.4 m, was used to compute the peak surge and inland which matched well with the reported44 surge of 5 m. inundation associated with two tropical cyclones – The 1978 Batticoloa cyclone affected the coasts of the November 1977 and May 1990 Andhra cyclones. Sri Lanka and Tamil Nadu. This cyclone crossed Sri

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Lankan coast of Batticoloa on 23 November, 1978 maximum storm surge of 3.2 m was simulated by the and struck the coast of Ramanathapuram district, model in the coastal belt between Veraval and Diu. Tamil Nadu on 24 November. Model simulated However, the reported40 surge at Veraval was about maximum surges were 2 m close to Batticoloa and 2.6 m. In another case, in the morning of 18 June, 3.7 m north of landfall point in Tamil Nadu coast. 1996 a severe cyclone struck the south Gujarat coast Reported45 maximum surge in Tamil Nadu coast was located close to Diu. The model computed peak surges 3-4 m. On 13 November, 1992 a cyclone hit the Tamil of about 3.2 m at Diu and 2.5 m at Mahua were Nadu coast near Tuticorin. Storm in fact lost intensity comparable with the observations49. On 9 June, 1998 when it crossed the Sri Lankan coast on 12 November, a cyclone crossed the Kandla coast of Gujarat. The 1992. However, after crossing Sri Lanka, it maximum surge computed by the model south of rejuvenated before hitting the Tamil Nadu coast. Kandla was about 5 m, whereas at Kandla it was 2.1 Model simulated surges were rather insignificant m, which agreed fairly with the reported surge42. At along the Sri Lankan coast, while the coastal belt of Porbandar and Dwaraka, the model computed surges Tuticorin was flooded with a maximum surge of more were about 3.5 m and 2 m respectively. than 1 m, which compared with available reports. On Conclusion 4 December, 1993 a cyclone, the 1993 Karaikal cyclone hit the north coast of Tamil Nadu. Maximum Most of the casualties from tropical cyclones surge simulated by the model near Karaikal reached occur due to storm surges. Nowadays especially, the about 1.6 m. The 1994 Madras cyclone struck the storm surge topic is becoming increasingly important coastal belt of Madras on 31 October, 1994. Model in coastal states of many nations because of the rapid simulated peak surge near Madras was 2 m, which development of coastal regions for industrial, matched well with the reported surge of 1-2 m. residential, and recreational utilizations. Numerical simulation and prediction of storm surges has been It is important to be aware of the maximum total an active area of research in many nations during the water level that could possibly occur at a particular last three decades. Many advanced nations have coastal station for a certain return period as a result achieved ample success in this. For example, of the combined effects of storm surge, tide, and wave improved surge warning systems are already in place setup. By considering this fact, in a recent in many coastal regions of the Atlantic and Pacific investigation46, a location specific storm surge model Oceans50. In the tropical North Indian Ocean (NIO), was used to compute the maximum probable surge storm surge generating tropical cyclones normally amplitudes for a 50 year return period along a few occur during pre-monsoon and post-monsoon seasons, selected locations along the east coast of India. resulting in heavy loss of lives and property. In Further, the total water level was computed as a linear particular, the Bay of Bengal is an ideal place for the addition of simulated surge amplitudes, local tide genesis, maintenance, and intensification of tropical amplitudes, and wave setup at each coastal grid point storms, which normally strike the eastern and northern of the numerical model. In another recent study47, a coasts of India. Although the frequency of storms is 2D non-linear model was used for simulating storm less in the Arabian Sea than in the Bay of Bengal, surges associated with cyclones hitting the east coast major storms occur occasionally in the Arabian Sea of India during the 27 years period from 1974 to 2000. and make landfall on India’s west coast, generating That study estimated the return periods and return destructive surges. Thus, better prediction of storm levels of extreme sea level events at 26 selected surges along India’s east and west coasts can stations along the east coast of India. considerably minimize the casualties. Arabian Sea Before the evolution of numerical models, In a recent investigation, Chittibabu et al.48 used empirical formulae, which correlate the sea surface a high resolution location specific model to simulate height with sea level pressure and strength and surges associated with the tropical cyclones hitting duration of prevailing winds in the storm, have been the Gujarat coast. For example, on 8 November, 1982 used for storm surge prediction. In the NIO, though a tropical storm hit the Veraval coast, Gujarat. A some researchers51-52 initiated the empirical

2172-feb 144 INDIAN J MAR SCI VOL. 43 (2), FEBRUARY 2014 nomograms approach, it was found to be not very Later this model was extended to the coasts of West useful as the method demands large series of Bengal and Orissa and computed surges due to observations to establish meaningful correlations. realistic cyclones19. In the ensuing attempt20, Nevertheless, it is worthwhile to note the significant simulations were improved by incorporating non- contribution of Rao9 in 1968, which used the linear advection terms and tide-surge interactions in nomograms to delineate those sectors of the east and the numerical model. After Das and co-workers, Johns west coasts of India which are vulnerable to dangerous and co-workers developed a series of non-linear storm surges into types A, B and C (see Fig. 10 and Tables surge models for the Bay of Bengal. Their remarkable 3 and 6). Later, it was recognized that the most viable contributions are as follows: a) alternative to empirical methods is to use numerical development of a surge model by Johns and Ali21 for models for the simulation and prediction of surges. the simulation of surges along the coast of the northern Numerical storm surge models are built on the shallow Bay by incorporating the dynamic effects due to the water theory, in which the vertically integrated Ganga-Brahmaputra-Meghna river system and islands hydrodynamic equations governing the motion in the near the Meghna estuary, b) surge simulations by ocean are used. Since analytic solution of the shallow Johns et al.6 using the actual geometry of the Bay of water equations is difficult without major assumptions Bengal coastline, c) development of a surge model and dropping many terms, numerical solution is the by Johns et al.24 which used a continuously deforming practical alternative. Numerical solution is obtained lateral fluid boundary, instead of the solid wall through either finite difference or finite element boundary at the coast, to investigate the inland discretization of the shallow water equations. Since inundation, d) surge simulations using a 3D model the non-linear advection terms have significant effect by Johns et al.5, which confirmed that instead of a on the final results, especially in shallow coastal 3D model, a 2D model is good enough for surge regions such as the Bay of Bengal, it would be better computations, and e) investigation of non-linear tide- to retain these terms for better surge computations. surge interactions by Johns et al.26. After the initial To understand the significance of non-linear advection contributions made by Johns and co-workers, Dube terms in surge computations, see the studies made by and co-workers upheld the storm surge work both in Das et al.19 and Das20. Thus, the surge models are the Bay of Bengal22-23 and Arabian Sea32-33. usually non-linear and are forced by wind stress and Subsequently the modeling initiated by Johns and co- quadratic bottom friction. Moreover, the final model workers has undergone several modifications for results are very sensitive to realistic coastal geometry improved surge computations. Since the frequency and bathymetry. In general, the numerical model of cyclones is relatively less in the Arabian Sea results are used in two ways to predict surges. In the compared to the Bay of Bengal, only fewer storm first approach, called the nomograms approach, model surge studies are found in the Arabian Sea31-34. results are used to generate surge scenarios for various Recently, on a line similar to that of Dube et al.28, possible combinations of cyclone parameters several location specific models have been developed including its tracks and coastal bathymetries. In the for elsewhere along India’s east and west coasts (see second approach, numerical surge models are section 4). effectively used to compute surges for each individual storm separately. Ghosh16 attempted the nomograms In summary, over the last two decades several approach to study storm surges along India’s east research groups made substantial progress in the field coast. It has been found that researchers prefer of numerical modeling of storm surges in the NIO. numerical modeling as compared to nomograms Yet, compared to the Pacific and Atlantic oceans50, approach because the latter requires accurate the availability of an efficient warning system in the meteorological inputs and coastal bathymetries to NIO region is still far from reality. In fact there are generate meaningful surge scenarios, which obviously many areas where more progress can be made to is not always available. Credit goes to the work of ensure that a reasonable reliable early warning system Das18 for using the first numerical storm surge model is right in place. Our views for improving storm surge in the NIO, in which he computed the surge generated simulations and predictions in the NIO region for the by an idealized cyclone striking the Bangladesh coast. foreseeable future are given below.

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• Accurate storm surge prediction critically by orthogonal straight line segments, called as depends on the quality of several model inputs the staircase coastal representation. But and parameters. Especially important among simulations have shown that the straight line these are meteorological forcing and the segments can over-reflect the wind driven water6. parameterization of air-sea interaction, open Second, to overcome this difficulty, continuous boundary conditions, bottom friction and deformation coastal representation has been bathymetry. Errors in any of these may lead to tried5, where the co-ordinates have been significant deterioration in numerical transformed into a new co-ordinate system. It has computations. been found that although this method is good to • The precise computation of wind field in a study inland inundation, it cannot yield good cyclone is an essential prerequisite for the realistic results without detailed onshore topography. computation of storm surges. Meteorological Third, Wanstrath53 suggested a conformal fields characterizing the cyclone, viz., the transformation coastal representation method, pressure drop (i.e. difference between pressure which converts a curved domain into a rectangular at the centre of the storm and ambient pressure one. In effect, this approach will use an orthogonal surrounding the storm), maximum sustained curvilinear co-ordinate system. A detailed winds, and radius of maximum winds can be comparative study of all these three approaches obtained from satellite imagery. The usual is required to understand which among the three practice in most of the storm surge models is that will stand as the best suited for Indian coastal it uses a dynamic cyclone model developed by representation in numerical surge models. Of 34 Jelesnianski and Taylor for the computation of course, to get correct surge values, detailed cyclone wind field, for which the cyclone model onshore topography is an essential prerequisite. requires the above mentioned cyclone specific meteorological fields as input. Thus, more efforts • The total sea level elevation occurs as a combined are required to see whether replacing the above effect of the non-linear interaction of storm surge, noted cyclone wind model34 with reliable wave set-up, and high tide. Non-linear numerical tropical cyclone prediction models interactions of tide and surge have been attempted developed by meteorologists would be a good earlier in storm surge modeling26. Tide and surge alternative to get accurate cyclone wind field. are long gravity waves, whereas wind waves are Very focused sensitive experiments in this short period gravity waves. Thus, how the direction need to be done. The other cyclone superposition of a high frequency oscillation on variables such as vector motion of the cyclone, a low frequency oscillating system affects the place of landfall, and duration of the cyclone is total water level elevation needs to be usually provided by the national weather services. investigated. To consider these complex • Since storm surge computations near the coast interactions, development of coupled surge-tide- are very sensitive to the coastal geometry and wave models is essential. offshore bathymetry at the location of the landfall • 54-55 of the cyclone, storm surge models should have River discharge can significantly modify the correct incorporation of such effects. In particular, surge, especially in the northern Bay of Bengal, the location of the highest surge is very sensitive where the world’s largest river system carries lot to the manner in which the coastline is represented of freshwater annually. Similarly, heavy in the numerical model. The curving coasts23 not precipitation associated with the tropical cyclone only shift the peak surge position but also affect can also influence the total water level. its height. Three different approaches are Precipitation effect, which has so far not been available to insert coastlines in a numerical storm considered in surge computations of any model, surge model. First, most of the models follow the need to be incorporated into the storm surge conventional method of coastline representation models.

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• A good network of real time tide gauges along and Gulf of Mexico Coasts of the United States, Dredging the coast as well as satellite observations of the Research Program Technical Report, DRP-92-6, U.S. Army sea level are required to get accurate sea surface Engineers Waterways Experiment Station, Vicksburg, MS, (1994), pp. 298. height data for the purpose of model validations 8 Crutcher, H.L., & Quayle, G., Mariners worldwide climatic and issuing warnings. guide to tropical storms at sea, U.S. Govt. Printing Office, • Most of the storm surge models used in the Indian Washington D.C., (1974), 311 charts and pp. 114. Ocean is finite difference models. Efforts are 9 Rao, N. S. B., On some aspect of local and tropical storms needed to use finite element models as well for in India, Ph.D. thesis, University of Jadavpur, Calcutta, India., (1968). surge computations. Inter-comparisons of the performance of both types of models, and 10 Swaminathan, D. R., The extra-ordinary path of the Bay of Bengal storm of December 7-15, 1965, in relation to the identifying their merits and limitations have to Tiros 10 satellite observations and the upper tropospheric be done. Eventually, for surge prediction, instead wind field, Indian J. Meteorol. Geophys., 20 (1966) 357- of a single model, using different models in the 360. line of the ensemble method would be a good 11 De Angelis, R. M., Hurricane alley, Mar. Weather Log., 22 approach. (1978a) 21-23. • To minimize the impact of surges, it is important 12 De Angelis, R. M., Hurricane alley, Mar. Weather Log., 22 (1978b) 182-183. to asses in advance the coastal stretch that is likely 13 De Angelis, R. M., Hurricane alley, Mar. Weather Log., 22 to be inundated in case of surge occurrence. (1978c) 265-266. Acknowledgements 14 De Angelis, R. M., Hurricane alley, Mar. Weather Log., 22 (1978d) 337-339. Authors are so thankful to the anonymous 15 Winchester, P., Disaster relief operations in Andhra Pradesh, reviewers and the Editor of the journal for valuable southern India, following the cyclone in November 1977, suggestions which helped us to improve the quality Disasters, 3 (1979) 173-177. of the manuscript. 16 Ghosh, S. K., Prediction of storm surges on the east coast of India, Ind. J. Meteo. Geophys., 28, 2 (1977) 157-168. References 17 Jelesnianski, C. P., SPLASH (Special Program To List 1 Smith, K., & Ward, R., Floods: Physical Processes and Amplitudes of Surges From Hurricanes) I: Landfall Storms, Human Impacts, Chichester: Wiley, (1998), pp. 382. NOAA Technical Memorandum, NWS TDL – 46 (1972). 2 Gönnert, G., Dube, S. K., Murty, T. S., & Siefert, W., Global 18 Das, P. K., Prediction model for storm surges in the Bay of storm surges: theory, observations and applications, Die Bengal, Nature, 239 (1972) 211-213. Kueste, (2001), pp. 623. 19 Das, P. K., Sinha, M. C., & Balasubramanyam, V., Storm 3 Ali, A., Storm surges in the Bay of Bengal and some related surges in the Bay of Bengal, Quart. J. Roy. Met. Soc., 100 problems, Ph.D. thesis, University of Reading, U. K., (1974) 437-449. (1979), pp. 227. 20 Das, P. K., Storm surges in the Bay of Bengal, Proc. Symp. 4 Welander, P., Numerical prediction of storm surges, Adv. Typhoons Proc. Symp. Typhoons, Shanghai, China, October Geophys., 8(1961) 315–379. 6-11, 1980 (1980). 5 Johns, B., Sunhat, P. C., Dube, S. K., Mohanty, U. C., & 21 Johns, B., & Ali, A., The numerical modelling of storm Rao, A. D., Simulation of storm surges using a three- surges in the Bay of Bengal, Quart. J. Roy. Met. Soc., 106 dimensional numerical model: an application to the 1977 (1980) 1-18. Andhra Cyclone, Quart. J. Roy. Met. Soc., 107(1983) 915- 22 Dube, S. K., Sinha, P. C., & Rao, A. D., The response of 934. different wind stress forcing on the surges along the east 6 Johns B., Dube, S. K., Mohanty, U. C., & Sinha, P. C., coast of India, Mausam, 32 (1981) 315-320. Numerical simulation of the surge generated by the 1977 23 Dube, S. K., Sinha, P. C., & Rao, A. D., The effect of coastal Andhra Cyclone, Quart. J. Roy. Met. Soc., 107(1981) 915- geometry on the location of peak surge, Mausam, 33 (1982) 934. 445-450. 7 Scheffner, N.W., Mark, D. J., Blain, C. A., Westerink, J. J., 24 Johns, B., Dube, S. K., Sinha, P. C., Mohanty, U. C., & & Luettich, R. A. Jr., ADCIRC: an advanced three- Rao, A. D., The simulation of a continuously deforming dimensional circulation model for shelves coasts and lateral boundary in problems involving the shallow water estuaries, report 5: a tropical storm database for the East equations, Computers and Fluids, 10 (1982) 105-116.

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