Storm surges in the Bay of Bengal

Quart. J. R. Met. SOC. (1974), 100, pp. 437-449 551.465.755 : 551.515.1

Storm surges in the Bay of Bengal

By P. K. DAS, M. C. SINHA and V. BALASUBRAMANYAM Meteorological Ofice, New Delhi-3, India

(Manuscript received 13 June 1973; in revised form 1 I March 1974. Communicated by Mr. H. Charnock)

SUMMARY

Estimates of the surge generated by a model storm impinging on three coastal areas of north Bay of Bengal are presented. A linear shallow-water long-wave model was used to evaluate the surge. Surge amplitudes were computed for different values of storm intensity and speed. The effect of the astronomical tide was estimated by specifying initial boundary conditions on the open boundary of the computational grid. For a devastating storm, which struck Bangladesh (formerly East Pakistan) on 12/13 November 1970, we show that superposition of the surge on the tide leads to an overestimate of sea level elevation at the time of landfall.

1. LIST OF SYMBOLS

Cartesian co-ordinates (oxyz) with the x and y axes pointing to the east and north respectively have been used. The depth (oz) is negative downwards from the sea surface, and the curvature of the earth has been neglected. The other symbols not defined in the text are as follows:

elevation of the sea surface from its undisturbed state the maximum surge at the time of landfall atmospheric pressure Coriolis parameter acceleration due to gravity density of sea water and air depth of the sea bed zonal and meridional component of velocity transport vectors defined by

i i U =s udz, V =s vdz

- h - h

x and y components of wind stress x and y components of bottom stress coefficient of eddy viscosity indices to represent the co-ordinates of grid points, i.e., X = mAx, Y = nAy where Ax, Ay are small increments in x and y . index to represent the time-step in numerical integration, i.e., t = kAt.

2. JNTRODUCTION

In an earlier paper (Das 1972) we computed the surge generated by an idealized storm striking the north-eastern coast of Bangladesh. The experiment was designed to simulate the surge produced by a devastating storm which struck Bangladesh on 13 November 1970. In this paper we present the results of similar computations for three coastal areas of the Bay of Bengal. While the earlier work was concerned with the north-eastern sector, the study is now extended to the northern and north-western sectors of the Bay of Bengal.

437

438 P. K. DAS, M. C. SINHA and V. BALASUBRAMANYAM

The purpose of this paper is to compute the surge generated by a model storm which moves with uniform speed. The idealized storm has concentric isobars and an inner ring of strong winds, but no cross-isobaric flow. By changing the parameters of the model we relate the surge at the time of landfall to the intensity and speed of propagation of the storm. This has predictive value if the storm parameters are derived from weather satellite data.

Surge estimates for the storm of November 1970 had shown reasonable agreement with the tide gauge readings of Chittagong (22*3"N, 91*8"E), a harbour 60 km to the south of landfall. The maximum predicted surge (3.2 m) was higher than the gauge observation (1.5 m), but there was uncertainty about the latter because of defects in the tide gauge. In addition, there was a difference of 2 hours between the computed and observed time of peak surge. We wish to examine whether these features could be explained by interaction between the surge and the astronomical tide.

3. BASIC EQUATIONS

The basic equations express the propagation of long waves in shallow water. The main assumptions are: (a) the amplitude of the surge is small compared to the depth of the sea ( ( /h < 1); and (b) the horizontal scale of the surge is large compared to h, so that the hydrostatic assumption for pressure is valid (Charnock and Crease 1957). External forces, such as atmospheric pressure, the Coriolis force and frictional effects at the surface and the sea bed are included in the model. It also includes other driving forces like the astronomical tide through specified boundary conditions. By integrating the linearized long wave equations (Stoker 1957) over depth, we express the basic equations in the form

(1)

where A is a column vector whose elements form the following set of three dependent variables,

A=[!] . (2)

1 - (3)

B is a matrix of coefficients containing partial derivatives with respect to x and y. We have

f -gha/ax B = -f -gha/aY r" -a/ax -a/ay O o

The product (C . D) represents external forces due to atmospheric pressure and frictional stress. C and D are defined by

(4)

(5)

c = [ - (h/P)a/ax ( /P) (Fs - Fb)

- (h/p>a/aY ( l / d G s - Gb)

The surface stress components (Fs, G,) due to the action of wind are provided by

Fs = kpal valua, Gs = kpal Valoa (6)

where ]Val is the scalar wind (u,2 + u,2)* at 10 m above sea level, and u,, u, are the components of V, to the east and north. k is a non-dimensional coefficient which, for simplicity,

STORM SURGES IN BAY OF BENGAL 439

we set at 2.8 x lo-,. This is an average value for strong winds blowing over water (Wilson 1960).

To compute the frictional stress components at the bottom (Fb, Gb), we assumed a steady Ekman spiral over the sea bed. For this current system the stress components are directly proportional to the transport vectors (U, V ) and inversely proportional to h2 (Welander 1961). We have

where c1 is a constant with the dimensions mz s-', when h is expressed in metres. For engineer- ing problems other forms of frictional stress have been used. In this context we refer to Manning's formula or the Chezy-Kutter formula (Bretschneider 1967). These formulations use a quadratic law for bottom stress, but the choice of frictional coefficients for a specific problem is best determined by field data. In the absence of field data in our region of interest, we felt the Ekman spiral would be adequate for this study.

It was realized that the expressions for Fb, Gb were only partially representative of the total dissipative force, because the time taken to build up a surge need not be sufficient to develop a steady Ekman spiral (Bowden 1953). But, as we can see, a dissipative force is always necessary to balance the input of energy through the surface wind, although its manner of representation may differ. If the overall representation of dissipative force is correct, at least to order of magnitude, then it should lead to a surge comparable to what is observed in nature. On the other hand, total neglect of dissipative forces, in any representative form, would result in large surges not observed in nature.

It was interesting to see how the typical viscous force compared with the Coriolis force, one of the other forcing terms in Eq. (1). The ratio of these two forces is the Ekman number (vlfh'). Considering an eddy coefficient (v) of 10' cm2 s - l , and a depth (h) of 20 m at 20"N, where f = 5 x lO-'s-', the Ekman number was 0.5. Consequently, for small values of h prevailing over the northern and north-eastern sectors of our grid, the dissipative force was comparable to the Coriolis force. This was indirect support for retaining bottom stress in the model.

4. METHOD OF COMPUTATION

The surge was computed for three storm tracks. Two different grids, shown in Figs. 1 and 2, were used. A 37 x 17 point grid (Fig. 1) was employed: (a) for storms moving north- eastwards towards Bangladesh; and (b) for those moving north towards the deltaic regions of the Indian coast. For storms that move north-west and strike the coast of Orissa in India, the second grid (Fig. 2) was used.

Isopleths of h in Fig. I were manually smoothed to prevent computational instability due to irregular variations of h. A noteworthy feature of this figure was the shallow conti- nental shelf on the north-eastern sector of the Bay of Bengal. But, by way of contrast, there was a sharp gradient of h just off the coastline in the north-western sector. Conse- quently, it was necessary to employ a finer grid (Fig. 2) for storms impinging on the north- western coast. Computations for the nested finer grid were matched with those pertaining to the coarser grid away from the coast.

This is illustrated in Fig. 3(a). Grid points on the coarse grid have been shown by circles, while crosses denote points on the nested grid. Points on the coarse grid which lay within the overlapping zone between the two grids are shown by a cross within a circle. Let us consider a segment of the transition zone bounded by double lines in Fig. 3(a). U, V and 5 were first computed for the points A,, A, and A, of the coarse grid. Values for M


Figure 1 . Grid for storms moving north-east (I) and north (11). Isopleths represent depth of the sea bed in metres.

the intermediate points A2 and A4 were obtained by linear interpolation between Al, A, and A,. Thereafter, we evaluated U, Vand I at B1, B2 . . . B, by treating the computations at A,, A 2 . . . As as boundary values for the nested grid. We note that to determine the dependent variables at Al, A, and As we need the values at C,, C, and C, of the nested grid. Thus, the larger and the finer grids act as boundary values for each other in the transition zone. We found by trial that this procedure did not create abnormal distortions in U , V and [ in the transition zone.

The finite difference analogue of Eq. (1) was solved with prescribed initial conditions on the open boundary (AB). We computed U, V, [ at each grid point, because our bottom stress formulation required U, V, [ at every grid point. Along the land boundaries, the boundary conditions prescribed no transport of water normal to the coast, while at corner points the transport vectors (U, V ) were made to vanish. To compute I along a land boundary, fictitious points inland were used. This is shown in Fig. 3(b). To compute at the

Figure 2. Grid for storms moving north-west (111).


P

Figure 3. Computations for nested grid and boundary conditions.

point 0, we used the values of U, V at P, Q, R and S to evaluate their space gradients. At Q we set U = V = 0, while at P and R, U vanished because of the coastal boundary condition. Along the open boundary AB in Fig. I, we assumed that V remained unchanged between the boundary and the adjoining interior point of the grid. Thus, in Fig. 3(c), Jet Y (m, 2) be a point on the open boundary. We assumed that V at 2 was the same as V at Y. This implied no space gradient of V between Y and 2, but the assumption was reasonable, because values of V along the open boundary were very small. 5 was then computed at Y by using the values of Vat X and Z. These boundary conditions were simpler than the more detailed finite difference scheme of Heaps (1969), but trials indicated that errors on this account would be small.

A predictor-corrector scheme was used to compute new values of the dependent variables in A . Let 4 (m, n, k) represent the right-hand side of Eq. (l), where we represent grid points by the indices m, n, and each time step by the index k . For the first time step (k = l), a new value of A was calculated by forward extrapolation

A(m, n, k + 1) = A(m, n, k ) + 4(m, n, k) x AZ . (8)

For simplicity in notation, we drop the indices (m, n) hereafter with the understanding that these operations refer to each element of A for all points in the range 1 < m < M , l < n < N .

Thus, for subsequent time steps (k > 1) the predictor for A was

Ao(k + 1) = A(k - 1) + 4(k) x 2At (9)

where the superscript (0) denotes the predictor. This provides a new calculation for 4, which is 4O(k + 1). The corrector for A is then

A'(k + 1) = A(k) + [ {4(k) + 4O(k + 1))/2] x At (10)


By repeating the process, the ith iteration yields

A'(k + 1) = A(k) + [{+(k) + 6' - ' (k + 1)}/2] x At (1 1)

We found that after the fifth iteration, the difference between successive values of A and 4 were less than one per cent, Consequently, the computations were stopped after the fifth iteration.

5. ESTIMATE OF TRUNCATION ERROR

0 3

By the mean value theorem, the truncation error of the predictor is - (At)3, where 0 is

the third derivative of A at some time between (k + 1) and (k - 1). Similarly, the corrector

may be seen to be an expression of the trapezoidal rule. Its truncation error is - - (At)3,

where q is the third derivative of A in the interval between k and k + 1. For small time steps we assume 0 2~ q , so that if A be the true solution at k + 1, we have

v 12

0 A(k + 1) = Ao(k + 1) + 5(At )3

0 12

A(k + 1) = A'(k + 1) - -(At)3

On eliminating A , we have 0 1 . - - (At)3 = [A'(k + 1) - Ao(k + l)] 12 3

Consequently, the final value of A(k + 1) is

A(k + 1) = A'(k + 1) + +[A'(k + 1) - Ao(k + l)] (13)

It was observed that the second term on the right-hand side did not exceed 5 % of the corrected value of A in these computations. The truncation errors were largest near the coast.

The unit time step and grid spacing were chosen to ensure computational stability (Fischer 1959). We put

(14)

(15) f' - A t < 1. . CI

These conditions were satisfied by a time step of 2 min, and a unit spacing of 13.8 km in the first grid (Fig. 1). For the second grid (Fig. 2) a spacing of 15 km was used for the nested fine network, while the coarser network had a unit spacing of 30 km. The unit time interval was 3 min for computations on the second grid.

Apart from facility in computation, At and A s were chosen in a manner best suited to the storm track. We could not, for example, use the grid in Fig. 1 for a north-west track, without considerably enlarging the size of the open boundary. This would have increased the number of grid points. We found by trial that the grid shown in Fig. 2 was best for a north-west track because, although it had a larger open boundary, the number of grid points was not substantially increased. This was achieved by employing a fine grid near the coast and a coarse grid elsewhere. The fine grid had the added advantage of incorporating


the sharp gradient of h near the coast even though the unit grid (15 km) here was slightly larger than the earlier grid spacing (13.8 km).

Slight smoothing was used to eliminate the growth of spurious computational modes in these computations. The smoothing operator on U, V and 5 for any time step k was (Welander 1961),

Rm, n) = y5(m, 4 + - - Y [c(m + 1, n) + ( (m = I , n) 4

+ 5(m, n + 1) + Hm, n - 111 (16) where y = 0.95

6. INITIAL CONDITIONS

Initial values of [ were specified on the open boundary (AB in Figs. 1 and 2). Two different initial conditions were specified for this experiment; they refer to the astronomical tide and the storm surge.

(a) The astronomical tide

This was designed to study the interaction between the surge and the tide for the devastating storm of 12/13 November 1970. From the Indian tide tables 1970 (Datta 1970) we calculated 5 along the open boundary by linear interpolation of the tidal amplitude and phase at Paradeep (20*3"N, 86.7"E) and Akyab (20.1 ON, 92-9"E). These two coastal stations were located at the western and the eastern edges of the grid (Fig. 2). With these values of [ on AB at t = 0, we solved Eq. (1) without any forcing due to pressure or wind stress. The only driving forces were sea-bed friction and interpolated tidal values of 5 along the open boundary. While making these computations, 5 was calculated on the open boundary not only at t = 0, but also at subsequent intervals. The computations were continued for four tidal cycles.

Apart from the first cycle, which was not fully representative of the astronomical tide, the computations reproduced the subsequent tidal cycles fairly well. They also provided an independent check on the appropriate value of the frictional coefficient (a). We found the best agreement between the computed and predicted tidal range was obtained by setting a = 0.020 m2 s-'. Table 1 shows a comparison between the tidal amplitude computed by the model and the values predicted in the tide tables for four coastal stations.

TABLE 1. COMPUTED AND PREDICTED TIDAL AMPLITUDES

Tidal amplitude (m) Station Latitude Longitude Computed Tide Table

- __ -

Balasore 21.5 86.9 1.9 1.5 Chittagong 22.3 91-8 1.7 1.7 Sagar Island 21.7 88.1 1.9 1.9 Shortt's Island 20.8 87.1 1.2 1.2

- _ _ - ~ ~~

The maximum tidal currents computed by the model varied from 0.5 to 1.0 kt in our region of interest. The transport vectors at the time of high water in Chittagong harbour for example, are shown in Fig. 4.


Figure 4. Transport vectors at time of high water in Chittagong. Inset represents magnitude of a transport equivalent to 20 mz s- '.

(b) Surge generated by a model storm

In the second experiment there was no forcing on the open boundary, that is, we put

The forcing terms in Eq. ( I ) were computed for an idealized storm with the following = 0 along AB at t = 0 and subsequently.

pressure profile

where r is the radial distance from the storm centre, and ro is the radial distance at which the wind attained its maximum velocity. Ap is the difference in pressure between the centre of the storm and its outer periphery. The cyclostrophic wind for Eq. (17) is

pa = 1010 - Ap/[l + (r/r,J2] . (17)

V 2 = 4 V i x / ? / ( I + p 2 ) 2 . (18) where p = r/ro, and V , is the maximum wind at r,. By Eqs. (17) and (18) the maximum wind speed (V,) was related to the pressure deficit (Ap) by the approximate relation,

V; = (13)2 x Ap. (19) when V, is expressed in kt and Ap is in millibars. For a central pressure of 960mb or Ap = 50 mb Eq. (19) provides a maximum wind of 100 knots at ro (30 km) from the storm centre. The wind stress due to the storm was determined by Eqs. (6) and (18).

The model storm was moved with a uniform speed of C along three different tracks. The first was a north-eastward track which made the storm strike the coast of Bangladesh north of Chittagong. The second track moved the storm north towards the deltaic regions of the Indian coast, while the third moved it north-west to strike the Indian coast near Paradeep. The three tracks are shown in Figs. 1 and 2. The tracks were chosen in this manner because they were representative of regions most affected by storm surges in north Bay of Bengal.

7. RESULTS

(a) Surge computations

The change in water level with time differs from place to place along the coast. By Cr.


we define the highest water level at the time of landfall. Generally, we observed, the maximum water level, or peak surge, occurred a little after the time of landfall for all three tracks. The location of the peak surge was approximately 30 km to the right of the storm centre. This was not unexpected because the maximum on-shore winds in the idealized storm was 30 km to the right from its centre.

We computed c L as a function of Ap and C for each track.' The results are depicted in Fig. 5(a), (b) and (c). An estimate of Ap, and the speed of propagation (C) enables us to compute .cL from these figures. t L was computed in the range 30 < Ap < 50mb and 15 < C < 40 km hr-'. We excluded very slow moving storms from this study; it is our intention to consider them in a subsequent paper. In Fig. 5(a), (b) and (c), the downward slope for low values of C suggested that the variation of c L with Ap and C was not linear. Consequently, we computed parabolic curves of best fit to relate c L with Ap and C. This is expressed by

C L = a&p + u ~ ( A ~ ) ~ + a2C . (20)

where a,, a, and a2 are constants. Their numerical values, determined by least squares, are shown in Table 2.

TABLE 2. NUMERICAL VALUES OF a,, al AND a2

a0 a1 a2 Track ( x 102) ( x 104) ( x 102)

North-east 939 - 0.9 1 -4.60 North 2.88 3.08 -1.20 North-west 8.24 -1.60 -5.15

The r.m.s. error of representing C L by Eq. (20) was 0.3 m. The largest deviations between tL generated by the model storm, and the values computed by Eq.(20) occurred towards the extreme edges of Fig. 5(a), (b), (c), but for practical purposes Eq. (20) provided a good representation of C L . It was interesting to note that the surge was largely determined by Ap; the contribution of storm speed ( C ) was small compared to the effect of storm intensity (Ap).

It was difficult to check the accuracy of Eq. (20) against actual observations because reliable tide gauge data were difficult to come by. Only in recent years storm parameters such as Ap and C, are estimated with some confidence from coastal radar stations and satellite data. What we have by way of past data, therefore, do not provide a conclusive test. But there was evidence to suggest that Eq. (20) or the values in Fig. 5(a), (b), (c) predicted cL of the right order of magnitude. In Table 3 we reproduce surge data for storms in the Chittagong area of Bangladesh from Flierl and Robinson (1972). The last two columns of Table 3 provide values of the sea level elevation predicted by Flierl and Robinson (1972) and by our computations for a north-east track. The total elevation of the sea level was obtained by adding c L to the astronomical tide. From records of the maximum sustained wind in column 3 of the Table, we computed Ap by Eq. (19) and then used Eq. (20) to compute cL.

Flierl and Robinson's values were usually higher than ours. Their model included the non-linear effect of bottom topography, but did not consider sea-bed friction. In our work, sea-bed friction was included, albeit in a crude form; this could be a reason for higher values of C obtained by Robinson and Flierl.


50 ~

t 45-

35 -

30 -

15 20 25 30 35 40 SPEED (Km hi')-

Figure 5(a). Sea level elevation ( 5 ) in m, storm intensity (Ap) in mb and speed (C) in km hr-'. North-east track (I).

L s 15 20 25 30 35 40

SPEED (Km h:)-

track (11). Figure 5(b). Sea level elevation ( 5 ) in m, storm intensity (Ap) in mb and speed (C) in krn hr-'. Northward

50 ! t 4 5 - n n E

- 4 0 -

35-

30 -

15 20 25 30 35 40

SPEED (Km h7')-

Figure 5(c). Sea level elevation (5 ) in m, storm intensity (Ap) in mb and speed (C) in km hr- l . North-west track (111).


TABLE 3. STORM SURGES IN THE CHITTAGONG AREA

Sea level elevation (m) Predicted

Speed V, Astronomical (Flier1 and Predicted Date (krn hr-') (kt) tide (m) Observed Robinson) (Das ef al.)

1 1 October 1960 31 October 1960 9 May 1961

30 May 1961 29 May 1963

5 November 1965 15 December 1965 13 November 1970

20 87 38 104 38 87 22 87 40 113 42 87 32 87 20 87

1 .5 0.0 1.2 0.6 0.3 1.2 0.3 1.8

6.0 5.7 6.6 8.7 4.8 6.9 - 3.3 - 11.4 - 5.7 - 4 8

6.0 to 9.0 8.7

4.7

3.6 3.7

3 -4 3 -0 5 .o

-

-

(b) Tide-surge interaction

The tropical storm of 13 November 1970 generated a surge which inundated the coastal areas of Bangladesh. There was much devastation. Flier1 and Robinson (1972) estimated a water level of 9 m in Chittagong at the time of landfall. Their estimate was obtained by adding the surge to the tide. But, in view of the coupling between the surge and the tide, superposition of one on the other need not always provide a realistic estimate.

It is difficult to extract the surge from the observed tide because, in shallow seas, the interaction between the astronomical tide and the meteorological surge is non-linear. Different methods have been described by Corkan (1950), Groves (1955) and Rossiter (1959) to compute the surge from the observed tide. Unfortunately, a filtering technique is not always helpful because the filter could modify the spectral band in which the energy of the surge is located. In this study we attempt to introduce the astronomical tide as an initial condition on the grid.

We had earlier computed the tidal cycle for Chittagong by imposing an initial tidal elevation on the open boundary of the grid. It is possible to introduce the model storm at different phases of the tidal cycle, so that the computed value of 5 at landfall provides a measure of the integrated effect of tide and surge. In effect, we solve Eq. (1) as before, but with initial conditions appropriate to the tidal cycle. The initial conditions were chosen suitably so that the time of landfall synchronized with different phases of the tide at Chittagong.

The track of the storm is shown in Fig. 1. It was assumed that the distance from P to Q was covered by the storm in 18 hours, with a uniform speed of 20 km per hour. This agreed reasonably well with the observed track and movement of the storm. Evidence from satellite pictures gave a reliable fix for position P in Fig. 1, but there was less certainty about the storm centre at Q. From the available winds and synoptic data, we inferred that this position could be in error by 50 km. The effect of this error on the speed of the storm would be small. The maximum wind at the time of landfall was 100 kt (Frank and Hussain 1971), while the estimated central pressure was 960 mb.

In simulated time, the storm was introduced on the open boundary at 0900 GMT of 12 November 1970. The storm was moved with a uniform speed at 20 km per hour, so that the time of landfall was 0300 GMT of 13 November 1970, in agreement with synoptic observations.

Considering different rates of lag between the time of landfall ( t = tL) and the initial point in time ( t = 0), we determined the initial conditions on 5 . Let us represent the astronomical tide at Chittagong, a station very near the point of landfall, by

9- L I L 5 = Hcos- T ( t - p )


where H is the tide amplitude, T is its period, t denotes time and p is the phase angle. From Eq. (21), we have at t = 0 and t L

and

where [ = H C O S ~ , .

From the uniform speed of the storm we set t L = 18 hours, while T was determined by the tidal cycle at Chittagong (12.2 hours). We assigned four separate values to $L, and computed the appropriate phase angles (p) from Eq. (24). Substitution of the phase angle in Eq. (22) yielded an initial value of [ at t = 0 for Chittagong. From earlier computations of the tidal cycle over the grid, described in Section 6(a), we evaluated the initial 4' (for t = 0) at all the other grid points, including the open boundary.

The storm was introduced on four sets of initial [. Thereafter, we determined the peak value of [ and its time of occurrence, with the origin in time (t = 0) set at 0900 GMT of 12 November 1970. The results are presented in Table 4.

TABLE 4. MAXIMUM SEA LEVEL ELEVATION FOR DIFFERENT TIDAL PHASES

Time of maximum Maximum Tide + surge Difference elevation (GMT) c

4 L on 13.11.70 (m) (m) (m)

0 03.24 4.5 5.3 -0.8 4 4 04.12 4.6 5.4 -0.8 4 2 06.18 3.9 4.8 -0.9 x 04.12 1.2 2.3 -1.1

Column 4 of Table 4 provides a measure of the maximum elevation obtained by super- posing the tide on the surge in the manner of Flier1 and Robinson (1972). We note from the last column of Table 4 that superposition of the surge on the tide led to an overestimate of the sea level elevation. The difference qt the time of high water (4L = 0) was 0.8 m. The highest elevation occurred earliest (03.24 GMT) when the surge coincided with the time of high water, but even this was slightly after the time of landfall (0300 GMT). The maximum sea level elevation for this storm was observed at 0100 GMT on 13 November 1970. Tidal interaction was thus unable to account for the difference of two hours between the observed and computed time of maximum elevation. We attributed this difference to our assumption of uniform speed of propagation for the storm as it approached the coast.

8. SUMMARY AND CONCLUSION

The main results of this study may be summarized as follows: (i) Idealizing a storm by a simple profile of pressure and wind, the surge at the time

of landfall could be related to storm intensity (Ap) and speed (C). The highest surges were observed for storms moving towards Bangladesh.

(ii) The contribution of storm speed (C) was small compared to the effect of storm intensity (Ap) on the peak surge ( c L ) at landfall.


(iii) The maximum surge was generally observed about 2 hours after landfall. It was

(iv) The total sea level elevation depended on the phase of the tidal cycle. The maxi-

(v) Superposition of the surge on the astronomical tide led to an overestimate of sea

located about 30 km to the right of the storm centre.

mum elevation was recorded earlier if the surge coincided with the time of high water.

level elevation by 0.8 to 1.1 metres.

ACKNOWLEDGMENTS

The authors wish to thank Dr. S. K. Roy from the Central Water and Power Research Station, Poona, for many helpful discussions on this subject. They also thank Dr. P. Kotes- waram, Director General of Observatories, for his kind interest in this work.

Bowden, K. F.

Bretschneider, C. F.

Charnock, H. and Crease, J. Corkan, R. H.

Das, P. K.

Datta, M. M.

Fischer, G.

Flier], G. R. and Robinson, A. R.

Frank, N. L. and Hussain, S. A.

Groves, G. W.

Heaps, N. S.

Rossiter, J. R.

Stoker, J. J. Welander, P.

Wilson, B. W.

1953

1967

1957 1950

1972

1970

1959

1972

1971

1955

1969

1959

1957 1961

1960

REFERENCES

‘ Note on wind drift in a channel in the presence of tidal

‘ Storm surges,’ Adv. Hydroscience, Academic Press, New

‘ North sea surges,’ Sci. Prog., 45, pp. 494-51 1 . ‘The levels in the North Sea associated with the storm

disturbance of 8 January, 1949,’ Phil. Trans. R . SOC.,

‘ Prediction model for storm surges in the Bay of Bengal,’ Nature, 239, pp. 211-213.

The Indian tide tables for 1970, Survey of India office, Dehra Dun, India.

‘ Ein numerisches Verfahren Zur Errechnung Von Wind- staund Gezeiten in Randmeeren’, Tellus, 11, pp. 6&76.

‘ Deadly surges in the Bay of Bengal, Dynamics and storm tide tables,’ Nature, 239, pp. 213-215.

‘ The deadliest tropical cyclone in history,’ Bull. Amer. Met. SOC., 52, pp. 4 3 8 4 .

‘ Numerical filters for discrimination against tidal periodi- cities,’ Trans. Amer. Geophys. Union, 36, pp. 1073-1084.

‘ A two dimensional numerical sea model,’ Phil. Trans. R.

’ A method of extracting storm surges from tidal records,’ Deut. Hydrograph, Z . 12, pp. 117-127.

Water waves, Interscience Publishers, Inc., New York. ‘ Numerical prediction of storm surges,’ Adv. Geophys.,

Academic Press, New York, Vol. 8, pp. 315-379. ‘ Note on surface wind stress over water at low and high

wind speeds,’ J. Geophys. Res., 65, pp. 3377-3381.

currents,’ Proc. R. Sac., A, 219, pp. 426-446.

York, Vol. 4, pp. 341-418.

A, 242, pp. 493-525.

SOC., A, 265, pp. 93-137.

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Storm surges in the Bay of Bengal