A storm surge during a cyclone inundates coastal areas and off shore islands, which causes most of the loss of life and property. Information on storm surge height is very scarce in Bangladesh, Available literature provides a range of 1,5 to 9.0 meter high storm surges, during various severe cyclones.  However a SMRC report shows the surge height for 1876 cyclone was 13.6 m at Bakerganj and the surge height for 1970 cyclone was 10m. Locations of these surge heights are not known. Therefore, it is difficult to compare maximum wind speed and corresponding surge heights. Displacement of water surface during a cyclonic storm surge also depends on the height of tide.

Tide is a periodic phenomenon with a period of about 12 hrs 25 min. while cyclonic storm surge is an occasional phenomenon. The storm surge wave moves along with the tide and the displacement of water surface is due to combined action of tide and storm surge. The resultant displacement depends on the phase difference between peak surge and peak tide. In the Bay of Bengal, the displacement of water surface is largest when the cyclone storm reaches the coast at the time of spring tide. The interaction between storm surge and tide is non-linear. The displacement of water surface during a cyclonic storm surge also depends on tide.

The displacement of water surface is largest when the cyclonic storm surge reaches the coast during the time of spring tides. Such coincidence occurred during cyclones of November 12, 1970, December. 10, 1981 and April 129, 1991.

The cyclonic storm surge is a gradually varied unsteady flow. The amplitude of cyclonic storm surge wave amplifies during its progression towards the coast of Bangladesh due to long and shallow continental shelf. The height of the surge at a point depends on the origin, track, forward speed and strength of the wind field associated with each particular cyclone event. Because of the large length of the cyclonic storm surge wave, movement of a huge mass of water accompanies it. When a cyclonic storm surge reaches the coast, the surge water floods the adjacent area.

Local waves can form over the surface of cyclonic storm surge wave and of tidal wave due to blowing wind. The height of wind , generated waves riding on top of the surge wave at the time of a severe cyclone may be large. As a rule of thumb, height of wind waves in allow coastal water could be 1/2 to 3/4 of the surge height.

The wind waves rapidly attenuate across inland terrain flooded by the surge. Another important factor is the path of cyclones, Due to its geographic location, cyclones hitting the Khulna regionin in the South West have comparatively lower surges than those hitting the Meghna estuary.

Tropical cyclones accompanied by storm surges are one of the major disasters in Bangladesh. These cyclones form in the Bay of Bengal, which is situated to the south of the country and is a favorable breeding ground of tropical cyclones. They hit the littoral countries namely, Bangladesh, India, Sri Lanka and Myanmar. Bangladesh is the worst sufferer of all cyclonic casualties in the world.

On the basis of wind speed, the cyclones can be broadly divided into two categories: depressions (with wind speed less than 17 m/s or 62 km/hr) and tropical storms (with wind speed greater than or equal to 17 m/s). About 80 tropical storms form in the world waters every year (McBride 1995). Of these, about 6.5% form in the north Indian Ocean and the Bay of Bengal share comes out to be about 5.5%. A country-wise break up shows that Bangladesh is hit by about 0.93% (-1%) of the world total of tropical cyclones, India by 3.34%, Myanmar by 0.51%, Sri Lanka by 0.22% and 0.50% die in the Bay of Bengal without hitting any country.

Apparently it would seem that Bangladesh is not a high-risk cyclone-prone area. The situation is however otherwise. If the world's tropical cyclones due to each of which death tolls were in excess of 5000 are considered, it is found that about 53% human deaths of the world total due to cyclones occurred in Bangladesh. This shows the gravity of the situation. It has been found that major cyclone disasters are still continuing in Bangladesh.

Storm surges are generated by the winds and the atmospheric pressure changes associated with a cyclone. Wind is the main contributing factor (about 90%). It exerts stress on the water underneath, and surge is generated. In Bangladesh, storm surge heights in excess of 10m are not uncommon. For example, in 1876, the great Backerganj cyclone had a surge height of 13.6 m and in 1970 the height was 10m.

Storm surges once generated are modified, mostly amplified by a number of factors. Some of these factors are:

• Shallowness of water in the northern Bay of Bengal in the Bangladesh coast

• Convergence of the Bay of Bengal towards Bangladesh in the north

• High astronomical tides

• Presence of islands

• Presence of large number of inlets and channels in the coast area

The Meghna estuarial region is the area where most of the surge amplifications occur. The country's low and flat terrain in easily flooded by amplified surge waters, thus converting the coastal area into a vast sea.

The dependence of cyclone formation and intensification on Sea Surface Temperature (SST) has led to the speculation that any rise in SST due to climate change is likely to be accompanied by an increase in cyclone frequency and cyclone intensity, which in turn will affect the storm surges generated by cyclones. A somewhat in-depth study on the impacts of climate change and SLR on cyclones and storm surges in the Bay of Bengal and in Bangladesh has been made by various authors (e.g. by Ali, 1996, 1999). Using SST data for the Bay of Bengal for the period 1951 to 1987, Joseph (unpublished) has shown that SST has been increasing in the Bay of Bengal since 1951. Singh et. el. (2000) have also shown that both SST and the intensity of Bay of Bengal cyclones in May and November have been increasing.

An analysis by of all the cyclones that formed in the Bay of Bengal during the period 1877-1997 showed that there has been no increase in cyclone frequency in the Bay of Bengal, rather an oscillation of about 40 year period has been observed.

The frequency of striking or land-falling cyclones is likely to increase at an increased SST in the Bay of Bengal. During 1877-1997, about 366 cyclones forming in the Bay of Bengal did not strike any littoral country (Bangladesh, India, Sri Lanka and Myanmar); they died in the Bay of Bengal. The percentage of such cyclones is about 25. If there were higher sea surface temperatures in the Bay of Bengal, these so-called dead (or non-land falling) cyclones could have further developed and struck the littoral countries, thereby increasing the frequency of 1and- falling cyclones and thus causing more casualties and damages. The frequency of striking cyclones would have increased by 32%. It may, therefore, be said that climate change is very likely to increase the frequency or number of land-fa11ing cyclones by a big percentage even if the actual frequency of cyclones in the Bay does not increase. An analysis has shown that though the frequency of tropical cyclones in the Bay of Bengal is not changing much, the number of intense tropical cyc1ones hitting Bangladesh have increased in the recent years.

There does not seem to be any study on the impacts of SST on cyclone intensity in the Bay of Bengal. However, there is an apparent observation that number of  more intense cyclones in the Bay of Bengal is on the rise. It is almost certain that an increase in SST will be accompanied by a corresponding increase in cyclone intensity (wind speed). An increase in SST is likely to cause greater convective instability, leading to an increase in the wind speed of a cyclone. Emanuel (1987) has developed a relationship between maximum sustained wind speed (MSWS) and SST. According to him a 2°C rise in SST would increase the MSWS by 10% and a 4°C rise by 22%. Henderson-sellers and Zhang (1997) estimate that even 1°C rise in SST could increase tropical cyclone intensity by 10%, a higher estimate than that of Emanuel (10% for 2°C rise). But theoretical considerations show that a 1°C rise in SST will increase the cyclone intensity by 4%, 2°C rise by 10% and 4°C by 22%.

A modeling study using variable SST has shown that the tropical cyclone intensifies more rapidly and acquires higher intensity at a warmer SST.

SLR may also cause an increase in the destructive effect of cyclones in Bangladesh. The SLR will force the shoreline to retreat. As a result, everything else remaining unchanged, the path distance of the cyclone will increase and the cyclone will have more time in the sea to acquire energy to increase its intensity.

The SLR may change the circulation pattern in the Bay leading to changes in the SST distribution pattern. Any change in SST distribution may affect the place of actual formation, cyclone intensity, cyclone tracks, etc. The SLR may also cause changes in the pressure distribution (one way is through redistribution of SST as mentioned above) and hence a change in cyclone activity and storm surges.

An increase in cyclone frequency (if at all) due to climate change will expose the coastal area of Bangladesh to more frequent flooding due to storm surges. An increase in cyclone intensity would increase the storm surge heights, which will increase the depth, extent and duration of flooding.

The stress exerted by wind on water underneath is proportional to the square of the wind velocity. Thus an increase in SST due to climate change will lead to higher storm surges. Using a numerical model for storm surges for the north Bay of Bengal and covering the coast of Bangladesh developed a few storm surge scenarios under two different temperature increases (2°C and 4°C) and 2 SLRS (0.3m and 1.0m rise). The 1991 cyclone that caused a death toll of about 138,000 people was used as a model case. The maximum wind speed was 225 km/hr. The results correspond to a grid point near Chittagong, the largest seaport of Bangladesh in the northeast comer of the Bay of Bengal. With no sea level rise, surge height increases by 21 % and 49% respectively for SST rise of 2°C and 4°C respectively, with respect to the present. Surge height is inversely proportional water depth. Thus apparently, SLR will cause a decrease in surge height. But, SLR will simultaneously, inundate the present land area which will then become shallow water. area due to SLR, and surge height will increase in the newly inundated shallow water area. The convergence of the Bay will also increase through SLR, thus making the surge height to amplify further. A corresponding increase in inland penetration of surges will increase by 13% and 31 % respectively.

A preliminary investigation shows that tidal ranges will increase due to SLR and compound the interaction between storm surges and'tides and hence increase the vulnerability of the country. In an earlier experiment found that an increase in sea level increases the tidal amplitudes in northeast Bay and a decrease in the northwest Bay.

Modelling cyclone hazard in Bangladesh
M.C.J. Damen and C.J. van Westen
Department of Earth Resources Surveys,
International Institute for Geo-Information Science and Earth Observation (ITC),