139736eo.pdf (20MB) - Japan Oceanographic Data Center

low pressure centres over the Bay intensify into depressions, some of which intensify further intocyclonic storms. A few of these develop into severe cyclonic storms of hurricane intensity. Tropicalcyclones capable of generating surges usually occur during the pre-monsoon months (April to May) orthe post-monsoon months (Sepember to December), but rarely during the monsoon seasons (June toAugust and January to March).Low pressure areas mainly form in the Andaman Sea (Figs. 1-3). Occasionally lows that areremnants of typhoons from the China Sea move into the Bay from the east (MOOLEY, 1980).MOOLEY (1980) estimated that the probability of a severe storm striking the coast of the Bayof Bengal in any year is 0.345. More recently MOOLEY and MOHILE (1983) identified significanttrends in storm frequency, notably a significant increase in the mean annual frequency of storms on theBangladesh coast during the period 1965-1980. RA0 (1968) suggested that for the pre-monsoonstorms (April to May) the strongest winds are usually in the southeast or eastern sector, whereas forthe post-monsoon storms (Sepember to December) the region of maximum winds is to the north of thestorm centre.In Burma, storms occur mainly in the pre-monsoon season and affect the Arakan coast. If thestorm occurs during 1-10 May, the landfall is north of 18'N. During the post-monsoon season thestorms landfall north of 18"N.Consider now the sequence of events as a typical storm approaches the coastline. In view ofthe water depth, atmospheric pressure effects can dominate over the deeper part of the Bay.Disturbances generated there propagate much more quickly than the speed at which the cyclone moves,and therefore reach the coasts ahead of the storm. Wind-generated currents (ANON, 1966) caninfluence sea level where they impinge on the continental shelf. However, the main surge effect isgenerated by the wind acting over the continental shelves. RATNAM and NAYAR (1966) suggestedthat the resonant coupling mechanism, shown by PROUDMAN (1929) to occur when the speed of atravelling atmospheric disturbance matches that of free gravity waves, could be significant in theshallow coastal waters of the Bay of Bengal. However, wind stress acting over shallow water is themain generating mechanism.The maximum onshore winds, and hence also the maximum surge levels, tend to occur to theright of the storm track as it crosses the coast. A negative surge associated with offshore winds on theleft of the storm centre also may appear. The surge maximum occurs about the time the storm centrecrosses the coast. The speed and angle of approach of the storm, relative to the coastline, influence themagnitude of surge, and convergence of coastline in a bay or estuary can produce substantialamplification.The extreme shallowness of the water in some near-shore areas, the focussing effect of inletsand estuaries and the absence of shore defences to protect the low-lying coastal lands may lead to theformation of a 'storm wave', frequently described by eye-witnesses as a 'wall of water', which cantravel several kilometres inland, flooding affected areas to a depth of as much as 13 m (see the accountof the Backergunge cyclone of October 1876 in ELIOT, 1900). Some accounts suggest that theaccumulating water associated with the surge may be held back by the river flow and ebbing tide inestuaries such as the Hooghly, eventually advancing upstream as a bore, overtopping the river banksand inundating low-lying land as a storm wave (e.g. the Calcutta cyclone of October 1864, ELIOT,1900). Clearly, nonlinear effects play a vital part in this stage in the development of the surge.In the northern reaches of the Bay of Bengal and in the Andaman Sea, the tidal range iscomparable with probable surge amplitudes. The relative timing of high tide and peak surge thenbecomes very important. For instance, in the disastrous inundation of Bangladesh in November 1970,the surge peak occurred at high tide and killed more than 200,000 people. In contrast, a comparablesurge on October 31, 1960 (No. 22, Table 2) occurred at low tide and caused far fewer casualties.The apparent importance of nonlinear effects suggests that dynamical tide-surge interaction must alsoplay a part here.RA0 (1968) classified the Indian and Bangladesh coast along the Bay of Bengal coast intothree categories based on the combined amplitude of the storm surge and wind waves (Table 4, Fig.4). Rao's C-type belt, which is the most dangerous zone, occurs in three locations:(1) The coastal belt around the head of the Bay of Bengal, approximately to the north of20"N. The frequency of cyclones is high, and storm tracks are usually favorable for generatingmaximum surges, especially in the Sunderbans.(2) The south Coromandel coast around Balk Bay. Although the frequency of stormsstriking this region is somewhat smaller than for the first belt, the major storms that strike this coastusually produce major surges.207