Cyclone and Storm Surge - Iczmpwb.org
Cyclone and Storm Surge - Iczmpwb.org Cyclone and Storm Surge - Iczmpwb.org
2.14 Strong winds known as cyclones occur along the coasts of West Bengal and Bangladesh. Most of the tropical cyclones manifest themselves in April/May or October/November. Wind velocities of 200 km/hour can occur during strong cyclones. The wind velocity reduces rather fast when it blows over land. This is discussed I n the next section. When the wind waves reach the coast there are two effects that can cause damage to the shore or to the embankments built by mankind to protect the land. The first effect is known as wave rush-up. By the force of the waves water is rushing up the slopes to a higher level than the level of the top of the waves in open water. The second effect is wave dash. Water is smashed with force against the slope of the natural or artificial embankment which might cause damage to these slopes and eventually the entire embankment. 2.4.5. Cyclonic storms The reasons for the development of tropical cyclonic storms are many amongst which the most important is the prevalence of moist warm winds. Hence they occur mostly during the period front April to the end of November when South westerly winds blow more or less steadily over the Bay. Cyclonic storms grow gradually till the maximum intensity is reached. The depth of the lowest barometric pressure and the extent of the wind field are the most important parameters influencing this intensity. Some stormy extend over a wide area but have only a weak pressure gradient while others are more concentrated over a small area but have a great pressure gradient. High wind velocities occur only in, regions of limited extent along the storm track through which the cyclone passes. The wind pressure causes an upsurge of the water levels and lead to high water levels much higher than the predicted High Water 1evel based on tidal influence. As such cyclone storms and the resultant surge levels are very important from the hydraulic point of view in as much as they influence the levels up to which water may rise and the levels up to which protection is desired severe cyclones may cause heavy devastation especially if the storm surge occurs in synchronisation with high tides. The high waves generated attack the shore and embankments at great velocities. The most severe cyclone within recorded memory occurred in October 1942 when there occurred wide devastation along the West Bengal coast. The maximum wind speed recorded during this storm was about 165 km per hour in Sagar Island. The maximum height of water level was about 3.5 meters above the predicted high water level and about 2 Meters above the highest high water level recorded during other storms. This storm is of course one of an extremely rare occurrence.
2.15 During the months of November to May depressions are formed in the lower region of the Bay and hence rarely reach the coast of the Sundarbans. The period during which cyclone storms affect Sundarbans is between June and October out of which October is the most dangerous month of the year when South West monsoon starts receding and North West wind starts stepping in. Most of the tropical cyclonic storms cross the shore having average speeds varying between 25 and 75 km per hour. During the 60 years from 1907 to 1966 there were as many as 108 tropical cyclonic storms of velocity above 25 km per hour crossing the Sundarbans coast. The wind – speed wise and month – wise distribution is as given in Tables 2-3 and 2-4. Table 2-3. Wind statistics recorded at Sagar Island Wind speed in Number of Occurrenc Percentage km per hour 25 31 29 50 30 28 75 25 23 100 11 10 125 10 9.9 150 1 0.1 175 - - Total 108 100 Table 2-4. Number of occurrences of tropical cyclonic storms at West Bengal coast Month Number of Occurrence Percentage January 0 0 February 0 0 March 0 0 April 0 0 May 4 4 June 9 8 July 28 26 August 31 29 September 16 14.5 October 17 15.5 November 3 3 December 0 0 Total 108 100
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2.15<br />
During the months of November to May depressions are formed in the lower region of the Bay <strong>and</strong><br />
hence rarely reach the coast of the Sundarbans. The period during which cyclone storms affect<br />
Sundarbans is between June <strong>and</strong> October out of which October is the most dangerous month of the<br />
year when South West monsoon starts receding <strong>and</strong> North West wind starts stepping in.<br />
Most of the tropical cyclonic storms cross the shore having average speeds varying between 25 <strong>and</strong><br />
75 km per hour. During the 60 years from 1907 to 1966 there were as many as 108 tropical cyclonic<br />
storms of velocity above 25 km per hour crossing the Sundarbans coast. The wind – speed wise <strong>and</strong><br />
month – wise distribution is as given in Tables 2-3 <strong>and</strong> 2-4.<br />
Table 2-3. Wind statistics recorded at Sagar Isl<strong>and</strong><br />
Wind speed in Number of Occurrenc Percentage<br />
km per hour<br />
25 31 29<br />
50 30 28<br />
75 25 23<br />
100 11 10<br />
125 10 9.9<br />
150 1 0.1<br />
175 - -<br />
Total 108 100<br />
Table 2-4. Number of occurrences of tropical cyclonic storms at West Bengal coast<br />
Month Number of Occurrence Percentage<br />
January 0 0<br />
February 0 0<br />
March 0 0<br />
April 0 0<br />
May 4 4<br />
June 9 8<br />
July 28 26<br />
August 31 29<br />
September 16 14.5<br />
October 17 15.5<br />
November 3 3<br />
December 0 0<br />
Total 108 100
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