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.