Cyclone and Storm Surge - Iczmpwb.org

2.46
The other type of failure may occur due to reasons like the presence of excess pore pressure
generated by a quick falling river water level. When a bank fails in this kind of geotechnical mode, the
probable slip lines or slip circles may be predicted, which depends also upon the type of material
forming the banks.
2.7.2. Embankments/dykes and their failures
Agriculture is carried behind the nearly 3500 Kilometer of protective earthen embankments. Those
are constructed by local earth of surface layers which consist mostly of very fine coherent. It is
generally of low liquid limit. This soil has got the property of low erosion resistance because this
material becomes liquid in coming with contact with water. Previously a forest belt of minimum width
of 100 meter was kept as Foreshore land in aligning on the embankment for the purpose of damping
the wave action. But by this time the increase of population and want of fuel has caused unregulated
felling and now practically there is no protection to embankment from high waves and suiting
currents.
These dykes were originally constructed by the Latdars with the gradual rise of high water level over
the years; the Latdars increased the height of the dykes correspondingly. The constant erosion of the
soil due to wave wash was also attended by them. But since late forties of this century the Latdars
started neglecting the maintenance due to rise in labour cost as well as the apprehension of
imminent state purchase of the Latdar's interest. The dykes at the time of taking over by the Irrigation
Department in 1960 were found to have inadequate section and strength. Breaches and overtopping
was of quite common phenomenon.
Due to paucity of funds, this enormous length of embankments could not furthermore be brought to
adequate section and height to ensure safety. In many cases these dykes are aligned bordering the
scouring bank of meandering rivers. This necessitated another set of embankments, some dist
inland, called the retiring embankments.
Generally during flow tide accompanied strong southerly wind from second fortnight, of March, to
October, the river water splashes over the earthen embankment causing continuous erosion of soil.
This phenomenon occurs twice a day and the poor soil give very little resistance to it. On an average
two major tropical cyclonic storms occur in each year in Sundarbans. During such cyclone storms
high waves are generated in sea face and which attack on the embankments at great storm velocities
and thus overtop embankment without free board, erode river side slope and crest and cause breach.
For want of cover protection works (Brick or Brick Block Revetment works) in strength of vulnerable
zone, the breach cannot be prevented.