Abstract:
The response to wave dynamics on coastal erosion and deposition in the southern sea front of Sagar Island of Sundarban delta, Bay of Bengal, during post- and pre-monsoon periods of 2003 and 2004 was critically studied. Two transects across the beach were selected at the western and eastern parts of the Island. Basic data like measurement of wave height with corresponding wave period, trends of wave crest and swash mark, inter-tidal width of the beach, tidal range and still water depth were collected along two transects during 10 lunar days of which five alternate days in each transect. From these basic data, different wave parameters like wave length, wave steepness, beach face angle, wave form velocity, long shore current, surf scaling factor, wave energy and rate of sediment transport were also derived using some working formulae.
The major findings are as follows:
1. The coastal belt belonging to Sagar Island is a macro-tidal coast. The tidal height varies from 3.008 (post-monsoon) to 3.427 m (pre-monsoon) in the western part and 3.586 (post-monsoon) to 3.814 m (pre-monsoon) in the eastern part of the beach. The inter-tidal width at its western part varies from 264.8 to 319.28 m and 104 to 133.6 m at its eastern part during post- and pre-monsoon periods respectively, Beach slope is <5°. Average wave height varies from 0.464 to 0.520 m at the western part and 0.520 to 0.741 m at the eastern part with corresponding wave periods of 8.393 to 8.41/s and 9.825 to 9.012/s respectively. At the western part, the significant wave height varies from 0.665 (post-monsoon) to 0.768 m (pre-monsoon) while at the eastern part, it varies from 0.953 (post-monsoon) to 1.019 m (pre-monsoon), the average wave
length varies from 13.377 to 21.459 m at its western part and 22.06 to 25.975 m at its eastern part for post- and pre-
monsoon periods respectively, the average wave steepness varies from 0.036 to 0.024 at western part and 0.037 to 0.03 at eastern part during post- and pre-monsoon periods respectively, the velocity of long shore current varies from 0.174 to 0.229 m/s at western part and 0.705 to 0.295 m/s at eastern part during post- and pre-monsoon periods respectively, wave energy varies from 286.471 to 226.293 Jm at western and 687.377 to 313.629 Jrn-2 at eastern parts during post- and premonsoon periods respectively.
2. The eastern part of the island is very vulnerable to erosion. Shoreline transgresses further inland causing devastating land erosion and a great danger to the inhabitants.
3. The beach comprises a mixture of very fine sand and silts with a bimodal grain size distribution having the prominent mode at 2.5 phi with secondary mode at 4.0 phi. The secondary mode becomes more prominent gradually from east to west of the beach indicating evidence of strong long shore current from east to west.
4. Frequent embankment failure, submergence and flooding, beach erosion and siltation at jetties and navigational channels, cyclones and storm surges are all making this area increasingly vulnerable.
