ABSTRACT

Patterns of sand movement based on the results of repeated seasonal surveys have been analysed in the foreshore region of an actively eroding beach around Digha, West Bengal, India. Two-dimensional trend surface analyses on the seasonal changes in the patterns of erosional and depositional envelopes between beach zones revealed a cyclicity in the offshore-onshore movement of sediment. An intefering effect of two sets of movements was clear. A major transfer of sediments towards offshore with gradual lowering of the beach and fluctuations in the direction of alongshore current were recognised.

Offshore transfer of sediments started in the early summer months (February to June) characterised by short period waves and strong onshore winds. During monsoon (July to September) further removal of beach sediments either towards offshore or towards alongshore was registered. In winter months (October to January) long period swells and mild offshore winds nourished the beach possibly at the expense of the offshore reservoir of sands. Occasional storm surges causing extensive collapse of coastal dunes gave rise to deviations from this general cyclicity.

The net seasonal material balance calculation for any set of complementary erosional and depositional zones was worked out. The pattern of volume change during different seasons displayed a clear tendency to settle down gradually with time. The impact loadings of cyclonic storms interfered and reinforced the oscillation. The overall nature of sand movement in this foreshore was fitted roughly into a mathematical model of an underdamped second order mass-spring-damper system.