COASTAL HYDRODYNAMICS AND SEDIMENT TRANSPORT ALONG SOUTH WEST COAST OF INDIA WITH SPECIAL REFERENCE TO HEAVY SAND
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The south west coast of India is famous for its rich heavy sand, also called placer, deposits. One of the world’s richest heavy sand deposits occur along this coast at Chavara and are being mined for their commercial utilities. These deposits are ultimately developed from the sea by the continuous action of waves and currents in the region. The mechanisms of formation of these deposits are studied in this dissertation by making field measurements and numerical computations. Waves and currents are measured during the monsoon and post-monsoon seasons. Bathymetric surveys were carried out in the innershelf in addition to the monitoring of beach erosion. The textural and mineralogical characteristics of the beach and innershelf sediments were studied for different seasons. Nearshore wave energy distribution and wave induced sediment re-suspension characteristics were studied by computations using numerical models. The onshore-offshore and alongshore sediment transport rate for the coast were estimated using transport models.

Short period high waves are observed during the south-west monsoon period, whereas, long period waves with SWH of the order of 1 m are observed during post-monsoon season, along this coast. The monsoonal coastal currents are southerly in direction with a maximum value of 25 cm/s whereas, the currents are in the north-easterly direction during the post-monsoon with a maximum value of 16 cm/s.

The threshold criteria for the re-suspension of individual sediment types shows that monazite, the densest mineral, needs higher shear stress and velocity for re-suspension whereas the lightest quartz needs the minimum shear stress for transport. The other heavy constituents have the threshold values in between. The zone of re-suspension extends up to a distance of approximately 50 km from shore during the monsoon. Onshore-offshore transport computations for the monsoon season give a maximum transport rate of 0.028 g cm^-1s^-1 in the southerly direction with an onshore component. During the post-monsoon the rate of transport is less with a component in the north-easterly direction. The dominant onshore-offshore transport during monsoon with a component towards coast helps in the transport of heavy sand from the submerged palaeo beaches in the offshore. During this onshore transport the individual sediments undergo different sorting processes, which ultimately results in the enrichment of heavies along the beaches. Also, the persistent erosion of the barrier island coupled with the seasonally reversing longshore sediment transport helps in the enrichment of heavy sand along the beaches. The onshore enrichment of individual heavy sand components along two shore normal transects are also discussed. A conceptual model for the enrichment of heavies along this coast is also presented.