Open coast monsoonal beach dynamics

On the coast of Kerala, southwest India, oceanographic measurements were made at the beach, in the surf zone region, and on the inner shelf and then were combined with mechanism-focussed numerical modelling. The goal was to investigate the sedimentary dynamics of an open coast monsoonal beach. A closed sedimentary circulatory system (here described as a step-ladder) was identified. Strong annual cycles were observed in waves, winds, and currents. Regional-scale dynamic sediment equilibrium is maintained by an annual net northerly sediment flux in the nearshore, driven by wave-induced currents. This is balanced by a net southerly flux on the inner shelf, but driven instead by wind-induced currents that transport the wave-induced suspended sediment. The two counterdirectional sedimentary pathways are linked by cross-shore bridging transport. A clear association between the annual beach erosion and accretion patterns and the cross-shore direction of the near-bed bridging currents on the inner shelf was recorded and was associated with seasonal and shorter oscillations in the cross-shore component of wind, leading to inner shelf downwelling during the onshore wind periods and upwelling at other times. This occurs in synchrony with wave-induced beach erosion during the stormy onshore wind periods and accretion during the periods of narrow-banded swell and offshore winds, thereby delivering the sediment to or from the beach to be further transported on/offshore by shelf currents. Overall, the synchronous and supportive behaviour of the physical factors results in an unusual sedimentary equilibrium that explains how a strong net littoral drift can be sustained when supplies of nearshore feeder sands are absent.