We investigated the seasonal spreading and transport of buoyant plume in the shelf off Kochi using Finite Volume Community Ocean Model (FVCOM). The modelled river plume typically consisted of an offshore bulge and a coastal current. The spreading of the bulge extended up to a distance of 19 km from inlet during the summer monsoon to <10 km in the spring inter-monsoon. The Kelvin number varied between 0.1 and 0.9 which revealed that the plume exhibited both the features of small and large scale plumes, resulting in a highly complicated plume pattern. During the southwest monsoon the plume fringe twisted towards the south, while during the northeast monsoon it twisted towards north according to the reversal of monsoonal winds. The fresh water transport with respect to coastal currents varied in accordance with seasonal river discharge such that the value peaked in the wet season and dropped in the dry season. During the non-realistic (no wind) condition the plume initiated barotropic and baroclinic flow, after which it was acted upon by earth’s rotation so that the plume propagated in the direction of Coriolis force (towards north), as geostrophic currents. The model run ‘with wind’ and ‘without wind’ condition revealed that in the shelf off Kochi the plume is transported in accordance with monsoonal winds/currents by nullifying the effect of earth’s rotation. The categorization of plume influenced area and realization of the direction of plume transport can be used for interpreting the dynamically and potentially active zones in the shelf off Kochi.
Estuaries with poor flushing and longer residence time retain effluents and pollutants, ultimately resulting in eutrophication, a decline in biodiversity and, finally, deterioration of water quality. Cochin Estuary (CE), southwest coast of India, is under the threat of nutrient enrichment by the anthropogenic interventions and terrestrial inputs through land runoff. The present study used the FVCOM hydrodynamic model coupled with the Lagrangian particle module (passive) to estimate the residence time and to delineate site-specific transport pathways in the CE. The back and forth movements and residence time of particles was elucidated by using metrics such as path length, net displacement and tortuosity. Spatio-temporal patterns of the particle distribution in the CE showed a similar trend during monsoon and post-monsoon with an average residence time of 25 and 30 days, respectively. During the low river discharge period (pre-monsoon), flood-ebb velocities resulted in a minimum net transport of the water and longer residence time of 90 days compared to that of the high discharge period (monsoon). During the pre-monsoon, particle released at the southern upstream (station 15) traversed a path length of 350 km in 90 days before being flushed out through the Fortkochi inlet, where the axial distance was only 35 km. This indicates that the retention capacity of pollutants within the system is very high and can adversely affect the water quality of the ecosystem. However, path length (120 km) and residence time (7.5 days) of CE were considerably reduced during the high discharge period. Thus the reduced path length and the lower residence time can effectively transport the pollutants reaching the system, which will ultimately restore the healthy ecosystem. This is a pioneer attempt to estimate the flushing characteristics and residence time of the CE by integrating the hydrodynamics and Lagrangian particle tracking module of FVCOM. This information is vital for the sustainable management of sensitive ecosystems.
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