Numerical Simulations of Lagoonal Flow Field and Pollutant Advection in Funafuti Atoll, Tuvalu

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Daisaku Sato, Hiromune Yokoki, Masamitsu Arita

Thursday 2 july 2015

14:35 - 14:50h at Asia (level 0)

Themes: (T) Hydro-environment, (ST) Impacts of pollutants on the water environment

Parallel session: 12G. Environment - Impact

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Water contamination due to insufficient effluent treatment of domestic wastewater is one of a concern in the enclosed lagoon of Funafuti atoll, Tuvalu. However, the fundamental characteristics of water flow field in the lagoon are not understood enough. This study conducted the numerical simulations of the lagoonal flow field and virtual particles advection in order to estimate the characteristics of water flow field and water contamination condition in the lagoon of Funafuti Atoll. Lagoonal water flow field was simulated by Princeton Ocean Model (POM). Boundary condition of water surface elevation in outer ocean was calculated from short-period ocean tide model (NAO99b) which provided by National Astronomical Observatory. Boundary condition of wind statistics was calculated from observed data sets in Funafuti Atoll which are collected by Bureau of Meteorology of Australia Government. Simulation domain of whole of Funafuti atoll was made at 333m grid resolution. The vertical resolution in _ space was set at 10. Lagoon and ocean bathymetry was arranged by chart and ETOPO2, respectively. Advection of weightless and massless particles which assume pollutant was calculated by simulated flow field. Initial location of particles is set in the nearshore of Fongafale Island which is the main island of Funafuti Atoll. Comparison of the calculated current velocity and water surface elevation and the observed data showed good agreement. The calculated lagoonal flow field showed that dominant currents were driven by wind and dominantly towards east to west. On the shallow reef-flat the high speed orbital currents were generated by tide. However the orbital currents influenced the limited area near the open reef-flat. In the nearshore zone of the Fongafale Island the flow velocity was low through a year. Numerical simulation of virtual particles advection indicated that the particles which were installed in the nearshore of Fongafare Island were transported to the west-side of the atoll through a year. In the non-trade-wind season, the high retention of installed particles was calculated in the lagoon. The simulated result provides important information for lagoonal environment conservation because the west-side of the atoll is established as the preservation area by Tuvalu.