Numerical modelling of released dredged sediment dispersion from Pandop harbour (New Caledonia)

The Pandop harbour is located in the New Caledonia North-West lagoon. This site is subject to sedimentation processes requiring dredging operations in order to maintain its activities. The present work aims to evaluate the released sediment impact on outside coral reef. The first step of this work consists in evaluating oceano-climatic conditions impacting the study site, i.e. tide, global circulations, waves, wind and upwelling phenomena. Offshore wave conditions were analysed from WW3 numerical model over a 23 years period. The tide is taken into account using the TPXO database (Egbert et al., 2002). The offshore general currents around New Caledonia are studied from results of Mercator model (Brasseur et al., 2005) at different depths over a 10 years period. Circulations along the west coast of New Caledonia show a seasonal variability in terms of intensity and direction (weaker currents during austral winter). These circulations are used to force the TELEMAC3D model. The mesh covers the North-West part of New Caledonia and is refined in the lagoon, on the coral barrier and around the disposal zone. The vertical mesh is only composed of 10 planes (fixed and sigma planes) to limit computation time and is refined in the upper part of the water column. The numerical model is calibrated and validated with tide gauge measurements from SHOM and ADCP campaign. Friction coefficients are assigned depending on the bottom type especially on the coral reef. To model the sediment disposal, a simplistic but realistic solution was used following the bibliography (KBR (2002), Alzieu (2000), Boutin (2000)). Indeed, in order to represent the sediment resuspension during the fall, sediments are injected in the water column at different depths and especially at the pycnocline. The results allowed to define a disposal zone that gives the best compromise between a relatively close disposal area and a low impact on the coral reef. Moreover, the cumulative impact of successive disposals is studied with long-term simulations. Finally, the currents around the disposal zone are relatively low and thus sediment transport is limited