Numerical study of wave and sediment dynamics at dredge disposal sites near the Otago Harbour Entrance, New Zealand

To assist the management of existing and future channel dredging and disposal activities in the vicinity of the Otago Harbour entrance region in New Zealand, a numerical study of the nearshore wave, circulation and sediment processes was undertaken. The area is a complex coastal environment, with strong wave-driven and tidal circulation. The implementation of a coupled model system with boundary forcing by regional waves, water levels and currents is presented. The modelled physics encapsulate wave transformation, wave-driven currents, wave-current feedback, and sediment transport due to wave and currents. A morphological component optionally updates bed levels based on sediment transport calculations. The model application to investigate the sediment dynamics in the region of existing dump sites is described. Time domain simulations of historical events show that a major feature of the region is the intense wave focusing developing over the submerged ebb delta near the harbour entrance, redirecting wave energy towards an adjacent beach and a dredge disposal ground. The wave energy drives a net onshore directed sediment flux from the disposal mound. The model successfully replicates the progressive onshore migration and spreading of the mound, as observed from bathymetric surveys.