Modelling of the Gold Coast Seaway tidal inlet, Australia

The Seaway entrance is a tidal inlet located on the Gold Coast (Queensland, Australia). Before the 80s, the entrance was highly variable in terms of inlet location and sand bar characteristics. The Seaway stabilisation with two training walls combined with an artificial sand bypassing system were completed in 1986 with the aims of fixing the entrance, maintaining a safe navigable channel, preventing shoreline erosion to the north and a buildup of sand to the south. Despite these training works, the dynamics of the Seaway is still poorly understood: channel infilling problems and navigation issues remain. For these reasons, the present study aims to develop a comprehensive model of the entrance to be used for further dredging and training work issues. The present investigation is carried out in two stages. The first stage is based on historic aerial photograph analysis of the Seaway before training works. It shows that the mouth was periodically driven northward by the longshore drift, with an average cycle time of 10 years. The second stage is based on numerical modelling after training works. Refined Delft3D modelling is undertaken with a 2DH approach on the Seaway area, taking into account the training walls and the sand bypassing system. This local model is coupled with MIKE21 implemented on a regional scale to provide accurate tide and flow forcing at the boundaries. After calibration, the analysis of flow patterns shows that the Gold Coast Seaway is ebb-dominated and that the more intense flow velocities are observed in the northern channel. Morphological evolution of the inlet is also investigated with a qualitative approach. Results indicate the pathways and rate of the sand movement within the tidal inlet in its current configuration and provide information about a planned 400 m extension of the southern training wall. A significant calibration work, involving sediment transport and bathymetry measurement, is required for the model to be used as a comprehensive tool for further dredging and dumping strategies within the entrance.