Numerical simulation of the morphodynamics of the Gold Coast Seaway

Abstract

Sediment transport and morphological change in the Gold Coast Seaway area, located on the east coast of Australia, were studied numerically. The main reason for the study was to understand the pattern of morphological evolution in the area with a focus on the ebb-tidal delta to examine the processes leading to its morphological change and gradual growth; despite the fact that a sand bypassing system has been implemented and operated since the entrance stabilization in 1986. The model used was MIKE 21 which is a depth averaged flexible mesh model which couples flow, wave and sediment transport. The model was run for a ten month period during which significant morphological evolution and delta growth was observed based on two subsequent bathymetry surveys. Model sensitivity to various input parameters such as wave directional standard deviation, wave breaking coefficient, sediment grain size and wave theory formulation were discussed. The simulated accumulative longshore sediment transport showed a good agreement with previous estimates. Although the model was unable to account for the movement of sediment resulting from the artificial sand bypassing system, it still simulated gradual natural sand bypassing around the offshore edge of ebb-tidal delta consistent with studies of other ebb-tidal delta locations devoid of an artificial bypassing system. The predicted trend of seabed level changes during the ten months, ignoring the artificial sand bypass system, also showed a tendency for northward migration of the inlet channel due to the dominant direction of the littoral current. This finding was consistent with the previous historical analysis of the entrance before stabilisation. During low energy wave conditions, the majority of the littoral drift accumulated to the south of the jetty. This resulted in erosion of the downstream coastline as well as the central part of the ebb-tidal delta. The cross shore distribution of the littoral drift confirmed the occurrence of a significant volume of longshore sediment transport further offshore from the seaward end of the bypassing jetty as well as the southern training wall, especially during storm events. It is suggested that this leakage partially passed the ebb-tidal delta through natural sand bypassing, but was mainly trapped on the ebb-tidal delta in response to the interaction of longshore, wave induced and ebb currents. Therefore, it is suggested that the growth of the ebb-tidal delta is mainly due to the leakage of littoral drift past the sand bypassing jetty and southern training wall. The results also showed that storm events, and in this case mainly the East Coast Low (ECL) in May 2009, play a major role in controlling the morphological evolution of the ebb-tidal delta. The model skill was therefore assessed quantitatively for simulated bed level changes in response to ECL 2009 event in comparison with the observed morphological changes over the ten month period using an adjusted Brier Skill Score. The skill results in this comparison show that the model performed well.