Right-angle low-profile sandbag groyne—beach stabilisation assessment: technical note

The aims of this study were to conduct an investigation into the placement of right‐angled low profile sandbag groynes on the Western Beach, which will inform the impact on the stabilisation of sand at the eastern end of western Beach to: (1) Confirm Model Capacity to Represent Processes; (2) Key Event Simulation; (3) Oceanographic Drivers; and, (4) Qualitative Wrack and Sediment Transport
The key findings are summarised below:
(1) Confirm Model Capacity to Represent Processes
The key processes included the westward transport of wrack along the western breakwater during northerly wind event. Model simulations of selected events in 2015, 2016 and 2021 indicated that the westward wrack transport was simulated by the model and confirmed through local imagery. It was possible to define the wind directions that contributed to the westward wrack transport (Figure 3.10).
(2) Key Event Simulation
The key processes included the westward transport of wrack along the western breakwater (see 1 above). Another process that was to be investigated was the ability of the model to include wave damping by high density of floating wrack. This was a challenge and although several approaches were investigated – the lack of data on the actual attenuation preluded implementation in the wrack transport model.
(3) Oceanographic Drivers
Field measurements from offshore Port Geographe during 2009‐2021 was undertaken and the finding are summarised below:
• Based on water levels, the largest storm surges occurred during May‐June. Relatively
larger events were also recorded in July, whilst several moderate surges occurred from April and through to the end of September. The annual maximum (peak) water level occurred during April–June, with most occurrences in June. However, maximum/minimum water levels varied greatly from year to year in response atmospheric and oceanographic conditions. Relatively low mean water levels were recorded from 2016 to 2020.
• The swell wave heights are relatively larger during winter, exceeding 2m during winter storms. The swell wave direction offshore Port Geographe Busselton was mainly from the north‐west, but changes occurred during storm events.
• The wave climate and storm events suggested that 2010 and 2015 were calm years. Wind‐generated waves from the north were large from 2017 to 2020. Maximum number (38) of strong wind events (speed > 16 ms‐1) was recorded in 2020, in which 14 of them arrived between NNW and NEE. All strong events during late winter and spring arrived from NNW and NEE.
It is recommended that although the model was able to reproduce the observed westerly transport of sand and wrack along the western breakwater, the complexity of the processes and the resolution of the model would not provide any confidence in the performance of right angle groynes thus no additional modelling is required.