Port Mandurah sedimentation study: Modelling investigation

The Port Mandurah Canal Estate (Port Mandurah) enables its residents to enjoy water frontage with private jetty and boat mooring facilities. The northern entrance to Port Mandurah is situated on the entrance channel to the larger Peel Harvey Estuary system. Sedimentation of the Peel Harvey Estuary entrance has been an on-going issue for decades, predating the construction of the estate, and this has had flow on effects for the management of the Port Mandurah canals. Due to the cost and required frequency of maintenance dredging, the City of Mandurah (the City) is exploring options for improved management; this study assessed the performance of three possible mitigation measures for this ongoing issue. Sediment transport in the proximity of the Port Mandurah entrance is predominantly driven by tidal currents flowing into and out of the Peel Harvey Estuary carrying sediment bypassing the entrance. This process is possibly supplemented by sediment from offshore areas transported onshore during storms. These processes are responsible for the formation of a large sandbank ('Fairbridge Bank') within the Peel Harvey Estuary channel, and the northern entrance to Port Mandurah is situated adjacent to this sand bank. For this study, a hydrodynamic and sediment transport model for the Peel Harvey Estuary entrance channel and Port Mandurah canals was developed using Delft3D. Measured time series of currents and water levels were used to force the model; the model was successfully calibrated and was verified against the changes in bed elevation recorded by hydrographic surveys. The use of measured data allowed the model to be forced by a range of larger scale hydrodynamic processes that are costly and difficult to resolve with a larger nested grid approach. In consultation with the City, three siltation mitigation options were identified for testing against the control scenario: a training wall, a sediment trap with maintenance dredging, and channel realignment. Overall the sediment trap option performed best of the three tested: after 5 years the channel maintained a navigable depth, and average sedimentation was halved compared to the present situation. The outcomes of the study assisted the City in identifying options for further detailed analysis and design.