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Portsea Beach Erosion

Erosion at Portsea Beach



There has been community interest in whether erosion in the Bay was either caused by or exacerbated by the effects of the Channel Deepening Project, particularly in relation to Portsea front beach.

The potential for effects from the project, including beach erosion from Entrance dredging, was assessed through a public environmental assessment prior to project approval. Approval was conditional on extensive monitoring before, during and after the works were completed.

The Office has considered all available monitoring data and historic sources of information. Analyses of data on changes to Bay tides, currents and waves in the Entrance to Port Phillip Bay shows that changes are consistent with the limited effects that were predicted. These limited effects were not considered sufficient to cause the beach erosion seen in the Bay.

The Office has scrutinised results from further work by PoMC on the stability of the Great Sands and adjacent beaches, including Portsea, following dredging (Project Delivery Standard 23 of the EMP). Project approval conditions required this work. Two peer reviewed reports have also been considered. They are a refinement of sediment transport modelling and a synthesis of information on tides, waves, currents, changes to bathymetry, sediment transport and the implications of these changes for the stability of the Great Sands and adjacent coast and beaches in the south of the Bay. This work finds that:

  • There are no identifiable changes in the stability of the Great Sands as a region which can be attributed to channel deepening activities.
  • The pattern and rate of change on beaches in the southern part of Port Phillip Bay during the past 2 years (during and post project works) is consistent with that documented over the past 150 years.
  • In relation to Portsea, swell waves from Bass Strait that reach Portsea do not pass over the portion of the Entrance which was dredged and therefore it is implausible that the erosion of Portsea beach has resulted from increased wave energy due to the project.
Analysis of wave and sea level data over the last decade indicates that 2009 was marked by a coincidence of unusually high sea-levels (storm surge) and unusually large waves in Bass Strait. Heavy swell reaching Portsea, on top of the high sea-level, was probably sufficient to start the erosion seen in 2009. This erosion continued into 2010. Once a beach has been eroded, the process of recovery, if it occurs, may take years to decades.

In Victoria, erosion is often conspicuous and severe following major storms and it may take some time following a storm to see the full effect. Erosive forces can be amplified by a combination of strong winds, the effects of water being pushed into the Bay by storms in the Southern Ocean and occasional higher than normal tidal levels when air pressure is low.

Human activities also affect beaches. Adding sand to beaches through renourishment programs replaces the sand lost by erosion. We have also learnt from experience that some actions can accelerate erosion by disrupting the supply of sand or exacerbating the erosive effects of storms.

Relevant reports



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