Storm surges occur during storm events where meteorological forcing of wind, atmospheric pressure and wave set-up leads to an increase in sea-level over a number of days. Subtracting the predicted water-level variation due to the astronomical tide from the observed water-level record determines the tidal residual. This provides an estimate of the magnitude of the meteorological component in the observed water-level variations. Positive tidal-residuals are commonly referred to as storm surge.

The meteorological forcing can cause very long period waves to move across the ocean and, in particular, along the edge of continents where they interact with the continental shelf to form “shelf waves”. There are a number of types and propagation modes for such waves and all involve a variation in sea level at the coast. Since all these mechanisms combine to make up the water-level variations other than the astronomical tides, they have been grouped under the term storm surge in this report since their combined effect will be greatest during a storm event. CSIRO determined surge heights under various exceedance probabilities (McInnes et. al., 2009a,b). This was done for a number of locations along the Bass Strait coast (where tide gauges exist) as well as twelve locations in Port Phillip Bay, from Point Lonsdale to Sorrento. Relevant locations for this study include Lorne on the open coast and Point Lonsdale, Queenscliff and Geelong within the bay.

Figure 3-4 shows the 1 in 100 year (i.e. 1% AEP) surge heights for Port Phillip Bay and the Bass Strait coast. These levels were determined through hydrodynamic modelling of surge events, forced with wind and atmospheric pressure. The data set used to determine these values was for 38 years from 1966 to 2003, therefore an extremal analysis was undertaken to obtain values for the less frequent surge events.

A combination of large positive tidal-residuals with high astronomical-tides results in extreme water-levels. This is termed a ‘storm-tide’. Values of extreme storm-tide levels for the study area are discussed in Section 4.3.1.

Figure 3-4     Spatial pattern of the 1 in 100 (1% AEP) year storm surge height for Port Phillip Bay and Bass Strait (McInnes et.al. , 2009a,b)