‘average tide’ hides local reality

The fact that the tidal range at ‘the Entrance’ is around twice that at Port Melbourne highlights the fact that there are very different tidal zones in the Bay. Incoming tides are impeded by the Bay’s narrow Entrance, and then by ‘the Sands’, the vast triangular area of shallow sand flats which are a barrier to waters moving to or from the north of the Bay. Wind strength and direction, and barometric pressure also affect sea level in localised areas.

Many people raised concerns that deepening the shipping channel at the Entrance (completed in September 2008) would open up the Bay to greater swell wave force, increasing tide levels, scouring ‘the Sands’, and eroding beaches. Swell waves generated by storms over Bass Strait can be quite destructive on the seabed and coast. In turn, changes to the seabed affect tidal current speeds and direction.

Baykeeper and friends have found that average daily highest tides at 4 stations in the south of the Bay are all different, between 48 and 100mm higher than before dredging the Entrance. According to our humble methods, the increase at Williamstown has only been 2mm. Much longer periods (at least 5 years) of monitoring tidal current speed, direction, and levels are needed before channel deepening could possibly be ruled out as a cause of beach erosion. Meanwhile, talk of a “10mm increase” in tides across the whole bay does nothing to explain local realities. Our findings are available as Pdf downloads on this website.