Port Phillip Baykeeper » Advocacy

‘average tide’ hides local reality

BayKeeper — Wed, 07 Jul 2010 12:39:09 +0000

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.

Portsea - a swell place for a beach!

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.

Portsea paradise lost

BayKeeper — Sat, 29 May 2010 13:58:47 +0000

The disappearance of thousands of tonnes of Portsea Beach sand since the dredging at Port Phillip Heads has left locals looking for answers. Was it the wind, higher tides, or the swell that took the beach? The mystery has deepened with the recent dumping of hundreds of tonnes of rock near both sides of the pier, especially as there’s no project information on-site to explain what’s happening and who’s responsible.

Portsea Beach - gone for good?

Government bodies are generally proud to declare their role in high profile public works. Apparently not on this occasion! But they’ll have to overcome their shyness sometime soon. The works so far are but a band aid compared to what’s required to stop continued erosion.

Singing for the Saltmarsh

BayKeeper — Sun, 16 May 2010 00:24:17 +0000

Saltmarshes on the western shores of Port Phillip Bay provide important habitat for endangered Orange-bellied Parrots and a host of other fascinating plants and animals. These Singing Honeyeaters were recently sighted in saltmarsh shrubland at Jawbone Marine Sanctuary, Williamstown.

Singing Honeyeaters on Shrubby Glasswort - pic by Jim Walker

A southern population of Singing Honeyeaters (Lichenostomus virescens) is recorded from Mornington Peninsula to the South Australian border. They eat seeds and berries, nectar and pollen, and a whole range of animal life (insects, damselflies, praying mantis, grasshopper, bugs, lerps, beetles, larvae, ants and spiders). Pretty versatile!

Incoming tides can bring sediments into a saltmarsh area and outgoing tides can remove them. Over time, this causes small changes in the elevation and slope of the soil, creating different zones of soil salinity and waterlogging. As different saltmarsh plants are adapted to different zones, a longterm increase in sea level could favour some species and eliminate others.

This wouldn’t be a problem if the saltmarsh was able to migrate inland as sea levels rise. But the landward edges of many saltmarshes have already been filled in for residential development or agriculture. There’s no time like the present to start planning for substantial areas of saltmarsh to be protected into the future.