Channel Deepening Project: Submission

 

The Dangers involved in Channel Deepening where Global Warming induced Rising Sea Levels are an established fact.

 

Port Phillip Heads provides an important restriction on tidal movement between the Southern Ocean and Port Phillip Bay. The result is a ‘modulation’ of tidal rise and fall, so that the upper and lower limits outside the heads are different to the limits inside the bay, which gives us a lower maximum and higher minimum bay sea level. This phenomenon is due to the fact that the water volume of ‘the bay’ is such that there is not enough time during the period of ebb and/or influx of the tide for enough water to pass through the restrictive channel of ‘the heads’ to allow equal sea levels to become established on both sides. Conversely, Westernport Bay is smaller with large openings to the Southern Ocean, and has larger tidal variations.

 

Any deepening of the channel through the heads must necessarily reduce the restriction and thus increase the tidal flow, and this inevitable outcome should be measured in light of our new-found acceptance of the fact that sea levels are remorselessly rising due to global warming. Rising sea levels are undeniably a future threat to coastal cities such as Melbourne.

 

The EES for the Channel Deepening Project makes the following claims for tidal variations following the projected modifications to the heads:

 

Change in average low tide       - 15 mm

Change in average high tide       + 8 mm

Change in tidal currents (presumably velocity) <2%

 

It is stated in the EES data, that 550,000 cubic metres of rock will be removed in the process of deepening the reefs of Pt Nepean Plateau (9.5 Ha) from 14m to 17.3m, plus the Rip Bank reef (‘patches’ area not specified) taken to the same depth. From the EES plans, the latter works appear to represent a further area of approx 10-15 Ha.

 

In the absence of specific geometric detailed information, I have made some fluid flow calculations based on a conservative estimate of the cross-sectional area of rock being removed from an area of near maximum tidal current flow rate, averaged over a tidal cycle. I arrived at a figure of 0.0065% volume change in the bay as a result of the changes in geometry at the heads. This equates to a potential +6 cm change in flood tide peak, which is in contrast to the EES claim of +8 mm. Although my figures are based on a mixture of values taken from the EES and Appendices, and on some necessary careful estimates, I believe there is serious reason for caution in this issue, and the EES figures should be independently reviewed.

 

 I believe a rise in general sea level will definitely change the water levels inside the heads as well as outside. The mysteriously rapid and threatening rise experienced by some low-lying South Pacific atolls such as Mortlock and Cartaret Islands gives us warning that the consequences of actual sea level rise can exceed the expert’s predictions.

 

With comparatively small changes of about a centimetre /decade this problem would not greatly threaten us in the short term, but long term uncertainties are a different issue, particularly if we are augmenting the problem by deepening the channel. However, if predictions of huge glacial discharges from Greenland and the West-Antarctic continent, resulting in changes measured in metres, were to happen in the foreseeable future, Melburnians would be wishing the heads had been left alone by the Bracks Government (and desperate efforts may be undertaken to re-instate tidal levies).

 

Further to the above considerations, the destruction of reef ecosystems at the heads over an area exceeding 20 Ha, plus the rock fall and rubble consequences in the chasm, are clearly undesirable and difficult to justify for a perceived short-term economic benefit. Surely it is logical to provide for the largest ships at Westernport, if their service is considered essential, leaving the majority of current sized vessels with good access to Port Phillip Bay. Future shipping transport developments will see larger capacity container ships with shallower draft than current vessels entering service, which will obviate the need for radical changes to the bay channels.

 

According to Dr Phillip Bock, Research Fellow of  Deakin University and world authority on Bryozoan species, the Port Phillip Heads, chasm and surroundings is an internationally famous biodiversity ‘hot-spot’ for these and other marine organisms, and deliberate damage to this area would be a tragedy and embarrassment for Australia and the world’s marine science community.

 

I trust the above matters will be seriously considered before a final decision is reached on changes to the bay that can still be avoided, but will have far reaching consequences if enacted.

 

Bernard Mace

Scientist, Marine Engineer

Hawthorn

7 May 2007

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