Submission to the Channel Deepening Project for Port Phillip Bay

Author: Mary Rimington

Victoria’s coasts are precious and locations like Port Phillip Bay are an intensively used environment. Activity and processes on the coast, inland in the catchment, and off shore in the ocean, have an influence on the health and sustainability of the coastal environment.

In view of the well documented evidence of sea level rise, the availability of alternatives, and the adverse environmental impact, we are opposed to the deepening of the Rip and dredging of the South Channel to allow larger container vessels entry to Port Phillip Bay.

Our objection to the deepening of the Rip is based on the fact that there is now sound evidence of sea level rise as a result of climate change, and that even a 1-2 cm rise in tide levels will intensify the beach erosion and flooding already occurring on the eastern, most vulnerable side of Port Phillip Bay.

GREENHOUSE EFFECT AND SEA LEVEL RISE

Erosion based on sea level changes

Areas around Port Phillip Bay likely to be submerged or eroded if the sea level rises by 1 metre (Source: E. Bird, 1989)

A recent CSIRO report presented at the Coastal Conference states that the scientific world has accepted “the evidence of increasing atmospheric temperatures and a rising global sea level”. Analysis of global tide gauges have recorded a sea level rise of 10 to 15 cms over 50 years (Dr Graeme Pearman Chief CSIRO Atmospheric Research, April 2000).

“Although tides within Port Phillip Bay are of a smaller range than in Bass Strait, because of the narrow entrance, a sea level rise will result in an increase in the tide range within the Bay” (Effects of Sea Level Rise in Port Phillip Bay, 1989, Dr Eric Bird p.33). Furthermore according to Bird, “The recurrent blasting of pinnacles of rock to enlarge the shipping channels at the entrance to Port Phillip Bay will have the effect of assisting the penetration of Port Phillip Bay by a global sea level rise” (Bird, p.33).

Any rise in sea level within the Bay will cause water levels to rise alongside the banks and levees of Mordialloc Creek, Patterson River and Kananook Creek, and by so doing the shores of the Bay “will be subject to an increasing depth of marine flooding at high spring tides and during storm surges of the kind often experienced in Port Phillip Bay during the passage of an intense depression from the Great Australian Bight” (Bird, p.35).

Bradley recorded that storm surges have raised tides in the Port of Melbourne by up to 1.94 metres (Admiralty Datum) and 1.06 metres above the maximum astronomical tide levels at Williamstown (Bradley J., Abnormally high tides in the Port of Melbourne).

Severe storm surges and high tide levels caused catastrophic floods in the Mordialloc, Chelsea, Carrum region in 1934 and 1952 due to heavy rain in the Dandenong Catchment, a high tide and a strong south westerly gale. Similar conditions in 1974 wrecked foreshore infrastructure at Chelsea and Bonbeach.

1994 saw a repeat of this weather pattern and the conditions were subsequently the basis of a study by the EPA and CSIRO of the impact of storm surges on Werribee, Hobsons Bay and Mordialloc. The conclusion was that given the same storm intensity
of 1934, Mordialloc Creek and environs would suffer flooding of the same magnitude or in fact, more severe (Extreme events and the impact of climate change on Victoria’s coastline, Publication 488, June 1996).

Bird’s conclusion is: “there is no doubt that the predicted sea-level rise will greatly extend and intensify beach erosion around Port Phillip Bay and… between Mordialloc and Seaford the beach, and dune fringed coastlines will recede and threaten foreshore developments”.

ENVIRONMENTAL IMPACTS

Southern Port Phillip Bay and the Port Phillip Heads area contain some of Victoria’s most treasured marine and coastal environments within easy reach of Melbourne and Geelong. Dredging the South Channel would put the unique sponge gardens and kelp forest colonies in the area in danger because of a change in tidal flows and temperature. Significant sites such as Pope’s Eye, Portsea Hole and the Lonsdale Wall fall within declared port waters. Of these Pope’s Eye is set aside as part of a marine reserve.

There is also danger in the disposal of sand and contaminated silt (from dredging the Yarra River) to spoil sites in the Bay. Sand and silt will be disturbed during storms and block the much needed sunlight to undersea plants, an important part of the marine food chain.

ALTERNATIVES

An economic study assessed the possibility in which Hastings in Westernport could be developed as an alternative to Melbourne. Port of Hastings is a natural deep water port with two deep water berths available. A rail line currently transports steel from Hastings throughout Australia. While the Hastings option is claimed to be too costly it should not be dismissed given that:

  • berths are available;
  • infrastructure (the rail link) exists; and
  • it would provide employment to one of the State’s depressed regions.

The preferred option however, is one already mooted by shipping companies. That is, the new generation of large ships be routed to the deep water ports of Fremantle, Darwin and Sydney. Containers could then be transported by standard gauge rail to other cities. The West Australian link is operating, and the Darwin-Adelaide and Brisbane-Mt Isa-Darwin links are under construction.

CONCLUSION

Port Phillip Bay is Victoria’s most valuable asset but is under threat from a number of sources. Deepening The Rip will not only hasten the changes already occurring to the Bay’s ecology, but irreparably damage the beaches and increase the risk of flooding in the low lying, flood prone areas on the eastern side of the Bay. While deepening the shipping channel may be technically feasible, the proposal is not environmentally sustainable or responsible.

Mary Rimington
Secretary, MBCL

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