In our previous post, we looked at the critical risk strong winds would pose for cruise ships berthed in Yarra Bay. In this post, we look at big seas – storms and ocean swells – and explain that while they can theoretically be controlled in Yarra Bay, the consequences are unacceptable – for Yarra Bay, for the Port Botany shipping channel and for Kurnell.
Let’s get started!
Yarra Bay is fully exposed to swells from the south east (red arrow). Southerly swells are also significant, as they curl into the bay (orange arrow):
To protect Port Botany’s Brotherson Dock from these swells, the Molineaux Point break wall was built. It arrests the swells and dissipates their energy as they run south-west to north-east along its face (green arrow in diagram above).
However large swells associated with storm events are only partially dissipated. When they hit the Molineaux Point break wall (red arrows), some of their energy is redirected across Yarra Bay (orange arrows):
This is why Yarra Bay has surf, on days when big seas cause Sydney’s ocean beaches to be closed:
And why the groyne in front of Yarra Bay Sailing Club can be over-topped by large breaking waves:
Information from port community sources indicates that the Berejiklian Government’s “Strategic Business Case” proposes a finger wharf to accommodate cruise ships extending into Yarra Bay from Bumbora Point.
Large swells, hitting the Molineaux Point break wall and rebounding across Yarra Bay would present a major risk for this finger wharf (dark red line), and for ships berthed at it (blue lines):
The ships would surge at their berths, and to prevent damage to them and to the wharf, they would need to leave their berths. Once they had departed, the wharf would be fully exposed to the swells and could suffer significant damage.
To protect the finger wharf, an island breakwater that would prevent swells entering Yarra bay would need to be constructed (green line), almost at right angles to the direction of the swells (red arrows):
The island breakwater could not be at right angles to the swells as it would protrude into the Port Botany shipping channel. But it would need to extend as close to the Molineaux Point break wall as possible, in order to prevent the swells reaching the break wall and being redirected towards the Yarra Bay finger wharf.
Locating a breakwater here has major consequences for Yarra Bay
Firstly, it would restrict access to the finger wharf to a narrow channel (orange arrow, below). Dredging of the (shallow) water close to the breakwall would be required and navigation of mega cruise ships through this tight access passage with strong tidal flows would be challenging. Keep in mind that cruise ships have inflexible schedules – to meet their passengers’ expectations, they must berth at dawn and sail at dusk.
Secondly, the breakwater would drastically restrict the tidal flow of water in and out of Yarra Bay, creating a narrow channel between the breakwater and the Yarra beach shore (green arrow, above). This would result in dangerous currents and deep channels close to the beach, scouring of the seabed and sand depletion from the beach. Yarra Bay beach would become dangerous, particularly on the ebb tide. The sailing club would no longer be able to operate and the Maroubra and Coogee surf clubs’ surfboat and IRB training would be compromised.
The consequences for the Port Botany shipping channel are equally severe
Sand dragged from Yarra Bay by tidal currents past both ends of the island breakwater (red arrows) would end up in the shipping channel (gray zone of chart) and would require frequent dredging to keep it clear:
Just as swells are redirected from the Molineaux Point break wall, they would also rebound from the island breakwater:
This would result in confused cross seas in the shipping channel and would cause passing ships to list side to side and plunge bow to stern. This is highly undesirable for large, deep draughted vessels with minimal Under Keel Clearance (UKC) and could cause hull damage and even groundings.
The confused seas would also greatly increase the risk of capsize faced by recreational fishing vessels.
And there are consequences for Kurnell …
After traversing the shipping channel, the redirected swells would strike the Kurnell shoreline:
Damage to the beaches could be expected in the vicinity of the Captain Cook monument. Further to the west, oil tankers at the Kurnell oil products terminal jetty and the the swinging buoy north of the jetty would also be impacted by the rebounding swells.
Yarra Bay is highly exposed to south and south easterly ocean swells that can occur at any time of the year. To protect mega cruise ships berthed at a large wharf in Yarra Bay, a massive breakwater would need to be constructed across the wide mouth of the bay. The cost of this structure and its impact on Yarra Bay, the Port Botany shipping channel and the Kurnell shoreline are simply too high. The ill conceived proposal to construct a mega cruise ship terminal in Yarra Bay must be abandoned now.
Note on options
The mega cruise ship wharf location design we have illustrated in the diagrams above is based on information we are receiving from the port community and our own calculations regarding depths in various parts of Yarra Bay and the orientation need to protect berthed ships from the extremely strong winds experienced at this exposed site.
Two other options are provided below. Both would require the protection of a massive island breakwater, identical to the one illustrated above.