Tidal Stream Predictions for Port Phillip Heads
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Tidal Streams

Port Phillip Heads

Due to the restriction of the tidal range within Port Phillip caused by the relatively narrow entrance, the tidal stream in the vicinity of the Heads does not turn at high and low water.

The force of the tidal streams depends upon the relative water levels inside and outside Port Phillip. The greatest difference in levels occurs at about the time of high and low water at Port Phillip Heads. This is when the inward and outward streams run at their strongest, up to six knots under normal conditions and may exceed nine knots in extreme conditions.

Slack water at Port Phillip Heads occurs at about three hours before and three hours after high water, which is when the levels inside and outside are the same. The ingoing stream runs from about three hours before to about three hours after high water and the outgoing stream at other times. On the average, it is high water at the Port Phillip Heads three and a quarter hours before that at Williamstown and slack water at the Heads when it is high or low water at Williamstown.

Tide Stream Signals

Tide stream signals are shown by night at the Point Lonsdale Lighthouse below the main light. Flood stream is denoted by one fixed green light for the first half of the flood and two fixed green lights vertically aligned for the second half of the flood. For the ebb stream, the first half of the ebb is denoted by one fixed red and the second half by two red lights vertically aligned.

Note: Tidal height information can be obtained by contacting Point Lonsdale Signal Station.

Explanation of Tidal Streams

As the tidal depths change the ocean water flows on to or away from the coast. This water movement is called a tidal stream.

Flood Tide
As the tide changes from low to high tide the increasing depth of water cause a tidal stream towards the coast. This is called a flood tide.

Ebb Tide
As the water depth drops due to an approaching low tide the water flows away from the coast. This is called an ebb tide.

Tidal Rips
The speed of the tidal stream can become quite fast as the water enters a bay or passes over a shallow or rough seafloor. These are sometimes called tidal rips. Tidal rips can occur at headlands, over ocean bars or in the entrance to bars and harbours. A notorious tidal rip occurs at the Port Phillip Bay heads where the tidal rip can flow at 5 - 8 knots (9 - 15 kph).

Slack Water
At the turn of the tides (at high tide and low tide) the tidal stream stops. This is called slack water.

Where the tidal flow passes through a constriction slack water may not occur at the high and low tides.
At Port Phillip Heads slack water occurs about two and half to three hours after high or low tide at the heads.

The animation below demonstrates the flow of water though the Port Phillip heads in relation to High and Low water inside the bay and outside the heads in Bass Strait. Extreme care must be taken when passing through the heads as the tidal stream exiting the bay can cause very rough water by interacting with waves entering the bay. The best time to pass through the heads is at slack water.

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