The satellite view below shows Port Phillip Bay with the three important tidal zones marked in. These assist with the discussion and understanding of the tidal flows through the entrance.
The "outside" tide zone is of course Bass Strait, where the maximum tidal range is up to 2.4 m. The "inside" tide zone comprises the "main body" of the Bay and includes its western arm. This has a very large surface area of around 1,700 km2. The maximum tidal range over this vast area is around 1 m.
Between the two lies the "choke zone" highlighted in yellow, with its tidal range varying from around 1 m near its northeastern edge up to 1.8 m near the entrance. It contains most of the areas that restrict tidal flows between the "outside" and "inside" zones and is directly responsible for the reduced tidal range in the large "main body" zone. The flow restricting regions of the "choke zone" include:
a) "Rip Bank" a medium depth rock bank just outside the entrance
b) The narrow entrance itself, known locally as "The Heads" or sometimes "The Rip"
c) "The Great Sands" which is a collection of large shallow areas showing up in the lighter blue colours
Tidal flows in and out of the Bay react to the presence of the narrow or shallow areas of the "choke zone" by speeding up the flow in an attempt to try to maintain the same volume flow rate. The yellow zone is therefore characterised by fast tidal currents (better known as tidal streams) that can create some level of danger for boating and various other activities in the yellow zone including; swimming, snorkelling, scuba diving, SUPs and using "short" (slow) kayaks.
Tidal flows are driven back and forth across the "choke zone" by the alternating level differences that occur between the "outside" and "inside" zones. At times this difference can reach more than one metre, producing tidal streams through the Heads that reach up to 6 knots. Further inside the yellow zone the maximum speeds gradually fall, reaching around 1 - 1.5 knots along the northeastern boundary. This reduction in speed is not the tidal stream "running out of puff", but is mostly due to the underwater cross-section of the "choke zone" expanding by about four times as we move inwards from the Heads. In actual volume terms, the water flow across the border with the "Main body" zone runs, for most of the time, at 90% or more of the flow volume through the entrance.
Beyond the "choke zone" to the northeast the waters quickly deepen, increasing the Bay's underwater cross-section by a further four or five times. Consequently the maximum tidal current here is only 0.2 to 0.3 knots so that even slow speed activities like scuba diving face little danger from the current. However things are not quite so benign on the western side of the Bay where the "tidal breathing" of the long and narrow western arm raises current levels somewhat above those in the central basin area.
Only in the strongest flood tides will water entering the Heads at the beginning of the flow make it across the "choke zone" before the flow reverses. All of the "old water" originally in the "choke zone" then gets pushed into the "main body" zone. However because of the much larger underwater cross-section there, all of that water can be accommodated in a thin strip less than 2 km wide along the border of the main body zone. There are no long distance tidal flows in the main body. Instead that thin strip of accumulating water behaves like a horizontally acting piston to push the original main body water into a slightly smaller area, and so raise its level. This happens all across the main body zone with a similar sized rise and almost in unison.
The numbered points on the above map mark the Bay's Tide Gauge locations. These gauges are used to take regular measurements of the height of the tide in the various parts of Port Phillip. Some of these measurement are available to the public as tide curve plots on the web. Often these locations also support some weather instruments giving wind strengths & direction, air temperature & pressure, etc.
Care must be exercised in interpreting the tide heights reported across this network because each gauge has its own different "Gauge Zero" height. The exception is that the Queenscliff and Hovell Pile tide gauges have very similar zero datums and so their readings can be directly compared to give the height difference across a large part of the "choke zone"
Another significant effect of the "choke zone" is to cause a delay of the "inside" Hi/Lo tide times behind those of the "outside" waters. In the case of a flooding (ingoing) tidal stream, when the "outside" height starts dropping after high tide there, it is still well above the "inside" level. Therefore the inflow continues, meaning the "inside" tide will still be rising for a further time period.
Even when the falling "outside" level drops below the rising "inside" level, the flow does not stop due to the significant amount of forward momentum still in the high mass tidal stream. This extra "up-hill flow" may last between 30 and 80 minutes.
On average High Tide in the "main body" zone occurs around 3 hours later than High Tide at the entrance. In the case of main body Low Tides, the delay is a little longer at around 3.5 hours later than Low Tide at the entrance. It is these long delays, plus boaters sometimes being unclear about the distinction between a tide and a tidal current, that can cause some dangerous confusion near the Heads.
For the record, the "tide" reading at a location is a measure of the amount of water accumulated around that location (ie. its vertical height), whereas the "tidal current" is a measure of the amount of water flowing past that location (ie. its horizontal speed). The timing relationship between a location's tide height and its tidal current can differ widely depending on the geography of coast around that location.
Although "the knot", which is one nautical mile per hour (or about 1.85kph), is the standard unit of speed in the nautical arena, it is not easy to estimate the speed of medium to slow tidal streams in knots. This is because both the distance and time units are really too big for the purpose. Better to reframe "the knot" speed unit into either of these more useful near equivalents:-
1 knot = 30 metres per minute (or 100 feet per minute in the old currency)
So a 5 knot current near the entrance means the water is moving along at 150 metres per minute (or 500 ft/min). For low powered vessels operating in "displacement hull mode", these current speeds are close enough to their maximum hull speed to make navigation around the Heads very tricky when the current is running hard. Even in the slower 2-3 knot currents further inside the Heads, you need to be fairly skilful in "vectoring" across a current to travel in a straight line to your destination.
Even planning runabouts with much higher maximum speeds may need to exercise care when they are forced to slow down to near displacement speeds due to poor sea conditions. These are usually caused by the current interacting with wind, waves and/or swell, producing the nasty short breaking seas that "The Rip" is famous for. Even a current of only two knots running into a strong breeze can force many fast boats off the plane for safety and passenger comfort.
For these reasons small and even mid sized vessels will usually try to time their passage through the Heads to coincide with "Slack Water" where the current stops running in one direction and is about to reverses to begin its run in the opposite direction. It is therefore important for safe boating in all of the "choke zone" to have a good understanding of how and when "Slack Water" comes about. Unfortunately there are three untrue but long standing "urban myths" about this topic that for various reasons still attract followers.
If you are going boating in this risky area, then for your own sake and that of your passengers and/or crew, you should try to understand the real behaviour of this region to reduce that level of risk as much as possible.
Presented below are three myths still in circulation about port Phillip's tidal streams that reduce the safety of those folks who believe them. It is a little difficult to order these myths into degrees of "badness", because that judgement involves thinking about this sort of calculation:-
Overall Badness = (the risk to an individual of the myth) x (the percentage of individuals who believe in the myth)Both of the bracketed terms involve quite some guesswork. I have chosen to weigh the "risk to individuals" more significantly than the "percentage of believers". My "Bad Boy" ordering from least harmful to most harmful is then:-
Unfortunately his myth is very widely believed. Part of this longevity is due to its long term promotion by the port authorities, which is then parroted by other government agencies and non-government organisations. Despite overwhelming evidence against it, officialdom seems to favour its "simplicity" and ease of understanding over a more truthful account that acknowledges the significant momentum effects inherent in Port Phillip's high mass tidal streams.
The risk to the individual stems from the untrue words of the myth which leads many folk to assume the wrong current vs time profile around slack water time. This may then lead to less safe situations on the water. An improved, more truthful understanding of the actual behaviour, also helps in unlocking other mysteries of this region.
While this myth is not likely to lead to very serious individual risk situations, the high percentage of believers (quiz showed around 80% - 90%), elevates the need for the myth to be quashed. Anecdotal evidence shows the risks of this myth can be quite real. This could lead to some bad decisions by small vessel skippers, which in turn can then be the start of a dangerous downward spiral.
The link at the bottom of this post leads to the Index Page of this website. There the "POST #1" link leads to an article that lays out a very comprehensive case why this myth isn't true, and what the real behaviour is. If you do believe the myth is in fact true, then you owe it to your family, passengers and crew to take the time to look at that document. Hopefully it will change your mind.
Unfortunately there are still a number of recreational groups who continue to insist that slack water suffers a progressive delay of up to 3 hours while crossing the southern part of the Bay "just like the High and Low Tide times do". This is both incorrect and dangerous. Tides and tidal streams are not the same thing !
Roughly speaking the High or Low tide moment ambles across the "choke zone" region at about 6 kph, whereas the true slack water moment zooms across it at more like 100 kph. Consequently any delays in slack water reaching "choke zone" areas further from the Heads are only in the range 5 to 15 minutes.
Myth 2) has serious risk to individuals, particularly for "scuba diver believers" operating well inside the Heads. It needs to be killed off completely before a life is lost. It has already endangered a number of lives.
Many years ago I rescued a drowning man who was right at death's door. He was in that pickle because during his paddle board outing he thought the current at that location would not reverse for several more hours (as per the myth). Instead the tidal stream reversed shortly after it did so at the Heads and he was knocked off his board by the rough tide-against-wind conditions that soon developed.
The wind whisked his board away, while the current dragged him along in the opposite direction. He was in the water for a long time and nearly spent ( no leg rope, no life vest, wearing only bathers, and in fading daylight. ) The rescue was a highly emotional experience (for both of us) as success looked uncertain in the very poor conditions. Fortunately I was able to haul him over the gunnel and then get an ambulance to attend the closest shoreline. After ten days treatment in hospital he was released to face "the Mrs". No more paddle boarding for him !
On the positive side, my judgement is that the "number of believers" of this myth has declined significantly over the last few decades and is now probably lower than say 10%. However hope for this decline to continue was put in doubt when long standing statements by the Port Authority and Sea Pilot Service that "only small delays of slack water occur" both disappeared in 2016.
This was due to some very silly "streamlining" (aka "jazzing up") of the relevant publications and websites. Those very important safety statements have never reappeared. My attempt to get the BoM to come to the party and insert similar wording into their slack water predictions for the Heads was unsuccessful.
If you still believe in significantly delayed slacks for locations well inside the Heads, think hard about this question:- Why do the tide gauges in the main body all start to fall within 15 minutes of "slack after flood" occurring at the Heads? The concept of water continuing to flow northward long after slack at the Heads is not compatible with this observed fact.
Also having "choke zone water" exiting both south through the Heads, and north into the main body at the same time, and for several hours, is problematic. It would create a severe fall in the choke zone level. If we put some reasonable numbers on these flows, the resultant fall of the choke zone level over these hours would be around four metres. This of course is never observed.
In the distant past this myth probably took many lives. Fortunately the "believer percent" is probably now down to single digit level. However there is still significant risk in the case of strangers visiting Port Phillip who are not across our tides and currents regime. This very bad myth promotes the absolute worst time to enter the Heads, with the outflowing ebb tide still running near full strength and giving dangerous short breaking seas even in periods of low swell.
Some years back I recall a sailing forum poster from NSW saying he couldn't see why the "Rip" at PPH was such a big deal. He then went on to quote this myth as his intended plan of action. Of course he was pounced on by many others, but those not familiar with the flow lags of a very large water body sitting behind a narrowish entrance might still accept this idea as true, and so put themselves in serious danger.
The real situation is that because "inside" (mid-Bay) tides lag behind "outside" tides by around 3 hours, a Low Tide "outside" may sit around -1m wrt MSL, while the falling "inside" tide still hasn't yet dropped below its mid-tide level of say 0m wrt MSL. This level difference is sufficient to drive an outward flow of around 5 knots through the Heads and is definitely not a good time to attempt entry into Port Phillip.
It takes a further 3 hours or so of the now rising "outside" tide to firsty wipe out, and then slightly reverse that level difference before the massive outflow stops. The flood stream can then begin, gently at first which is a good time to pass through "The Rip" while it is quiet. Then follows 5 to 7 hours of "flood assist" towards the northeast. This allows slow Melbourne bound vessels to get into the relatively current free "main body" zone before the tidal stream reverses again.
A basic flaw with all three myths is a failure to distinguish between tides and tidal streams as being very different things and who's timing relationship with one another can vary remarkably from place to place. This is further complicated by many folk using just the "tide" word for both things, expecting just the context to give a clue as to exactly which is being referred to.
At Port Phillip Heads the tide (ie. height) rises and falls in fairly close sync with the "outside" level. However the tidal stream (ie. current) floods and ebbs in close sync with the rate of rise or fall of the "inside" tides over the vast central, northern, and western parts of the Bay. Since these 40km distant tides are delayed by around 3 hours behind the "outside" ones, the actual Heads current strength and direction will often appear unrelated to the state and movement of the local tide level at the Heads.
Small vessels at the Heads see PPB waters as stretching all the way to their northeastern horizon. However beyond that horizon, and so often out of mind, lies the rest of the Bay with seven times more surface area. In the first half of a tidal stream, the tides in the two different areas are moving in the same way so the Heads experiences either a "rising tide with a flooding current" or a "falling tide with an ebbing current". Many folk see these combinations as a comfortable "normal".
However in the second half of a tidal stream the tide heights in the two areas are moving oppositely to one another. Of course it is always "the big area guy" who succeeds in determining the actual direction and strength of water flow, even though the height range of the northern tides is a little smaller than in the south.
So in the second half of a tidal stream near the Heads we always get the less comfortable combinations of either a "rising tide but with an ebbing current" or a "falling tide but with a flooding current". This can be confusing for some folk but you just have to suck it up! This is also why it is best to avoid using the single term "tide" without making it perfectly clear whether you are talking about water heights or water currents.
The final take-a-way is to remember it is the ocean tide just south of the Heads that (with some delay) determines the tide height where you are, but your current is determined almost entirely by the rate of rise or fall of the 3 hour delayed big area tides way up north.
Put another way, mid-tide time in the north gives the fastest streams (and highest/lowest tides) at the Heads, while mid-tide time at the Heads signals it is close to slack water and the highest/lowest tide levels in the north.
To return to the Index Page of this website use this link: To Index Page
--------------------------------------- The End ----------------------------------------