Uphill Tidal Flow Through Port Philip Heads - Are You Kidding Me?

Packo's prediction for the slack water times at Port Phillip Heads.

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Uphill Tidal Flow Through Port Philip Heads - Are You Kidding Me?

Postby packo » Fri, 28 May 2021 12:42 am

***** Uphill Tidal Flow Through Port Philip Heads *****

No kidding at all! It does happen - for about the last 40 to 80 minutes before slack water. This "last run" of the tidal flow is going uphill, and against a growing uphill slope!

Before the words "no-way" escape your lips, please take a long pause. That knee-jerk response would put you in good company with numerous recreational folk, and even many professionals. However the physics, the measurements, the data, the theory, and the numbers all line up to make this one true.

It turns out the old adage "water only flows downhill" is not actually "water tight". Given the right conditions flowing water is perfectly capable of overcoming gravity and flowing upwards, at least for a limited amount of time. Common examples might be water flowing out of an upward pointing garden hose, or water in a rapidly flowing river climbing up and over small boulders in the stream. It is all just a matter of trading off some forward speed (and momentum) for a gain in height.

Perhaps the only astonishing thing is that while the time frame of the uphill flow examples just given are less than a second or so, for the decelerating tidal flows near Port Philip Heads the time frame is in the 40 to 80 minutes range. The are several special circumstances about Port Phillip Bay's size and shape that play a key role in this unusual behaviour.

The lengthy uphill flow phase is possible because during the previous five or so hours of fast downhill flow, the Bay's waters have absorbed and partly retained an enormous level of momentum in the forward direction of flow. The water flow then continues on in the same direction well beyond the "equal levels point", (or "zero slope point", or "zero drive point"), where the ocean level "crosses over" the water level in the main body of the Bay.

This flow will only stop when the combination of the growing reverse force (from the growing reverse slope) and the diminishing frictional drag, absorb all remaining forward water momentum to finally yield "slack water". The tidal stream then reverses direction with that uphill slope immediately becoming a downhill slope which gives a very solid "push start" to the reversed tidal stream.

The reverse bay-to-ocean height difference attained by the time slack water occurs is a major factor in determining the current's "reversal rate" for each particular slack water event. At present the official BoM "Rip predictions" don't tabulate these reversal rate numbers but we all might be much better off if they did!

Depending on the particulars of each tide cycle, the rate at which the speed of the current changes near slack water can be as high as 0.7 knots per ten minutes, to as low as 0.15 knots per ten minutes. This wide range makes quite a difference in determining how much time you have got in and around Port Phillip Heads before things turn ugly. Short slacks might last only 15-20 minutes whereas for other tidal cycles slack water can be as long as 60-80 minutes. Knowing what you are likely to be facing at the Heads is useful knowledge in the quest to keep yourself safe in this potentially dangerous area.

The reverse level difference that develops during the uphill flow phase becomes the major player in stopping the last part of a tidal stream to produce slack water. In stopping the strongest of tides, the reverse height difference between the ocean and main body water levels may need grow to as much as 35-45cm before slack water is achieved. This degree of height difference takes the best part of an hour to build up because the level difference typically changes at a little less than 1cm per minute.

A characteristic of Port Phillip Heads is that slack water doesn't occur until roughly 3 hours after either high or low tide in the ocean outside the entrance. In very rough terms, this typical 3 hour delay is made up of roughly a little over 2 hours or so of slowing down-slope flow until the "equal levels point" is reached, followed by roughly a little under 1 hour of slowing up-slope flow (powered by stored momentum) until the tidal stream comes to a complete stop to produce "slack water".

***** "A Little White Lie" (or "ALWL") *****

Unfortunately for many decades various port authorities have been pushing out "A Little White Lie" ("ALWL") about Port Phillip Heads. They claim that the tidal streams reverse (ie. slack water happens) at the same time the Bay and Ocean levels become equal, and when the water surface slope is horizontal and changing from inwards to outwards, or vice versa.

Although extremely widely believed, this simply isn't true. "ALWL" assumes there are no momentum or inertia effects due to the mass of water being moved back and forth. This is rather silly given that for Port Phillip Bay around a billion tonnes of water are moved in or out on a single tide. This is a large mass in anybody's language and means it is not easy to slow the flow down to a complete halt and reverse it without applying a considerable reverse force!

All this is happening around slack water time when "ALWL" insists that either no, or only small driving forces are in play because the inside and outside waters are claimed (falsely) to be at the same level.

This nonsense has been peddled by the various authorities for over 50 years and it really needs to stop. Its legacy is that we now have thousands upon thousands of mariners, boaters, and various water sports folk being significantly misled about how and why the tidal currents reverse through Port Phillip Heads. Someday this misinformation may contribute to a serious incident, or even loss of life.

***** Why is "ALWL" for Port Phillip Bay so Widely Believed? *****

The main reason would appear to be that ALL the "official sources" simply claim it to be true, but without supplying any evidence. Exactly who is parroting whom is a little unclear. Probably the Federal bodies (BoM & AusHydro) are just accepting wording from state entities, with either the Port Authority or the Port Phillip Sea Pilots being the original source of "ALWL".

In general, bays and harbours can have widely different tidal behaviours and slack water timing depending on their size, shape, bathymetry, and the degree of entrance constriction. However there are two extreme "classic cases" worthy of note:

Case 1:
At one end of the spectrum, bays having wide and unconstricted entrances will see their inside tide peak just a matter of minutes after the outside tide does. The inside tidal range is also close to the outside range. Slack water after a flooding tide will occur shortly after the peak outside tide when the Bay becomes "full" and then starts to empty.

Case 2:
At the other end of the spectrum, bays with an extremely restricted entrance will have a very small inside tidal range and so continue to fill for around another 3 hours after the outside tide peaks. Slack water at the entrance does not occur until around 3 hours or so after peak outside tide. In other words the bay switches from "filling" to "draining" as the outside level falls below Mean Sea Level (MSL), which is the approximate inside level.

Since slack water timing at Port Phillip Heads matches closely to the second type of behaviour, and that the entrance is regarded as quite narrow in relation to the large size of the Bay, the authorities just assumed it was a "Case 2" situation. They then went on to claim that slack water occurs at the same time as the outside and inside levels "cross-over" each other, as per the classic "Case 2" situation.

What went unnoticed is that a true "Case 2" scenario demands a very small inside tidal range of less than 10% of the outside range. This condition does NOT apply to PPB where the average inside tidal range is around 40% of the outside range. While for strong tides this figure might drop to as low as 30%, for the weak tides it may climb as high as 65%. This is due to the increased "filling efficiency" for weak tides where frictional drag effects are very much reduced at the lower stream speeds. Clearly the level "cross-over" point must occur much earlier than the slack water time and the "ALWL" claim that these two events occur at the same time is false.

In past decades "ALWL" lurked around mostly in marine handbooks and the like with a fairly limited readership. Since the coming of the internet "ALWL" has infected just about everywhere you might look. It has become the most widely spread and widely believed piece of misinformation about Port Phillip Bay.

It is not just bloggers spreading it either but also a raft of federal and state government bodies. It appears in AusHydro's ANNT and Mariner's Handbook publications, as well as "VicPorts" (VPCM) "Port Information Guide". It also graces the BoM and the Port Phillip Sea Pilots' website as well as those of many, many other organisations. With so many sources singing from the same hymn sheet most recreational boaters simply conclude "it must be true".

With "ALWL" so entrenched, its followers then fall into making some untrue, unhelpful, and sometimes self-contradictory conclusions in an attempt to align this defective "ALWL story" with various genuine facts about the Bay. That makes the "official" Port Phillip Heads advice confusing and it is difficult for many marine folk to understand the true situation.

It also potentially reduces their safety with many encouraged by "ALWL" to believe a gentler "U-shaped" reversal of current speed occurs rather than the "V-shaped" reality where the current speed changes by roughly a constant amount over equal time intervals. It is high time to toss "ALWL" overboard and replace it with something more accurate, more sensible, more useful, - and a little bit safer!

I am working hard on this but Harbour Masters (and others) seem to be a rather stubborn bunch. On two occasions I did manage to convince key personnel that the conventional wisdom about Port Phillip Heads was defective and needed redrafting. However not long after, those folk either retired or moved to completely different areas of responsibility and nothing got changed. It is a long and frustrating process of "two steps forward followed by one and a half steps backward".

A series of other posts listed below expand on the relevant Port Phillip Bay issues in an attempt to get more folk to come onboard with this very different explanation of "what really goes on at Port Phillip Heads":-

2) The Equal Levels Moment

3) The Reverse Level Difference at Slack Water

4) The Slack Water Period

5) The Momentum Of Tidal Streams

6) The Bay's "Balance Line"


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