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.
(Excuse the long rant below from a frustrated VIC boatie in stage four Covid-19 lockdown. These words may be difficult for many but do need to be aired.)
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.
The reverse level difference that develops during the uphill flow phase becomes the major player in stopping the last 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 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.
A characteristic of Port Phillip Heads is that slack water doesn't occur until roughly 3 hours after either high or low ocean tide outside the entrance. In very rough terms, this typical 3 hour delay is made up of 2 hours or so of slowing down-slope flow until the "equal levels point" is reached, followed by roughly 1 hour of slowing up-slope flow (powered by stored momentum) until the flow comes to a complete stop to produce "slack water".
***** "A Little White Lie" (or "ALWL") *****
Unfortunately for 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 close to 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 "near 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? ******
I am not really sure. While all bay or estuary entrances must suffer some time delay between the "equal levels" time and the "slack water" time, it is apparent that for smaller, or more open ports and harbours, this delay might just be too small to be noticed. Port Phillip Bay seems to be a rather special case with significant up-slope flow times. Perhaps "ALWL" was just transferred over to Port Phillip from its smaller cousins and the port authorities thought no more about it. (Despite there being truckloads of evidence against it everywhere you care to look!)
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 Pilot Service websites 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 advice" confusing and it is then 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 persons 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 and one and a half steps backward".
***** The "Equal Levels" Moment *****
The equal levels moment at Port Phillip Heads occurs roughly every 6 hrs when the "outside" ocean water level matches the "inside" water level in the vast "main body" of the Bay. The "main body" region begins about 15km inside the entrance and is a whopping 1,700 square kilometres or so in area. It is the "tidal breathing" of the vast main body surface area with its fairly well synchronised tides that produces the high current speeds at the entrance.
Between the entrance and the large "main body" area there is a wedge shaped "connection region" which acts to restrict the flow of water to and from the ocean. This is due firstly to its narrow width at the entrance end of the connection. Secondly before the Bay's wide and deep central basin is reached there are extensive shallow areas (known as "The Great Sands") that play a nearly equal role in restricting the flow to and from the ocean.
Significant consequences of the constricting connection between the ocean and the Bay's vast "main body" region are that:-
a) Water level height differences of a metre or more are typical across this 15km connection at times of maximum flow.
b) Tidal currents through the entrance may reach 6 knots, and from 2 to 4 knots at other places within the connection.
c) The "main body" tidal range is only 30% to 50% of the ocean tidal range. (Lower % values for faster flowing tides.)
d) The High and Low Tide times in the "main body" region are delayed by around 3 hours behind the "outside" tide times.
During this 3 hour delay period the water levels in the two regions move in opposite ways with the ocean falling and the main body rising or vice versa. The tidal current through the entrance is primarily determined by the rate of rise or fall of the tide over the enormous "main body" region, with only a small contribution due to tide height variations within the connection itself. During the delay period areas near the entrance experience the minor "brain-bend" of an incoming tidal stream but a falling tide level, or an outgoing stream but a rising tide level.
While at first this might be confusing to those boating around the Heads, remember that the tide height at your location is dictated largely by the ocean level a few km away to the south, whereas the tidal current at your location is dictated largely by the rise or fall rate of the 3 hour delayed tides in the vast area northwards. This is well over your horizon and 40 km or so away. It is really a case of "out of sight - out of mind" that enables the confusion to persist as to why the horizontal and vertical components of tidal motions near the entrance behave at times in what at first glance may appear to be in "unexpected ways".
At the "Equal Levels Time" the gravity forces are balanced and so there is no net sideways force attempting to push the water one way or the other through the entrance. Note however this "zero driving force" moment does not imply zero velocity (or slack water) as "ALWL" claims.
Rather at that moment the water will just continue to "coast along". It is neither assisted by any downward slope between the two water bodies, nor impeded by any upward slope. Frictional drag is the only active force during these few equal level minutes. However a reverse slope force will then begin to grow because the two water levels are now moving apart with one rising and the other falling.
The timing of the brief equal levels moment is a little variable depending on the details of each tidal cycle. On average it occurs a little over 2 hours after high or low tide at the entrance. On a "big tide" day the entrance tidal stream is still running at around 2 knots at the equal levels time. This occurs because in the weakening tidal stream the decreasing frictional drag is simply insufficient to remove all the forward water momentum by the time the equal levels moment is reached.
All waters within the Bay will also be in motion at this time but with lesser speeds as the underwater cross-section of the Bay expands about 20 times as you move northward into the central basin of the Bay. However there is sufficient residual momentum stored in this now 25 billion tonnes of slowly moving water to allow the entrance stream to keep flowing in the same direction (and now slightly uphill) for around another 40 minutes on a flooding tide, or around 60 minutes on an ebbing tide.
These long lasting up-slope flows before a tidal stream is halted are in part due to the wedge shaped nature of the waters connecting the ocean to the main body of the Bay. This tends to focus the water momentum onto a smaller area of coastline around the entrance. In turn this means a much higher reverse level difference over this smaller area is required to generate sufficient force to halt the tidal stream compared to a situation where no focussing effect occurs.
The significant reverse height difference required to stop a tidal stream takes the best part of an hour or so to fully develop because it relies principally on the ocean rise or fall rate to create it. The effect we see in Port Phillip Bay is a form of "slow-motion water hammer" where the Heads in effect behaves like a slow closing tap. This "Heads Tap" slowly closes for outflow on a rising ocean level, or slowly closes for inflow on a falling ocean level.
Near the end of an outgoing tidal stream, the water levels rise above the central basin level as the water flow encounters the "closing downstream Tap" and is less able to escape as quickly as before (so it rises). Near the end of a incoming tidal stream, the water levels fall below the central basin level as the water moves away from the "closing upstream Tap" and is not replaced as quickly as before (so it falls).
These effects are just ordinary "moving water physics" in action, but where the rather unique size and shape of Port Phillip Bay enhances them to the point of being quite significant - and best not ignored.
***** The "Reverse Height Difference" at Slack Water ******
During the "up-slope flow times" the reverse height difference between the Bay's very large "main body zone" and the ocean grows to as much as 30-45 cm by the time slack water occurs. This difference is spread across about 15km from just outside Rip Bank to the northern edge of The Great Sands, so the "slack water slope" isn't visible to the naked eye.
It is however clearly evident when comparing the "outside" and "inside" tide curves once these are adjusted to a common datum level like MSL or AHD. The reverse height difference produces a high rate of slowing down approaching slack water and a high rate of speeding up once the stream reverses.
The commonly held belief that slack water at the Heads is a time of no height difference and no forces in play is totally false. Water speeds are in fact changing here at a faster rate than at any other point in the tide cycle. Boaters near the Heads need to appreciate that while the flow has momentarily stops at slack water, their vessels are actually floating on a sloped water surface.
Compared to craft floating in the "main body" region, their water line levels may already be around 30cm higher (slack after ebb) or 30cm lower (slack after flood), with that difference growing strongly. There are strong sideways forces acting on the water under the vessel to accelerate it in the new direction.
****** The "Slack Water Period" ******
Often there is talk of a supposed "slack water period" during which no water movement occurs. Scuba divers know this is misleading and with careful observation the current reversal can be seen to occur within just 1 or 2 minutes.
The reality is there is only a "slack water window". Its duration depends on what the water reversal rate might be for that particular slack water event, and what current strength is considered as acceptable as a "safe limit" at both the start and end of the window.
The safe current limit will depend on the activity being undertaken at Port Phillip Heads. Scuba divers might set safe limits of just plus or minus 0.7 knots (about +/- 20 metres/minute). This allows them to make headway against the current at all times within their safety window. Small yachts can accept larger current limits but must also factor in the need to travel for quite some time and distance, often at reduced headway, to escape from the danger zone in sufficient time.
Other watercraft will have their own safe current limits depending on vessel size, type, engine power, etc. They will also take into consideration the wind and swell conditions at the time to assess their safety in this potentially very dangerous area.
However the Achilles heel in all these safety considerations is that at present the official slack water predictions give only a reversal time. They don't say whether that particular slack water event will be a slowly reversing one or a rapidly reversing one. That information would improve the safety of many recreational folk operating in the vicinity of Port Phillip Heads.
Hopefully a wider appreciation of this issue among those recreational folk who operate in and around the Heads may encourage the relevant authorities to acknowledge the damage "ALWL" may be doing and amend their claims. While it is easy to ignore a lone voice, it is harder to turn a deaf ear to a chorus of voices.
The "Port Phillip Heads advice" offered by a host of players including, "VicPorts", Aus Hydro, Port Phillip Sea Pilots, and "BoM", needs to be amended. Hopefully the "Rip predictions" publications could also be expanded to include predictions of the tide stream's "reversal rate" to supplement the existing "reversal time" predictions.
cheers,
