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

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Created by JakeH5 > 9 months ago, 28 Sep 2020
JakeH5
9 posts
28 Sep 2020 10:49AM
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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,

FabulousPhill
VIC, 262 posts
28 Sep 2020 2:47PM
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Is this a question on a forum or a monolog article for a magazine?
These issues would be covered in the training course for the PPB heads as advertised here before.

tired
134 posts
28 Sep 2020 1:33PM
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Crikey.

john24
84 posts
28 Sep 2020 3:37PM
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Can you give a tl;dr summary for those of us not in lockdown. thx

saintpeter
VIC, 122 posts
28 Sep 2020 10:06PM
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Jake,
We have talked about those mushrooms, and we agreed to only eat the ones with the pink gills.

Water does not flow uphill through PPB Heads - momentum or no momentum.

However, there is a more simple and important observation to be made. The southern end of PPB, south of the Great Sands, is in effect a second smaller bay with respect to tides. That is why water can be flowing inwards through the Heads (after Slack Water), while there is still a strong outbound flow in the channels (West, Symonds & South etc). The equalising point is reached in that 'mini bay' between the Sands and the Heads.

When entering the Bay during mid-flood flow, one can certainly see that one is descending to water at a lower level in the bay.

The tides in these parts are a major determinant of sailing , as a good breeze is required to make progress against the tide, and the prevailing winds often do not help. And then there is the ferocious flow through the 'Cut' to enter the Queenscliff Harbour, driven by yet another bay (Swan Bay) - don't get me started.

And don't forget the influence of the wind on the timing of Slack Water!!!

r13
NSW, 1430 posts
28 Sep 2020 10:40PM
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Maybe more info is in this.................

www.vicports.vic.gov.au/publications/Documents/hm-directions-11th-edition.pdf

wongaga
VIC, 600 posts
29 Sep 2020 12:24PM
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Jake;

I assume that you might not be the first human to know about this, and also that PPB might not be the only body of water in the world that is subject to it. So can you refer us to any published accounts?

Cheers, Graeme

JakeH5
9 posts
30 Sep 2020 2:00PM
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Thanks for all the responses so far. I'll just have to finish my magic mushrooms before attempting to reply to you all (in no particular order).

Far out man!


******* TO "FabulousPhil" (VIC) WHO SAID:
"Is this a question on a forum or a monolog article for a magazine?
These issues would be covered in the training course for the PPB heads as advertised here before."

Yes while I did end the topic heading with a question mark, the topic itself wasn't posed as a question but rather I am just trying to undo some very common (and unjustified) assumptions. It had to be a little long-winded to cover all bases!

Forums should have room for both question and answer style posts - sharing info is what it is all about!

Rather than tack my post onto the end of other several years old "PPB heads thread", I thought I would start a new one with my answer first - and then wait for questions!

As far as the PPB heads training courses go, they would just parrot the conventional wisdom which is what I am trying to debunk.


******** TO "saintpeter" (VIC) WHO SAID:
"We have talked about those mushrooms, . . etc
and then:

"Water does not flow uphill through PPB Heads - momentum or no momentum."

Just like water doesn't overtop a bathtub with a wriggling kid in it?
Just like strong surf breaking on a steep beach doesn't run up higher than the ocean level itself?
Just like a skateboarder doesn't roll up the other side of the skate bowl?
Come on "saintpeter", boating and yachting is full of momentum effects!

Several tonnes of yacht motoring gently across a windless harbour towards a dock doesn't rely on drag alone to stop in time. It requires a judicious dab of reverse thrust to decelerate it to a stop at just the right time. Similarly, a billion or so tonnes of sea water rolling in downhill through the Heads can't be stopped quickly by frictional drag alone. Its burst of "reverse thrust" can only come from one possible source - a period of "reverse slope".

You are right about dividing off the southern end of PPB as a special area. However I have called this the "connection zone" (or the "choke zone"), rather than a "mini-bay" because it isn't very "bay-like" with water nearly always on the move anywhere within it once you get away from the shore.

As to oppositely directed currents at opposite ends of this zone, well my experience (excluding the Queenscliffe Cut) is that opposite currents occur for only a short period up to about 15 minutes around slack water at the Heads and both those opposite currents are very slight. Certainly you can sail through the Heads near the end a flooding tide yet meet a strong ebb flow by the time you reach Cole's channel. However these are not simultaneous observations.

As far as weather effects on slack water timing are concerned, well as a scuba diver they can be the bane of my sport. The main culprit via the inverse barometer effect are changing air pressure differentials between Bass Strait and the bordering Southern Ocean and Tasman Sea. Strong onshore or offshore winds are also contributors. The term "storm surge" is a little misleading in that tidal surges are always present, it is just a matter of degree. +/- 20cm are quite frequent even in good weather.

Of all water sports yachting is most affected because they can be out in weather that would see most people tucked up in bed. In severe cases the extra "weather current" through the Heads due to a tidal surge alone can be up to 3 knots. This can shift the slack water times by two hours or so away from the predicted times as well as altering stream strengths.


********* TO "tired" WHO SAID:
"Crikey"

Not sure if this was in the vein of "Crikey who would have thought that occurs. I must think about it a little more."

or more like:

"Crikey, what sort of juice is this guy on?"


********* TO "r13" (NSW) WHO SAID:
"Maybe more info is in this.................

www.vicports.vic.gov.au/publications/Documents/hm-directions-11th-edition.pdf"

Well that was one of the sources I was hoping to get amended. The HM seemed to agree and we exchanged several emails regarding some new wordings. When nothing further happened and the 11th edition was published unaltered I was disappointed. Turns out he had retired some months before that edition was published. This put me back to start all over again (once Covid-19 passes) with the new guy.

Naturally there is resistance to change because no organisation wants to look a little bit silly, but the Physics and observations are all quite clearcut. It is the inertia of entrenched (and at first glance reasonable) ideas that is the roadblock to change. Aus Hydro defers to Maritime Safety Victoria, who in tern defer to VicPorts, as do BoM.

I understand that "VicPorts" might not be particularly interested because for commercial shipping "slack water" is largely irrelevant and they are able to barge through in or out at any time. However for those down the "little guys" end of the spectrum, understanding slack water better could improve our safety.


******* TO "john24" WHO SAID:
"Can you give a tl;dr summary for those of us not in lockdown. thx"

Hi John, not sure how to decode your typo. A summary is:-

a) Parts of the official advice on Port Phillip Heads are incorrect and not supported by any evidence - only assumptions.

b) The claim that slack water occurs when the Bay and ocean levels are equal is not correct and a reverse height difference does exist by the time slack water occurs.

c) This difference may only be 10 or so cm for weak tidal cycles which give drives a relatively slow current reversal.

d) Strong tide cycles may produce a 40cm reverse height difference resulting in a much more rapid reversal.

e) The common conception of a more gentle "U" shaped reverse is incorrect and potentially dangerous. (Think "V" shaped!)

f) There may be a safety benefit for small craft operators if the official advice was based only on facts, and if reversal rate predictions were to be published alongside reversal time predictions.

g) These effects are probably too small to be noticed in other systems, so only VIC boaties or visitors need to think about it.

Enjoy your no lock-down status!


******** To "wongaga" WHO SAID:
"I assume that you might not be the first human to know about this, and also that PPB might not be the only body of water in the world that is subject to it. So can you refer us to any published accounts?"

Well probably not the first human, but seemingly the only one not to gloss over it! Thank goodness we have moved beyond the "burning at the stake" response to ideas that seem to come out of left field.

Some early papers from academics aligned with the Cardno consulting group (who do the slack water predictions), show it in their results but then seem to ignore the significance of it. A major problem is they concentrate only on the Ocean - Queenscliff level difference and with not too much data at slack water. Nevertheless they do see about a 20cm reverse height difference at slack water.

They fall into the trap of "its all about the Heads" and ignore the additional 20-25cm level difference between Queenscliffe and the northern border of "The Great Sands".

Modern coastal engineering firms are typically involved in computerised hydrodynamic models of PPB tidal flows in which the actual physics is buried deep within the computer code of the model and not clearly visible. They generally tweak various parameters until they get a reasonable fit the tide height data.

It all really boils down to Netwon's laws of motion that simply state that in order to change a body's speed or direction, you need to apply an external force. You can't decelerate a large mass to a stop, and then accelerate it in the reverse direction without applying a large force. This simply can't happen when the two connected water bodies have the same water level as "ALWL" claims.

All the official wording and "Heads advice" I have digested make these 4 basic claims:

1) Slack water at PPB Heads occurs around 3 hours after Hi or Lo tide. (Roughly MSL)

2) Slack water after a flood tide occurs around the time of Hi tide at Williamstown.

3) Slack water after an ebb tide occurs around the time of Lo tide at Williamstown.

While the previous 3 claims are easily observed to be "fairly true", they are incompatible with the 4th (and main) claim:

4) At Slack water the ocean and Bay levels are equal.

The only possible explanation consistent with "near facts" 1 -> 3, is along the lines I have outlined about additional momentum driven flows. That is:

a) At slack water after a flood tide, it is roughly MSL outside the Heads, but Hi tide (say MSL + 30cm) at Williamstown.

b) At slack water after an ebb tide, it is roughly MSL outside the Heads, but Lo tide (say MSL - 30cm) at Williamstown.

These height differences of typically 30cm create a slope between Rip Bank and just north of the sands. It is this slope that is responsible for the deceleration and following acceleration of the very large mass of water that occurs around slack water.

Graeme, There is no BS or magic mushrooms involved here. The physics clearly says there must be a reverse slope to reverse the current around slack water. The question for any semi-enclosed bay or estuary is how big is that slope? Well I have noticed for PPB, which is known world wide for massive and quite fast tidal flows, that reverse difference at slack water is quite sizeable and should not be ignored by those offering advice on "Safety at Port Phillip Heads". It is probably an unusual situation of size and shape.

If we take a high level difference of 0.4m averaged over say 15km, that's an average slope of 1 in 37,500. In still water, this slight tilt in the water surface will produce a sideways acceleration of the water of 1/37,000 x g, where "g" is the earth's gravitational constant of 9.8 metres per second per second. So in m/s/s units, the water will be changing its speed at 0.00026 m/s/s. This sounds small, but converting to more pragmatic "boating units":

0.00026 x 600 = 0.157 m/s per 10minutes
0.157 x 60 x 60 = 565 m/hr per 10minutes
565/1852 = 0.305 knots per 10 minutes.

This acceleration rate is an average so will be less than that at the Rip itself. It is likely to be double at say around 0.6 knots/10 mins for the stronger tides. This is in good agreement with what I have observed with a free drifting "GPS tracker buoy" and is close to what can be seen from the Cardno's current curve predictions.

All the science is there and it all forms a clear and consistent picture that is in good accord with all the observable data. Unfortunately virtually no one even bothers to look at this more accurate picture because of a stubborn "water only flows downhill" mentality.

I apologise to all readers that as a newbie to this forum I am not permitted to post images or links for some unknown time. These would make the arguments easier to make and understand. Perhaps some time in the future I can add these when allowed.

cheers,

tarquin1
931 posts
30 Sep 2020 4:35PM
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Couldn't read all of it. Too much for my small brain.
I sail sometimes with this guy.

www.winningtides.co.uk/pages/winningknowledge.htm

He spent years doing tidal research in the Solent because of small local anomalies and because the existing data wasn't accurate. Mostly using GPS trackers as you said. He has been using multiple trackers for a while. Drop multiple trackers at the same time and they can end up in completely different places an hour later!
When you are racing or at any point knowing there is a back eddy or something happening 10 mins later or earlier can change your day.
When using routing programs like Expedition you only get the best out of it if you are ontop of these small differences.
A good navigator will watch the tide changes and advance or slow down the routing to whats actually happening. If you don't you can be hours out after a day. Good navigators say they know at the first tide change which boats are just letting the program tell them where to go and which boats are doing whats actually happening or finding a local back eddy or something!
So many things have an influence on tide and water flow you can't just look at some tide table and go this is whats going to happen right here.

Karsten
NSW, 331 posts
30 Sep 2020 8:32PM
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Speaking for myself, discourse like this on any sailing topic is welcome any day of the week.

Trek
NSW, 1068 posts
30 Sep 2020 8:50PM
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Water can flow up hill. A fountain proves the extreme of that. You can demonstrate uphill tidal flow with a garden hose on an upward sloping driveway.

Achernar
QLD, 369 posts
1 Oct 2020 12:42PM
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Select to expand quote
Trek said..
Water can flow up hill. A fountain proves the extreme of that. You can demonstrate uphill tidal flow with a garden hose on an upward sloping driveway.


That is because the water coming out of a fountain or garden hose has momentum. It proves the rule - Water flows downhill, except when it doesn't.

(I'm trying to stay away from this topic because I do flood modelling for a living, so am fully acquainted with the hydraulics and the math).

Yara
NSW, 1250 posts
1 Oct 2020 7:19PM
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I don't have the patience to plough through all the text but my quick scan sees levels, momentum, air pressure, wind. No mention of the real driver, the good old moon.

JakeH5
9 posts
8 Oct 2020 11:01AM
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Thanks for the additional replies - a mix of scepticism, some encouragement, and some acceptance of discussion of odd-ball topics despite the heavy going at times. This reply, also a bit heavy going, may give the skeptical and doubtful readers a more easily digested view of things.

The target audience of the original post was VIC sailors (and interstate visitors) who may pass through PPB Heads on vessels of 10m LOA or under, and who might frequently be targeting the important slack water window. In adverse weather even a 15m boat length shouldn't exempt you from showing plenty of respect for "the Rip" at PPB Heads.

Seems "wongara" (Graeme) might be somewhere near that target group. By my reading, his reply showed some scepticism (that's OK), politeness (that's good), and some open-mindedness (that's excellent!). All that is needed to get to the facts about the real "equal levels" timing is for a cool, considered, and unbiased examination of the official "ALWL" claims.

An easier route for some folk might be to tackle the situation the other way around. That is, instead of arguing that slack water comes 40 - 70 mins after the "equal levels moment" due to residual momentum affects, we instead could just determine the correct time of the "equal levels moment" after ocean high tide, and then compare it to the known approximate 3 hour delay to slack water.

This can easily be done at least to a rough level of accuracy by simple trigonometry. While the rise and fall of tide curves are not exactly sinusoidal in shape, there are usually quite a few days per month where the agreement is pretty darn good. We can use the trig functions Sine, Cosine, & Tangent to analyse a simple mathematical model of just two curves that represent the outside ocean tide and the inside "main body" tide.

Let us assume an ocean tide curve of 100cm amplitude (200cm range), and a three hour delayed "main body" tide curve of 40cm amplitude (80cm range). These are typical real case values for biggish tides and can be checked via the tide tables. Further lets take the period of a full tidal cycle as exactly 12 hours for simplicity.

The 3 hour (180 minute) time delay then corresponds to a phase delay of 90 degrees in the trigonometric sense. This means each 1 degree of phase difference in the trig world equates to a 2 minute time difference in the real world. We will also set our time origin (t=0) to be when the ocean tide peaks and when the "main body" tide is at mid-tide or MSL (ie. Bay Height = 0cm, passing from -ve to +ve).

So starting from t=0 the height of the "main body" tide in cm at any later time "t" is:

H_bay = 40 x Sine(t/2) --------------- #1

Where "t" is the elapsed time in minutes. (and (t/2) is the trig angle in degrees where 360 degrees represents one complete tide cycle from one climb through zero to the next one.)

Since from t=0 the ocean tide starts to fall from its peak amplitude of 100cm, its height curve can be modelled as a Cosine curve using the same angle variable:

H_ocean = 100 x Cosine(t/2) ------------ #2 [ Trig identity: Sine(Angle-90) = Cosine(Angle) ]

The rising bay curve and the falling ocean curve will then have equal height levels at the later time "t" given by solving this equality:

40 x Sine(t/2) = 100 x Cosine(t/2)

By rearrangement we get:

[Sine(t/2)] / [Cosine(t/2)] = 100/40 = 2.5

Hopefully some might remember from high school trig that Sin(A) / Cos(A) = Tan(A)
If that memory doesn't jog you will have to take it on trust, or check it out using a handful of examples for angle "A".

So the formula for the equal levels time "t" at Port Phillip Heads for our typical outside & inside tide curves is:

Tan(t/2) = 2.5

From which we can get (t/2) = Inverse_Tan(2.5), or (t/2) = 68 degrees (approx).

The equal levels time "t" in minutes is then:

t = 68 x 2 = 136 minutes, or 2hr 16 mins after ocean high tide.

This time is some 44 minutes before our typical flood slack water time of 3hrs (or 180 mins) after ocean high tide. The 44 minute difference is the "up-slope flow time" and is typical of the observations for the real life tide curves. However these do show quite a bit of variation from say 35 minutes to 80 minutes of up-slope flow because of the different tide curve shapes that occur over a full lunar cycle.

Interestingly the weaker the maximum flow rate of a Port Phillip Heads tidal stream is, the longer the up-slope flow time is. This might sound a little counter-intuitive at first, but stopping weaker flows takes longer because both the braking forces provided by frictional losses and the reverse slope growth rate are lower.

The uphill flow times are also longer for ebbing tides than they are for a flooding tides. In part this is due to the outside tide which on average rises slightly more slowly than it falls. The slower rises means an outflow isn't stopped as quickly as an inflow. The Bay itself also drains slightly more slowly than it fills. On average, the outflow --> inflow reversal rates are lower than inflow --> outflow reversals.

Popping our 68 degree angle back into both of the original equations shows both water levels are equal to 37.5cm at that time. No matter how you look at this system, you will find that "equal levels" and the "slack water" do not occur at the same time. The widely spread official advice is wrong on this point.

If you take any slack water time roughly near 3 hours (180 mins) after high outside tide, then the corresponding trig angle (t/2) will be somewhere close to 90 degrees. Plugging 90 degrees back into both our original #1 and #2 tide curve representations gives:

H_ocean = 100 x Cosine(90) = 100 x 0 = 0cm (or MSL), and

H_bay = 40 x Sine(90) = 40 x +1 = +40cm (or MSL + 40cm)

Again this shows that at the end of a strong flood tide with a 200cm ocean range, the Bay's "main body" level sits somewhere around 40cm above the ocean level around the time of slack water. There is simply no way the widely believed notion of equal levels can be true. Port Phillip Bay has extraordinarily large momentum flows of up to 80,000 tonnes per second at up to 10kph, and an enormous surface area of nearly 2000 square kilometres. Both these contribute to its ability to store lots of momentum within its waters.

The funnelling shape of its waters between the "Great Sand" and the entrance also serve to concentrate its residual momentum flux onto a smaller area of coastline around the entrance making that flow fairly "unstoppable". Flowing upwards across a growing slope for some period of time is the only way to bring this slow moving beast to a halt. Sailors and other folk who operate in the Heads area should begin to change their thinking. Even when the official advice eventually gets corrected, statements like "PPB tidal streams reverse when the Bay and ocean levels become equal" will take some decades to dwindle away and disappear.

In a practical sense nothing dramatic needs to change in your approach to passing through the Heads, except to realise that the waters around the Heads don't "loll around" at a standstill while waiting for a level difference to develop before moving off again in the reverse direction. A sizeable level difference may already exist around slack water which leads to rapid changes in the current strength of both the old and reversed currents.

The most rapid reversals occur around 4-5 days after a full or new moon. Although the ocean tidal range normally peaks 2-3 days after new and full moon, slight changes in the shape of the tide curves add another day or two before the PPB reversal rates hit their peak. This rather long period of delay also gives the slightly surprising result that the weakest reversal rates (and longest slack water windows) occur around 2-3 days before a full or new moon.

cheers,

saintpeter
VIC, 122 posts
8 Oct 2020 9:39PM
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Interesting, Jake, but still not relevant.

Forget about the main part of PPB, the only part relevant to a Rip transit is the triangle between Point Nepean, Shortland Bluff & Point Lonsdale. This is the pool of water immediately inside the Heads, and therefore the only waters that can interact with the 'ocean' (aka Bass Strait).

In my experience, the important factors for transiting a smaller yacht through the Rip are:
1. The published Slack Water times.
2. Prevailing and previous winds.
3. Sea state - 'outside'.
4. The wind.
5. Was it a maximum or minimum tide?
6. The wind.
7. Take the right line - if you are on the Nepean side you are already in trouble.
8. The wind.

.. .. a pattern is emerging here.

Rather than debating trigonometry and Newtonian physics, let's just concentrate on the wind effect on timing of actual Slack water - it's far more important!

wongaga
VIC, 600 posts
9 Oct 2020 8:47AM
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I'd also add barometric pressure as another variable which can affect water height in the bay.

Notwithstanding the finer points of hydrology or whatever, arguing about whether slack water is precisely to the second of the published data is about as useful as being upset that a forecast 15 knots of wind turned out to be only 13. And if you haven't the knowledge to take all this into account, you've no business to be playing in the Rip.

Fortunately I've never noticed my cup of coffee sliding back along the cabin table as my little boat gamely climbs the hill into PPB.

saintpeter
VIC, 122 posts
9 Oct 2020 11:01PM
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Select to expand quote
wongaga said..
And if you haven't the knowledge to take all this into account, you've no business to be playing in the Rip.




+1

JakeH5
9 posts
11 Oct 2020 6:46PM
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Its Jake again:

Ok, Ok, The messenger has well and truly been shot! While I bleed-out from that fatal wound there is just enough time for one or two (thousand) final words.

The long and nerdy nature of my posts does not seem to have gone down well with VIC yachties. However the common ground is that the Heads of Port Phillip Bay can be a very dangerous place for recreational craft and the better we understand it the better off we are.

I merely laid out undeniable evidence that one very commonly held notion is incorrect. The fault for such misinformation lays not with the recreational boater but various government authorities. I have confidence this will eventually get corrected and my original post was intended as a "heads up" (no pun intended) for those journeying in and out.

Sure there are dozens of different factors that affect the safety of those transiting through the Heads, but the notion of the slack water time predictions being expanded to also give a current reversal rate seems to be just one more helpful thing for passage planning.

As pointed out in the latter replies, weather induced alterations of slack water timing away from the predictions is a significant issue for small craft. This is also a major concern of mine. However to unravel the mysteries of these processes first requires an accurate evidence based picture of how slack water comes about. Just tweaking the official story around towards the truth is all I am trying to achieve here. So far nobody has directly attacked that tweak, but instead just introduced a stream of other issues.

It turns out that while the inverse barometer effect and strong winds from certain directions create those troublesome "surge" height changes in Bass Strait, it is the reversal rate number for each slack water event that determines what the resulting time shift will be. So again knowing these numbers will be useful once the weather effects are better understood.

Interestingly the smaller time shifts of 10-20mins often found even in good weather seem to stem mostly from time shifts of the "equal levels point". There might be future scope for the VTS Lonsdale folk to detect that well in advance of slack water itself.

I am gobsmacked by the assertion that what happens tide-wise in the Bay's main body area is "not relevant".

Certainly it is the sea, swell, and wind conditions around and outside the Heads that determine the primary level of danger. However it is the fast currents surging through this area due to "main body" breathing that can quickly elevate that sheet of water from dangerous to treacherous.

If the "main body" could somehow be removed to leave a northern shoreline running from say St Leonards across to Rosebud, the infamous "Rip" would then become as gentle as a lamb.

Perhaps Graeme might be better off watching his coffee cup back at the marina while pushing it around the saloon table. While accelerating it forward he will see a surface tilt in the forward direction, and a reverse tilt when slowing it down.

It is the liquid height difference across the coffee surface that provides the force to speed up or slow down the coffee in the middle of the cup. That is how liquids with a free surface change their speed.

Sure at Port Phillip Heads both the surface tilts and sideways "g-forces" are tiny, but factor in a few billion tonnes of mass and the total force involved becomes huge. While the "driver" of currents through the Heads is the force produced by an inside to outside level difference, that force splits itself in a time varying way between the tasks of (i) overcoming flow friction and (ii) changing the speed of the water mass.

Around the time of maximum current, friction is very high and the speed isn't changing much so almost all the available force is devoted to task (i). Near slack water the flow friction virtually vanishes so it is almost all task (ii). In between these events it is a time varying mix of both tasks. The result is that current speed through the Heads is a complicated function of the level difference between Bass Strait and the "main body" of the Bay.

A rough summary is that the inertia and momentum effects of having to speed up and then later slow down a massive tidal stream means its "speed curve" is delayed behind its "drive curve" by around 40 to 60 minutes. This gives us slack water well after the "equal levels" (zero drive) point and the maximum current speed well after the point of maximum level difference.

I am contributing this point to increase the total "Rip knowledge" that "wongaga" suggests is important for all wanting to be "playing in the Rip".

( By the way, my "playing" has extended for over 45 years and I still seem to be in one piece. )

Goodbye to all, Jake H "over and out".



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"Uphill Tidal Flow Through Port Philip Heads - Are You Kidding Me?" started by JakeH5