Port Phillip Heads - "Packo Prediction Tables 2018" - Update #1: Jan,Feb,Mar

1 month 5 days ago - 2 weeks 4 days ago #303681 by packo
packo created the topic: Port Phillip Heads - "Packo Prediction Tables 2018" - Update #1: Jan,F
Hello,

Looks like I am signed up to continue these "slack water info" predictions for at least another year! Update #1 for 2018, covering the predictions for January, February and March, can be found in the updated original trial dive-oz post at this link:-

Packo's 2018 Port Phillip Heads Slack Water Info & Predictions for Jan, Feb, & March


I'm still dribbling them out in 3 month batches to allow for any tweaks and adjustments that may come to light through further testing or user feedback.

The intent was to do updates on the 22nd of each of December, March, June and September, but I am doing this one a bit early because of Christmas.

***** SYNOPSIS: *****

These predictions are derived from some water volume modelling of Port Phillip Bay. This uses high precision tide predictions at multiple locations together with the way the Bay's surface area is distributed to determine the rate at which the Bay's water volume changes in response to the celestial tides for each 6 minute interval over the entire year.

Some basic assumptions then convert these volume flow rates through the Heads into water current predictions for the Heads. The main aim of this project was to determine the rate at which the current through the Heads slows down approaching slack water, and then speeds up again after the flow reverses.

It turns out these rates are quite similar, and do not change too much across the whole of the "diving window". The situation can be usefully described by a single signed number which is called the "acceleration rate". This number is negative for the slack following a flood stream (called "flood slack") because the direction of the change in speed ie. the acceleration ("Accln") ,is always outwards through the Heads. Ebb slacks are characterised by a positive value because the acceleration direction is always inwards through the Heads.

It seemed that if scuba divers could see such a predicted "reversal rate" for each slack water event, then useful comparisons could be made as an aid in selecting which slacks to dive, likely duration of the "dive window" etc. Because of the significantly differing tide cycles, these quantities can change by a factor of nearly four times and so will significantly affect the diving experience.

The reversal rate or "acceleration numbers" are given in units of "knots per 10 minutes" as this seemed a sensible choice and a fairly pragmatic one for the rough mental arithmetic sometimes needed. In magnitude these acceleration rates range from as little as 0.15 knots/10mins up to 0.65 knots/10mins. This range reflects the wide variations that occur in different tidal cycles.

These rates are useful knowledge because they determine the duration of the "diving window", and may help predict when slack will occur should you arrive at the site way too early. They also indicate, in a reciprocal way, how vulnerable each particular slack is to having its timing messed about by the weather on the day. Therefore they are helpful in predicting the appropriate "weather allowance" divers should use to make sure they don't miss any particular slack should its timing be altered by the weather conditions.

These numbers, combined with other external information, can also allow approximate calculation of any weather induced timing shifts that might occur on the day. While this correction method is still in its infancy, the results show some promise.

The modelling also spits out the predicted slack water times as obtained from the maximum water volume time, and the minimum water volume time for each tide cycle. This is where the average flow rate through the Heads drops to zero and the Bay's water volume remains steady for a minute or two. These times are corrected to daylight savings time when it is in operation.


***** NEW THINGS: *****

The recent rebuilding of the forum website has corrected a problem that means the "plain text format" versions of the tables now display correctly aligned columns without me trying to fudge a fix by using full stops for some spaces. Note that if you choose to "copy and paste" the tables in this format, you may still need to force a fixed pitch font for printing, or displaying properly on your own device. The "month-by-month image" format option is provided through a set of jpeg files which can be download from the above post.

These images avoid any formatting problems. They are probably at a higher resolution than is really needed, but when downloaded to a phone they allow old guys like me to zoom in to read them, usually without needing reading glasses.

An extra "Dif" column has been added to the tables for 2018. This gives the minutes difference between the "Packo slack times" and the usually slightly later "official" prediction times. These are done by the Cardno company, and published in the VIC Tide Tables, on the BoM website, and in the Australian National Tide Tables (ANTT). These differences range from just a minute or two up to as much as 20 minutes for some very weak ebb slacks.

Originally I was a bit dismissive of the some of the very late Cardno times because I was aware the algorithm they devised ignored the flow resistance across the Great Sands. This does vary significantly between ebb and flood flows due to large relative water depth changes. It was also very clear that at some late Cardno times the Bay water volume was definitely rising significantly at the same time they claimed the ebb stream was just still running out along the main channel leads.

I have pulled my head in a little since then because on two occasions I have observed water flowing in quite strongly past Pt Nepean on the new flood tide, yet a fading ebb stream appeared to still be running out on the Pt Lonsdale side. There have also been quite a few observations the showed the Cardno times as definitely too late, even in the Great Ship Channel. It is an issue that needs more investigation but possible weather induced timing variations can make this difficult.

Cardno tries to model only on the main channel leads while Packo tries to model the average inflow and outflow currents. It is possible we can both be right because water momentum in different parts of a tidal stream can create some weird effects.

I hope to be able to get a definitive answer to this issue by late Autumn. This is the best time to study these differences as when the air and water temperatures are closer together, the chance of having pesky weather related timing shifts clouding the results is minimised.

In the meantime being aware of the when the time difference is large might be useful. Using the earlier times for dive planning might be wise until the issue is eventually resolved. This is particularly true for dive sites well inside the Bay as many observations have shown that ebbs slacks may occur well before the Cardno times.

BoM NOTE:- The BoM tide guys tell me that if you downloaded the 2018 RIP predictions pdf file from the BoM website before 25/11/2017, you should get an updated version as there was a formatting error that saw some stream rates go missing. I think they have to own that error because the bug seemed to relate to daylight savings corrections, and it is only BoM that adjusts the Cardno predictions from EST to "summer time".


***** OLD THINGS: *****

Unfortunately the packo tables still have to contain a warning that advice from the scubadoctor website on slack delays for sites more than 5km inside the Heads is not safe, and should be ignored. I have spent a lot of time and effort trying to get that bad advice removed, but have had to give up. The culprits won't test their own advice, and won't look at any evidence - including some live streaming video.

I just don't understand what is going on in that camp. If someone drowns because of that advice then at least I can say I tried damn hard to prevent it, but without anybody else's help I was ultimately unsuccessful. This dispute was hand-balled to DIVA who initially said "go away". However, some younger and more aware DIVA committee members may want that decision reviewed. I will leave it to DIVA to squabble over and just hope that in the end politics does not triumph over safety!

Later update: Ironically on the very day I posted the 2018 tables there was an incident with a scuba diver being swept miles away from his boat. However I didn't get to hear about until a week later. In the recent forum discussion of this it wasn't clear if this was a scalloping drift dive gone wrong or a misinformed "slack attempt". The boat was anchored and the diver alone. However the dive timing seems to match the bad LDS advice, and so demonstrates its dangers regardless of the diver's drift/stay-put intentions. Luckily the diver was rescued alive 4 hours later but it could have easily gone the other way.


***** MORE MYTH BUSTING EFFORTS: *****

More recently the myth in my sights is the common one that says slack water occurs at the Heads when the water levels outside and inside become equal. Unfortunately this "official myth" is promoted by far more serious players such as PoMC (now VPCM), VRCA, BoM, Port Phillip Sea Pilots, and Australian Hydro (through ANTT). I've been head butting some of these parties recently to get a more truthful story told, and one that does not have all the logical inconsistencies of the current "official line". Something supported by facts would be a good start!

While at first glance the "official line" might sound plausible enough, it totally misrepresents the true situation as the momentum and inertia effects of the very large mass of water involved are totally ignored. It is like being told a skateboarder who gains speed by rolling down the slope in a half-pipe will come to a sudden stop at the bottom of the pipe just because the slope there is zero. In the real world of course they will keep on rolling in the same direction and climb some way up the reverse slope whilst shedding speed.

We all know that while manoeuvring our boats around piers and the like at speeds of less than 2 knots, water friction is already quite small and will decrease further as the speed drops even more. It is then a case of "momentum rules", and the only way to stop in time is via a generous dab of reverse thrust. It is much the same in the Rip, and as the water slows down the level of bottom friction is simply insufficient to completely halt the flow of the tide by the time the "equal levels" moment is reached.

Instead the water's large momentum allows it to continue flowing in the same direction beyond the equal levels point and into the region where it is now flowing uphill against a growing reverse slope. This builds up principally because of the continuing change in ocean level and of course produces the "reverse thrust" needed to finally halt this "one billion ton juggernaut".

The clear facts are that the Ocean-Bay equal level moment is typically reached a little over two hours after high or low tide at Pt Lonsdale, whereas the slack water moment is typically reached a little over three hours after hi/lo tide at Pt Lonsdale. During this roughly 40 - 70 mins uphill flow phase, the bottom friction decreases as the speed falls, but the down-slope gravitational retarding force increases as the reverse slope builds up.

In the end it is the reverse slope in the water surface that does the lion's share of the "stopping". Interestingly the friction and down-slope gravity forces combine in a way that gives a slow down rate that is roughly constant during much of this time for each particular tide cycle. This is what makes these simple Packo acceleration numbers quite useful.

At the equal levels moment the water speed will typically be a little over 2 knots on a big tide cycle, or a little over 1 knot for a small tide cycle. The time needed to stop the stream is however generally similar because although the slower stream requires "less stopping", the rate at which the ocean level is changing is also smaller, so the "stopping hill" grows much more slowly.

When the water eventually comes to a halt to give slack water, the "stopping hill" has grown to somewhere between 45cm for the larger tide cycles down to as little as 12cm for very small tide cycle. It should be noted in the first case that this height is quite sizeable when compared to the 110cm downhill slope that got the streams running at 5 knots or more earlier in the tide cycle. Even for the smallest tide cycles the 12cm high "stopping hill" is still a sizeable fraction of the 30cm "starting hill" that is the maximum level difference on those days.

These water level differences are spread out over a horizontal distance of around 15km and so are generally not observable by eye. Anyone that tells you they have seen a 100cm level difference across the entrance reefs is bullshitting. At most it is around 35cm, but that is plenty big enough over a short distance to give a very fast current.


***** EVIDENCE? *****

Evidence that the above story is substantially correct is easily seen from any examination of the various ocean and Bay tide gauge curves after they have been adjusted to a common datum level such as MSL or AHD. Comparing curves whilst each is referenced to its own individual gauge zero level is useless in helping to understand what goes on. A week long set of such tide curves can be see at this link:-

7-days of tide curves

Additional support comes from simple high school trigonometry. For many tide cycles in the year, the ocean tide curve outside the Heads is close to a Sine Wave curve with equal positive and negative amplitudes around a zero level that lies close to mean sea level.

During such a Sine Wave cycle the tide height passes through zero at angles of 0, 180, and 360 degrees with the maximum height (high tide) reached at 90 degrees and the minimum (low tide) at 270 degrees. The whole cycle of 360 degrees in angle takes about 12.4 hours to complete, or close to 29 degrees per hour.

Tide tables tell us that on these days the maximum Bay height reaches about 40% of the maximum ocean height. For the ocean to fall from 100% amplitude down to 40% amplitude, the Sine Wave angle needs to change from 90 degrees to 156 degrees. This requires an angle increase of 66 degrees, or 66/29 = 2.27 hours for the ocean height to fall from its max down to the Bay's height, and so produce the true "equal levels" moment. This moment is typically between 40 and 70 minutes before the actual slack water time.

Note all normal, well adjusted readers, should now skip straight to the ***** marker below. This next paragraph is for nerds only.
SOME NERDY THOUGHTS: During the claimed uphill flow phase, won't the incoming water continue to raise the Bay level? (TRUE: it goes up by around 6cm in that time). Therefore shouldn't the "equal levels" calculation be targeting a slightly lower ocean level than 40% amplitude because the Bay level will be a bit lower at that time? (TRUE: a better figure to use would be 32% amplitude, giving an equal "levels moment" more like 71 degrees or 2.45 hours after ocean max.) Doesn't this mean your claim of "a little over two hours" is a bit off? (NO: That original claim was relative to Pt Lonsdale hi/lo times whereas your improved calculation of 2.45 hours is relative to ocean hi/lo times. These run about 15 mins earlier than Pt Lonsdale times so the original claim of "a little over two hours after Pt Lonsdale hi/lo" is still ok. Thanks for you sharp eyed attention, and yes I did gloss over that point in the hope no one would notice!

*****
Conversely if the "official line" was true and the ocean fell for a little over 3 hours before it equals the Bay level to create slack water, then that Sine Wave angle would be close to 180 degrees (or 0% amplitude) with its height at slack water being close to mean sea level. Since the ocean rising from its low tide point for a little over three hours would also give a level close to mean sea level, the implication of the "official line" is that the Bay should experience virtually no tides at all! The story just makes no sense whatsoever and yet so many organisations just keep on publishing it.

The "official line" also claims that:

1) At flood slacks: Bay level = Williamstown high tide level = Ocean level.
. . . . . . . and
2) At ebb slacks: Bay level = Williamstown low tide level = Ocean level
. . . . . . . but then also claims that:
3) All slacks occur around the Ocean's mid-tide level.

Points 1) & 2) are incompatible with point 3), yet simple observation of the flat reef platforms around Pt Nepean or Pt Lonsdale show that 3) is very close to the truth because no matter which type of slack it is, those reefs will either be just submerging (ebb slacks) or just emerging (flood slacks) with quite a small height difference. (Unless the high and low tide levels on either side of the slack water time were vastly different in amplitude.)

Assuming a sizeable tide cycle, the only story to fit all the facts is:

a) The ocean and Bay levels become equal a little over two hours after high or low tide at the Pt Lonsdale tide gauge.

b) Between this time and the slack water time approximately 50 minutes later, the stream runs slightly uphill and is slowed down to a stop mainly by this reverse slope.

c) Slack water occurs when the ocean is around its mid-tide level, but the water surface near the entrance slopes either up or down to a different water level in the main body of the Bay on the northern side of the Great Sands.

d) At a flood slack the level in the main body of the Bay is around 30cm higher than the ocean, and is near the Williamstown high water mark.

e) At an ebb slack the level in the main body of the Bay is around 30cm lower than the ocean, and is near the Williamstown low water mark.

The only problem some people might have in digesting this vastly better story is perhaps getting their heads around the fact that a slope in the water surface exists even when the water is completely stopped. The problem here is that the very low acceleration rates involved are way outside our normal everyday human experience. Lets look at something slightly more "everyday" that we might relate to more easily.

Consider driving a car down a level road at some speed, and then meeting an uphill slope with a gradient of 1 in 30. Suppose you slip the gears into neutral at the bottom of the hill and then watch what happens. The car looses speed as it rolls on up the hill.

The forces acting to slow the car include air friction and the car's rolling friction, but these will drop away quite quickly as the car slows down. What does not drop away is that roughly 1/30th of the cars own weight (or perhaps around 50Kgs weight of force) is acting horizontally in the direction of the down-slope and steadily slows the car down.

Eventually the car comes to a brief stop but then starts to roll backwards down the hill. The driver will have sensed a brief "slack motion" moment where the car's speed in either direction was never above 0.5 knots (or 0.9kph). This period lasts only about 1.6 seconds, during which the car has only moved just 10cm forward, and then 10cm backwards down the slope.

The rate at which the car decelerates to a stop and then accelerates backwards down the hill is the same, but physicists would call it a constant acceleration in the negative direction (opposite to the way the car is facing). Now lets return to Port Phillip Bay and compare the car journey to a tidal stream motoring through the Heads but approaching slack water time.

Near the Heads, typical average water surface slopes around slack water are 1,000 times less, with gradients around 1:30,000 (or 0.5m over 15km). The rate at which the water speed changes will also then be 1,000 times less. In turn this means the time period spent below the +/- 0.5 knot limit will then be 1,000 times more. That is 1.6 x 1,000 = 1,600 seconds or about 27 minutes - enough time for a safe dive in currents always less than 0.5 knots.

Note however that since these weak currents are in action for 1,000 times longer, the total back and forth distance the water moves in these 27 minutes is now 1,000 times more than the car moved, ie. 0.2m x 1000 = 200m. Some swimming is needed to "stay put", especially near the beginning and end of the dive. The numbers used in this example correspond to an acceleration rate of -0.37 knots per 10 minutes, which is a typical mid-range value for the Heads.


***** WHAT IS THE POINT OF CHASING DOWN THIS PARTICULAR RABBIT HOLE? *****

While this very commonly believed "official myth" is not really dangerous like the "very delayed slacks" myth mentioned earlier is, there are three reasons why I think it should also be killed off.

Firstly one would like to think that when government bodies attempt to explain something to the citizenry, the explanation given should be reasonably close to the truth, not contain obvious logical inconsistencies, and be supported by some easily measurable data. The "official line" fails on all these counts.

Secondly I am trying to promote the notion that slack water reversal or acceleration rate predictions are useful numbers to have alongside the tide reversal time predictions. They may improve the safety of those who "muck about" at or near the Heads around slack water time. This ambition becomes a rather hard-to-sell prospect while the "official line" maintains there is zero-slope in the water surface at slack water and hence zero acceleration at that time.

Thirdly the "official line" may lead Bay users into wrong and somewhat unsafe beliefs like:-

i) There is a distinct and sizeable "slack water period". (Wrong! - it is just a brief minute or two in time.)

ii) There is zero water acceleration at slack water and it will increase later. (Wrong! - water acceleration is near its maximum at slack water, and it will decrease [slightly] later.)

iii) A water sports person can "loll around" in the Rip for a while at slack water until the level difference develops sufficiently (at around 30cm/hr) to finally start the water moving in the reverse direction. (Wrong! - at slack water a 30cm level difference may already exist and the water is already accelerating in the reverse direction at close to its maximum rate. If you "loll around" for an hour in the Rip on a big tide day the level difference may have grown to over 70cm with a current of up to 3 knots flowing.)


***** LEFT STANDING AT THE ALTAR! *****

From late 2015 to mid 2016 these issues were taken up with PoMC. After a bit of back and forth, and them reanalysing their tide records after correction to a common datum, and talking with their external tide consultants, we got to a point where they agreed that the long standing explanation of how slack water occurs given in the VIC tide tables should be rewritten for the next edition.

That was to be the 2017 edition. Alas the Port Privatisation Act was passed in 2016 and PoMC got split into private and public (VPCM) parts. The guy interested in the rewrite got shifted into the private sector and away from the tides area. For a while nobody really wanted to assume responsibility for the VIC tide tables and the job eventually got hand-balled to the Victorian Regional Channels Authority (VRCA) based down in Geelong.

This is a much smaller outfit and a little short on manpower. Apparently there was some dispute with the privatised part of the Port of Melbourne over copyright issues to do with all the previous editions "how tides work" stuff, and maybe any new slack water words. The VRCA was unable to use any of it. Instead they wound up publishing a very stripped down (and rather Geelong centric) version of the VIC Tide Tables late in December 2016. It was as if I was left standing at the altar and the bride had done a bunk!

All explanations about tides that were a feature of the previous editions simply disappeared, leaving only a general "Tide Glossary". It's definitions of flood and ebb streams were also particularly unhelpful for the Heads area because they did not make it clear that the "rising tide" associated with a Rip flood stream is not the local tide level (which actually falls in the 2nd half of the flood stream), but the tide level in the distant central and northern Bay areas. This is always an area of confusion and the glossary only "muddied the waters" in my view.:(

Also missing was the important statement that "Slack water occurs at the Rip and throughout the South and West Channels at approximately the same time".:( I know that statement, appearing in numerous editions of the VIC Tide Tables, has saved many a diver (scubadoctor crew excepted) from believing in the dreadful "very delayed slacks" myth mentioned earlier.

It was a little tragic that all advice (even the slightly wrong stuff) has disappeared. I know from numerous discussions with a wide variety of people that some good guidance from an official and believable source is desperately needed for many who venture onto the waters of the southern end of the Bay.


***** A NEW BEGINNING? *****

In recent weeks I went searching for a new potential bride among Australian Hydro, BoM, and VRCA. Aus Hydro thought it was a state level matter, and that ANTT had carried its Rip explanation for many years. Besides ANTT was aimed at SOLASS class vessels with masters and crew who knew much more about tides than did your average recreational Joe Blog in his tinnie. (I took that to mean it is ok to lie to these well educated men because it won't cause confusion or impact the safety of their operations in any way.)

So it was one brush-off and two no replies. The "VIC TIDES 2018 Edition 2" from the VRCA is due out very soon and it will be interesting to see if there are any new inclusions, or whether it is a "bare bones" booklet again. They used to distribute it via "Boat Books" in St Kilda ($10), but this has now closed and not sure what new arrangements there are.

They do also kindly release a free pdf version downloadable from the VRCA website. I also understand the hardcopy version might be available as a free pickup from the VRCA's Geelong office. (Beware it is very Geelong centred, and didn't have an index in 2017 - with the RIP predictions somewhere in the middle of the booklet).

Among all those who publish Rip predictions, there seems to be a sense that because all outlets use very similar words in their description of the events around slack water, they all somehow "reinforce and confirm" that story. It is more likely that all of the similar sets of words are derived from one single "bad story", probably beginning way way back in the very long history of the Port Phillip Sea Pilots company.

It might be that the words are many dozens of years old and from a time when many things were unknown. It was probably an "it seemed ok at the time" situation, and nobody has really thought about it much since.

An added irony for me is that Cardno who actually do the Rip predictions that ANTT, BoM, and VIC TIDES all publish, are fully on board with the "better slack story" outlined above. Their acceleration numbers are pretty close to mine and they have published a number of academic papers that show this is the correct way we should be thinking about how slack water occurs.

However in some of their earlier papers the slope at slack water seems to be disguised in various ways because the author's (incorrect) intuition got in the way: "Intuitively, these times (ie. equal levels moments) might be expected to correspond to times of minimal water movement due to the lack of driving force through pressure gradients." This is not correct as the physics laws say only "no driving force = no acceleration" without reference to movement (ie. velocity), which can be any constant value.

So we see that at all levels it is this incorrect hangup that "no level difference should imply zero speed" (or vice versa) that seems to be the root cause of the frequent misunderstood writing about, and the widespread happy acceptance of the "official myth" that the equal levels time and the slack water time are one in the same. Can this ever be changed?

I realise that changing the mindset of the thousands and thousands of divers, boaties, yachtsman, fishermen, etc who have grown up on the "official story" has a multi-decade time frame. I just thought it should start somewhere, and since divers are the group most heavily reliant on accurate Rip information, they might just take the lead.

Try talking about it with your dive buddy the next time you have some spare time with nothing much else to say. I hope you have better luck than I did in finding someone who is the slightest bit interested! I'll have a chew at the Sea Pilots to see their reaction.

Anyway, see if any of the various table columns work for you in some way.

cheers,
packo
Last Edit: 2 weeks 4 days ago by packo. Reason: remove last of Dec 2017 info

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