Understanding Tides and Streams in the Bay

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

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Understanding Tides and Streams in the Bay

Postby packo » Sat, 24 Mar 2018 2:29 pm

Hopefully a post will appear here soon(ish) that looks at common problems in understanding tides and streams in Port Phillip Bay.

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Re: Understanding Tides and Streams in the Bay

Postby packo » Fri, 25 May 2018 7:18 pm

The aim of this post is to clear away some of the confusion about tides and tidal currents in Port Phillip Bay and its entrance. Getting a reasonable understanding of this system is not actually that hard, but has been made far more difficult than it should be by a number of unfortunate circumstances including:-

******** #1 - OFFICIAL BULLDUST ********
An incorrect account of current flow through the Heads, first published over 50 years ago is still being published today by the "official bodies"! These include the BoM, VPCM, the Port Phillip Pilot Service and in the Australian National Tide Tables (ANNT) published by the Australian Hydrographic Service.

I have tried hard in recent years get this rectified but have not succeeded yet. I came very close a couple of years back, but the part privatisation of the Port of Melbourne stymied that attempt at the last moment. Currently talking to AusHydro to see if they will fix it.

Update 11/09/2018: Well it seems AusHydro didn't actually investigate and because the Heads are in VIC state waters didn't want to "tread on toes" by doing so.:( So the issue was taken back to some higher ups in TSV/MSV and also sent back to VicPorts (VPCM).

After what seems like an eternity I received an email from the Melbourne Harbour Master a couple of days ago. The good news is that it says that after a comprehensive review of my long trail of correspondence going back several years:- "there would seem to be some merit in rewriting the Port's advice to mariners on Tidal Streams at Port Phillip Heads". Hallelujah!

I've been asked to draft a few paragraphs to form the basis of a new wording to replace the current wording in VicPort's "Port Information Guide". The Harbour Master goes on to say that the new explanation will also eventually be pushed out to the other publishers of "Rip info" such as BoM, the Port Phillip Pilot Service, and AusHydro. Hopefully the relevant websites can then be amended but I'm afraid the previous misinformation has spread so far and wide that it will be decades before it dwindles to just a bad memory.

So it looks like dogged persistence does eventually pay off. Not sure of any timetable for the roll-out of this change and I guess I'll have to show some more patience. In the meantime the trick will to balance what I would like to say, with what they might be prepared to accept, given they view these publications as aimed at ships master's rather than recreational boaters and divers .



******** #2 - DIVER BULLDUST ********
Some old and very persistent diver folklore exists about the size of the time delay between slack water at the Heads, and slack water at sites further inside the Bay. The delays are actually quite small and generally no more than about 15 minutes at Rosebud. However many older divers, including the ScubaDoc crew, insist they can be up to 3 hours.

This nonsense has confused many, is very unhelpful, and is also quite dangerous. Please ignore this advice if you hear it. It is a view that seems very hard to completely kill off despite the overwhelming evidence and science against it.

******** #3 - "LOOSE TALK" AND FISHERMEN BULLDUST ********
Many folk have a poor understanding of the proper distinction between Tides and Tidal Streams. This seems to have come about by a common practice of using the term "Tide" in an ambiguous way to refer to both things. They are certainly not the same thing, but relying only on the context to figure out which one is being talked about in different situations can be difficult.

This practice is very common among fishermen. Perhaps they are trying to sound like "old salts" rather than nerds. Often they can be heard to use the phrase "the turn of the tide". What does that mean, the reversal of the tide from rising to falling? Or the reversal of the tidal stream from inflowing to outflowing? It could mean either!

In the upper reaches of Port Phillip or Western Port these two events do often occur at nearly the same time so the ambiguity is not such a problem. However near the Heads these two "turn of the tide" events happen about as far apart as you can get!

For divers in particular it only breeds confusion to use the word "tide" for both things. So it is best to stick to the separate names "Tide" and "Tidal Stream" and sound like a nerd rather than an old salt. Other folk will secretly thank you for being clear in what you are saying.

Alternatively many divers use the term "Current" instead of "Tidal Stream". That can be ok, but oceanographers tend to use the term "Current" as meaning "water movements not necessarily related to the tides" (eg. the East Australia Current). They stick to either "Stream", "Tidal Stream", or "Tidal Current" for any rhythmical sideways water flow that is connected with the tides.

The important relationship between a Tide and its Tidal Stream is a little tricky in that it can vary enormously for different seafloor profiles and coastal geographies. We will explore this very important point later.

******** #4 - "IT'S ALL ABOUT THE HEADS" BULLDUST ********
Many people believe it is just the narrow Heads that controls water flow into an out of the Bay. Yes this is a narrow and spectacular constriction, but not the only area that controls the tidal currents. This CSIRO bathymetric map of the Bay clearly shows the vast shallow waters of the "Great Sands" that also play an equally significant role.
Image
We see that not only is the gap at the Heads quite narrow, but the area between Queenscliff and Portsea is not that much wider, and in the main only 10 - 15m deep. It too contributes to restricting the flow in and out of the Bay.

Further inside we meet "The Great Sands" a vast area of less than 5m deep. I will include the large "Southern Sands" area off Sorrento in this category although sometimes this area goes by that separate name. Although a number of deeper channels cut through this shallow "sand delta" and provide extra water pathways, much of the flow is still forced across the sands.

The whole roughly triangular region from the Rip to the northeast edge of the sand delta is responsible for "choking" and controlling the tidal flow. An enlargement of the "choke zone" with its approximate border in pink, is shown here:-
Image
Note the fainter orange lines that divide the "choke zone" region into three parts. The water level difference across the whole zone when a strong tidal stream is flowing divides itself roughly equally across these three parts with around a 40cm drop across each part.

Of course the drop across the thinner Rip section gives the highest water surface gradient and the tidal currents there may reach 6 knots. In the middle section the gradient is milder and the tidal currents range from 4 knots down to about 2 knots. In the Great Sands region the gradient is slightly milder again but the much shallower water restricts the speed of the currents to 1.5 to 1.0 knots across the sands. Tidal streams in the channels may run a little faster at up to 2 knots.

Along the northeast edge of the sands the water depth drops quickly from 5m to around 20m. This sudden four fold change in the Bay's cross-section means the current strength will decrease from around 1 knot to around 0.25 knots once you move into the "Main Body" part of the Bay. Scuba diving and snorkelling activities anywhere in the "main body" zone are unaffected by these weak tidal currents but inside the "choke zone" divers must be aware of the dangers, and particularly at locations closer and closer to the Rip.

After studying a large amount of detailed tidal data the effect of the "choke zone" can be summarized as: It reduces the range of the tide in the "main body" zone to around 40% of the tide range in Bass Strait. In doing this the main body tides are also delayed by around three hours behind the high and low tides just outside in Bass Strait.

However throughout the main body zone the tides have a very similar timing and a quite similar range. Across the choke zone the tides are progressively reduced in range and progressively delayed in time. However these significant tide delays are unrelated to the small delays in slack water timing across the choke zone.

******** OUT WITH THE OFFICIAL BULLDUST! ********
At least fifty years ago someone wrote a simplified and substantially incorrect explanation of tidal streams at Port Phillip Heads. That person was probably in the Port Phillip Pilot Service, or possibly in the Melbourne Harbour Trust of the day.

It is not known if "the simplification" was a deliberate act to make the explanation more understandable to "the common man" of the time (who may not have completed secondary schooling), or if that author just "fumbled the physics" to produce a particularly bad explanation.

Incredibly this still survives to this day and is published by the BoM, in the Australian National Tide Tables, by the Port Authorities, by Search and Rescue organisations, the Port Phillip Sea Pilots, and by a zillion (minus one) internet bloggers.

The "fumbled physics" totally ignores inertia and momentum effects. These are always important when any large mass of anything is moved back and forth. Typical tide cycles shuffle around a billion tonnes of water back and forth between the Bay and the ocean. These mammoth flows are not easy to start, nor easy to stop.

-------- ERROR #1 -------------
The "official line" states that the tidal streams reach their maximum velocity around either high or low tide at Pt Lonsdale. This is more or less true, but the given explanation is that this time is when the maximum level difference between "outside" and "inside" occurs. This is not correct for the following reasons:-

1) The real "driver" of the streams is the ocean level a mile or two outside the Heads and typically this peaks around 15 minutes before the high tide at the Pt Lonsdale tide gauge.

2) Even at this earlier peak ocean time we don't get the maximum "outside" to "inside" level difference because the strong inflow occurring means the mid-Bay level is rising quickly while the ocean level isn't rising at all. So the level difference is already decreasing at the time of peak ocean level.

The maximum level difference actually occurs around 30 to 45 minutes earlier than the ocean peak. This is the time when the initial rapid rise rate of the ocean tide has fallen to match the rise rate of the mid-Bay tide. The level difference, or "driving force" of the tidal stream reaches a peak at this time and will then slowly fall.

However there is an "inertia lag" meaning that at the "maximum drive" point about 10% of the driving force is still being used to accelerate the flow, and only the remaining 90% of the drive force is available to overcome the frictional resistance of water flowing across the "choke zone".

It takes another 40 to 60 minutes before the stream stops accelerating and 100% of the available driving force is used in balancing the frictional resistance. At this point the stream has reached its maximum speed and will thereafter decline in speed as the level difference continues shrinking.

It is simply a fluke that this "inertial lag" of 40 to 60 minutes just happens to be roughly the same amount of time by which the "maximum drive" point leads the Pt Lonsdale high tide time. Thus the "maximum speed" point may appear to coincide with the maximum Pt Lonsdale tide level, but the first is not a direct consequence of the second.

Although the flawed "official line" leads to a 45-60 minute error in the "max drive" time, there are really no safety implications in this. Around these times both the stream speed and drive levels are fairly close to their maximum values and hence changing relatively slowly over time. Because of this, the difference between the "real behaviour" and the "official behaviour" is small enough not to be that significant. However this is definitely not the case for the second error in the "official line".

-------- ERROR #2 --------
While the "inertia lag" ignored by the "official line" doesn't create a particularly bad misunderstanding while the stream is gaining speed, ignoring the "momentum lag" during deceleration of the stream does create a significant misunderstanding. Their claim that slack water occurs when the ocean and bay levels become equal is totally incorrect although it is very widely believed.

The guys who invented this yarn may have seen that on average slack water occurs a little over 3 hours after high or low tide at Pt Lonsdale. They might have then reasoned that if the water is not moving at that time, its surface must be level, and therefore the Bay and ocean levels must be the same at this time.

What's wrong with that logic? Well it turns out the laws of physics say a fluid will only have a level surface if it is not accelerating (ie. its speed is not changing over time - and it doesn't actually matter what that fixed speed is!). If the guy had looked more closely he would have seen that a bit before slack water time the water was moving in one direction, and a bit later than slack water it was moving in the opposite direction.

Divers in the Rip know that if you watch carefully, you will see the change in direction takes only a few minutes. This changing speed scenario means the water is decelerating and then accelerating in the reverse direction. This is enough to invalidate the conclusion that the surface is exactly horizontal and that the Bay and ocean levels are the same. There is in fact a slight tilt to the water surface at this time with some important consequences for diving at or near the Heads.

******** THE REAL "EQUAL LEVELS" TIME ********
Coming at the situation from another direction we know after peak ocean tide, the ocean level is falling from its 100% amplitude point. However the tide level in the Bay's "main body" zone is rising towards its maximum height of around 40% of of the ocean maximum. The falling "outside" and rising "inside" levels become briefly equal to each other at a level of approximately 35% of the maximum ocean amplitude.

Since these tide curves are approximately sinusoidal, trigonometry can tell us that the real "equal levels" time is a little over 2 hours after high tide at Pt Lonsdale. This is nearly 40 to 60 minutes BEFORE the "official claim", and 40 to 60 minutes BEFORE slack water. All this is clearly confirmed by tide height measurements adjusted to a common datum. It is a real pity nobody publishing the "official line" seems to have bothered to look.

The 40 to 60 minute range of the time delay between "equal levels" and "slack water" is to cover the range of maximum stream speeds produced by both the "strong" and "weak" tides. Perhaps a little surprisingly, it is the weaker tides that produce the longer times.

******** SO WHAT HAPPENS NEXT? ********
At the real "equal levels" moment and when the "driving force" drops to zero, we find that the stream is still running at typically a little over 2 knots for a strong tide and a little over 1 knot for a weak tide. The reason the streams are still running at zero drive level is because the frictional forces, which drop rapidly with stream speed, have simply been insufficient to halt the stream entirely by the time the "outside" and "inside" levels become equal.

At this point it is the momentum of a billion or so tonnes of slowly moving water in the southern end of the Bay that allows the stream to continue flowing in the same direction even though there is no level difference to drive it onward. In fact from this point on, the falling ocean level and the rising main body level mean the stream actually begins flowing slightly uphill.

******** WATER FLOWING UPHILL ********
During the 40 to 60 minute run up to slack water the reverse slope in the water surface grows and the "reverse drive" force aids the dwindling frictional force in halting the flow. Nearer slack water this reverse slope becomes the main "stopping force".

Divers will need to accept the fact that at slack water itself, when the stream velocity briefly hits zero, there is actually already a significant reverse slope in the water surface. The fact that this can exist for times around an hour or more may seem counter intuitive but is true none the less. It is just that in human terms, the water mass is so large and the acceleration rates are below our everyday experience. This is how the sloping situation can persist for what in human terms seems "quite a long time".

The level difference at slack water is approximately 1/3rd of the maximum forward slope that existed earlier when the stream was running around 6 knots. In actual height terms the slack water reverse level difference between Bass Strait and the "main body" zone 15km away may be up to 45cm after a strong tidal flow, and down to 12cm for the weakest tides of the year.

[A useful analogy might be to think of a car, bicycle, or skateboard rolling down a slope with no other driving force than gravity. On reaching the bottom of the hill it will have gathered sufficient momentum to push at least part of the way up a following rise. The brief pause that occurs as it comes to a stop on the uphill slope (before then rolling backwards down that slope) is the equivalent of "slack water" at the Heads.]

******** HIGH RATES OF CHANGE ********
Note that although this "reverse driving force" is only around 1/3rd of the previous maximum forward force, because the frictional forces are essentially zero at slack water, 100% of this force is able to be applied to reversing the stream. Therefore we find the stream deceleration/acceleration rates produced by this force ARE HIGHER NEAR SLACK WATER THAN FOR ANY OTHER TIME IN THE TIDAL CYCLE.

This is where the "official line" is so regretful because it incorrectly implies the acceleration rate at slack water is zero. The moral of the story is that following strong tidal streams, "slack water" comes and goes very rapidly with stream rates changing at up to 3.5 knots per hour (about 0.6 knots per 10 minutes).

******** REAL LEVEL DIFFERENCES IN A TIDAL CYCLE ********
The following diagrams show how the real "level differences" behave and is based on the tide curves for 9th January 2018:-
Image
Note that between the equal levels moment at 8:18 am and slack water 42 minutes later at 9:00 am, the reverse slope builds up to rapidly slow the stream from +2.4 knots to 0 knots.

Note also that in the following one hour from 9am to 10am the stream rapidly reverses and builds to -3.1 knots. The "official line" implies nothing like this sort of rapid change in stream strength should occur. It encourages a more relaxed attitude about slack water with the thinking that "things are level" and only changing slowly. That is definitely not the case.

Continuing to follow the ebbing stream up to the following slack water, the diagrams are:-
Image
It is a curious fact that on average the rate at which the current changes is generally lower for ebb slacks compared to the slack following the flood stream. This seems to related to the observation that on average the Bass Strait tide levels fall slightly faster than they rise, and it is the rapid fall that reverses a flood stream more quickly. So although the "ebb slack" may produce lower water visibility, a partial compensation is a longer "dive window" due the lower reversal rates.

******** A SUMMARY OF TIDAL STREAMS AT PORT PHILLIP HEADS ********
a) The "official line" disregards the mass of the water being moved about and assumes 100% of the "driving force" created by a difference in ocean and Bay levels will always be used in overcoming the friction of the moving water. It therefore claims "max speed" is achieved at "max drive", and that "zero speed" occurs at "zero drive".

b) This view is fundamentally incorrect as some fraction of the available "driving force" is consumed in accelerating the stream. This produces an "inertial lag" of around 40 to 60 minutes where the streams speed curve lags behind the drive curve. "Max speed" is achieved around 40 to 60 minutes later than "max drive". Since both stream speed and level difference are changing slowly around this time, it is difficult to observe much of a behaviour difference between the "real Heads" and the "official Heads". There are no significant safety issues around the "max speed" time.

c) As the ocean level heads back towards its half-tide value both the "driving force" and "stream speed" fall. However an even more rapid fall in "flow friction" occurs because a drop in speed down to 50% gives a drop in friction down to roughly just 25%. These low friction values are incapable of slowing the stream rapidly enough. The momentum of the stream keeps it flowing strongly despite low drive levels. When the drive force has dropped to just 10% of its maximum, the stream speed has only dropped to 50% of its previous maximum.

d) At a little over 2 hours after high tide at Pt Lonsdale the real "equal levels" point between Bay and ocean is reached. At this point the stream is effectively just "coasting along" with zero driving force and a fairly low friction value.

e) Over the next 40 to 60 minutes the stream enters an "uphill flow phase" where the changing ocean and "main body zone" levels create a growing reverse gradient which produces a growing "reverse drive". This combines with the decreasing friction values to apply a more aggressive braking strategy to the stream.

f) When the stream finally stops to give slack water, the reverse level difference has risen to about 1/3rd of the maximum forward level difference. Although this reverse force magnitude is a lot lower than before, the frictional force is then around zero so ALL this force is applied to reversing the flow. The result is that the rate of change in stream speed around slack water is in fact higher than at any other point in the tide cycle. This is what divers need to be aware of. Slack water can come and go very quickly on strong tide days.

******** HOW DO I KNOW ALL THIS STUFF ISN'T JUST "PACKO BULLDUST"? ********
That is a fair question! How can you check it for yourself? Probably the best way is to use the VPCM website to examine the real tide curves, or the WillyWeather.com.au website to look at the predicted tide curves for Point Lonsdale and Williamstown. Unfortunately a nearby true ocean location like Lorne isn't available on either of these sites. (Note: Only use the WillyWeather site for the two tide curves mentioned - all others for any nearby locations may have significant height, range, and time errors due to a very crude "estimation algorithm".)

You can use these websites to estimate the two tide readings/predictions at the predicted time of slack water. The catch here however is that each reading or prediction is relative to its own individual zero mark. To correct these numbers to a common datum level such as mean sea level, which allows for a true level comparison, subtract 60cm from the Williamstown reading and 97cm from the Point Lonsdale reading.

The adjusted Point Lonsdale reading should be roughly within +/- 15cm of the new zero (ie. mean sea level), whereas the adjusted Williamstown level may be up to 30cm higher for a flood slack, or around 30cm lower for an ebb slack. Your numbers will clearly show these levels are not equal to each other as the "official line" claims. It will also prove that some momentum driven "uphill flow" has occurred.

For a more direct observation, divers can also note that regardless of which type of slack water it is, the Point Lonsdale or Point Nepean reef platforms will either be just covering (ebb slack) or just uncovering (flood slack) with generally only a handful of centimetres between these levels. On the other hand we know that at these two times the water levels at Williamstown will differ by typically 60cm.

There are also direct contradictions in the "official line" because while it makes these three (roughly true) claims:-
1) Slack Water occurs when the ocean is around its mid-tide level.
2) Flood Slack Water occurs around the time of high water at Williamstown.
3) Ebb Slack Water occurs around the time of low water at Williamstown.
BUT when taken together, these claims are incompatible with its claim that Slack Water occurs when the Ocean and Bay levels are equal.

******** OUT WITH THE "OLD DIVER FOLKLORE" BULLDUST! ********
I don't want to spend too much on this having tried hard before to convince those who believe. Those who do still believe that slack water at Rosebud occurs 3 hours behind slack water at the Heads should think hard about the following points.

a) After a flood slack at the Heads, the tide level at any "main body" zone location starts falling within no more than 15 minutes. How is it possible to claim the tide stream at Rosebud is still running inwards for nearly three more hours when all Bay levels fall over this time?

b) In the three hours after a flood slack at the Heads, about 500gL leaves the Bay through the Heads. Three hours of a flood stream heading northeast across the border between the choke zone and the main body zone would remove a similar but slightly lower amount of water (say 400gL). How is it possible for both these things to be occurring at the same time when removing 900gL of water from the 250 km2 area of the "choke zone" would drop its level by -900,000,000m3/250,000,000m2 = -3.6 metres, (and all in just 3 hours!)

Note that while there are very large water flows through the "choke zone", there must always be a rough balance between the amount of water entering at one end, and the amount leaving at the other end. It is just impossible to contemplate large flows either entering at both ends, or leaving from both ends at the same time as the "very delayed slack" believers would claim. The surface area of this region is just far too small to support these hugely imbalanced flows without very large changes in water levels developing.

The correct situation is that in the three hours following a flood slack, around 500gL leaves the "choke zone" through the Heads, but is largely replaced by around 400gL flowing from the "main body" zone into the "choke zone". The calculated level falls in the two zones are then:-

1) Fall in "choke zone" level = (-500+400 = -100)gL/250km2 = -0.4m (Around the typical 3 hour level drop observed at Queenscliff.)

2) Fall in "main body" zone = -400gL/1720km2 = -0.23m (Around the typical 3 hour level drop observed at any north bay tide gauge.)

So apart from a very brief period following slack water at the heads during which very weak oppositely directed tidal currents will occur, at all other times the tidal currents at opposite ends of the choke zone, or at any mid-point, will all be in the same "inward" or "outward" direction.

******** THE RELATIONSHIP BETWEEN A TIDE AND ITS TIDAL STREAM ********
Tides are the slow vertical up and down movement in water level once the waves crests and troughs are averaged out. Tidal streams are the horizontal back and forth movements of water into or out of an area that bring or remove the extra water needed so the tide height can rise or fall over that area.

Most divers have an understanding of these basics, BUT many miss the vital issue that the location where strong tidal streams are observed is usually far removed from the area where the rising and falling tides responsible for this current are operating. Often this distance may be 20km to 50km away, and these tides often run to a different timetable to the local tides around the area where the tidal stream is observed.

Port Phillip Bay is quite an extreme example of this where after high tide at the entrance, water will continue to flow inward for around 2 hours before equal Bay and ocean levels are reached, and then continue flowing inward for roughly a further "momentum hour". This delay of around 3 hours between high tide at a bay or harbour entrance and slack water is close to the maximum theoretically possible and makes Port Phillip Bay stand out from many other large harbours.

Those operating near the Heads just have to get used to the fact that in the second half of any tidal stream you will have either a rising tide level but out-flowing water, or a falling tide level but incoming water. It also means that anywhere in the southwestern part of the "choke zone" you can't reliably determine if the tide is rising or falling by looking only at the water flow. Nor can you reliably determine if the water is inflowing or outflowing only by observing whether the tide level is rising or falling.

Many folk also have a hangup in the southern part of the Bay that the stream direction can be unrelated to whether the local tide is rising or falling. They might say "Surely if the tide rises to a maximum and then starts to fall there must be a current reversal"?

The answer here is that a change from a rising tide to a falling tide in the most general sense just means that "more water is departing from that location than is arriving". At the far northern end of the Bay the only direction options are "onshore" or "offshore" so the "departure direction" is opposite to the "arrival direction" and it can't do both things at the same time. So when a rising tide starts to fall, there must be a reversal of the tidal stream.

However for southern Bay locations water can arrive from one direction (eg. flow in from the Rip), but depart in the SAME direction (eg. flow onwards towards Melbourne). If originally the flow INWARD from the Rip was slightly stronger than the flow ONWARD towards Melbourne, then the tide height will rise as water accumulates near that location. Later on when the INFLOW from the Rip declines and drops below the ONFLOW towards Melbourne, the tide will start to fall although there is NO CHANGE IN THE FLOW DIRECTION. Only a tiny difference between INFLOW and ONFLOW rates is required to make the local tide rise or fall at the typical rates we see.

A crude test of whether a tidal stream will reverse or not when the local tide changes from rising to falling is to check if the stream directions are basically onshore/offshore, or if the streams have a large component simply flowing back and forth parallel to the shoreline. For any location in the "choke zone" the main stream flow is along the direction of the shoreline and not at right angles to it. This means the high and low tide times in the "choke zone" are not connected to a tide stream reversal.

While the high and low tide times in the "main body" zone are fairly closely connected to "the current reversal event", the very large cross-section of the Bay throughout this area means the tidal streams are so weak here that their reversal can be hard to detect. Indeed further northward they may be so weak as to be dominated by a wind driven current component, making the reversal in the tidal stream component unable to be detected without sensitive instruments.

I have combined the major "Port Phillip Bay Quirks" into this table:-
Image

Note that the table shows that the easiest way to think of tidal streams at the Heads is perhaps to relate them to the "Main Body Tide Change Rate". Indeed this is a simple and direct "cause and effect" relationship. While this is convenient, some deficiencies in the official predictions make this seem a slightly more exact relationship than it is.

Many yachtsmen and fishermen do take high and low tide at say Williamstown as indicating slack water at the Entrance. For those folk this timing is probably good enough. However divers need to be a little more careful. The simple logic here is in effect saying:-
"When it is high tide at Williamstown the Bay is FULL, and no more water can enter. Therefore it is slack water at the entrance".

However as the table above shows, at flood slack water the levels at and near the Heads are falling at their maximum rate. We can't really claim the Bay is "Full" at this time when it is obvious that while the water volume in the north is at its steady maximum, the volume in the south is falling. This means the total volume is already falling and the ebb stream at the Heads is already underway.

Some better logic says:- "At some time a little before high tide at Williamstown, the known volume losses in the south could be counterbalanced by the still slightly rising volume in the north." This would give a true time of "no change in total Bay volume", and so give a better prediction of slack water at the entrance.

Taking a typical estimate of a -12cm/hr fall rate averaged over the "choke zone" at flood slack water, and allowing for the fact the "main body" zone is around six times bigger in area, the compensating rise rate needed for the main body zone is +2cm/hr. Assuming the northern tide curve is roughly sinusoidal, a +2cm/hr rise rate for a typical Williamstown tide curve will occur around 17 minutes before peak tide.

When we factor in that there are large areas of the "main body" zone where tides are known to peak a few minutes earlier than Williamstown, it seems reasonable to expect slack water to occur in the range of 8 to 20 minutes before high or low tide at Williamstown depending on the exact tide cycle. Most of the "Packo Predictions" (using a much more complicated set of calculations) end up in this range.

Many of the "official predictions" do too. However the flood slack predictions are often a few minutes later, and many ebb slack predictions are closer to the Williamstown low tide times. Some days they do seem to get a bit whacky with times a few minutes after the Williamstown times, including a few days where the ebb slack predictions run up to 15-20 minutes behind the low tide times at Williamstown, #1 Point Richards Channel, and even Geelong low tide times.

It is hard to see how this is sensible. Nevertheless it is encouraging that even though the prediction methods employed use totally different approaches, on most days we are only squabbling over time differences of 10 minutes or so for slack water, and a tidal current difference of at most 0.6 knots.

Each approach has its weaknesses. For the "Packo Volume Modelling Method" it is the uncertain tide curve modelling in the southern end of the choke zone where tide gauges are few and far between and both the amplitude and phase of the tide are known to vary with location. Luckily because these areas are quite small compared to the very large "main body" zone, these uncertainties don't seem to affect the results much at all.

The "official predictions" don't have any underlying theory but instead that prediction algorithm has been developed by gathering about 130 days worth of Tidal Current measurements (made over Rip Bank), together with the corresponding Lorne and Queenscliff tide heights.

This data was then presented to a "neural network" computer program which was asked "to learn" how to best mathematically represent the tidal current variations in terms of the Lorne and Queenscliff tide levels. The "neural network" approach has often been successfully used in difficult problems where the details of the relationship are not obvious to a human observer.

My concerns over the "official predictions" are:-
1) Only the Lorne to Queenscliff level difference is used despite the fact that for a flood tide this is only around 55% of the total ocean to mid-Bay difference. For an ebb stream that difference represents around only 40% of the total level difference.

2) Not so sure that 130 days of tidal data captures all the relevant nuances since there are both yearly and multi-year cyclic components within our tides.

3) The original algorithm was actually developed for predicting how the wave climate in the Rip is altered by tidal currents greater than around a knot. It is not certain whether its accuracy was ever tested around zero current. The algorithms adoption for "slack water predictions" came at a later stage.

******** PHEW - THAT ABOUT DOES IT! ********
All of the above covers perhaps more than enough the keen "Rip diver" needs to know. However if you want to better understand exactly how the speed of a tidal stream relates to its tide, then I recommend reading this "jpeg blog":-
The Tides and Streams Relationship

cheers,
packo




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