How the Weather Affects Tides and Streams

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

Moderator: packo

DAE avatar
Forum Moderator
Posts: 58
Joined: Thu, 22 Mar 2018 8:34 am

How the Weather Affects Tides and Streams

Postby packo » Sat, 24 Mar 2018 2:33 pm

A post on how the weather affects tides and how to cope with this will soon be ported across from the old dive-oz site soon.

Ok now done! Be aware it is long and needed to be split across three posts! It might be best to tackle it in several parts at a time.

Understanding Port Phillip Bay Tides - How Weather Affects Tides, Streams, & Slacks

********* Intro *********

Actual slack water times at Port Phillip Heads can differ from even the best theoretical predictions due to the effects of the weather. Trying to correct for the weather is a very tricky area, and not well understood by most divers. Even armed with the best understanding of the factors involved, you may get only partial success at best.

Being a Bit Brutal: It is likely the majority of those who dive the Heads region still have at least one, (and possibly two), commonly held beliefs on slack water that are simply untrue and unhelpful. Despite it being said and written a zillion times, (even in government/shipping media), the fact is slack water at the Heads DOES NOT OCCUR when, or because, the Bay and Ocean levels become equal.

Until divers manage to shake off this historical and simplistic myth, there may be little point in continuing with this post right now. Instead they first need to read the last couple of screens of the top post in the packo dive-Oz "final smack-down" thread starting at the heading "*** OTHER STUFF: The Equal Levels Moment,. .".

For those who need to see it in the real world, simply observe water levels at Pt Lonsdale (or Pt Nepean) reef platforms at a mix of flood and ebb slacks. In each case the reef platforms will be either just drying, or just submerging, with the ocean level being roughly the same for any slack to within roughly +/- 10cm except on some days where very unequal tides occur.

In contrast we know the central and north bay areas have water levels at these times close to either high tide or low tide. The hi/lo level difference in the north is typically around 55cm. This shows bay levels are definitely not equal to the ocean level when slack water occurs.

In fact the very brief equal levels moment occurs roughly 40 to 60 minutes before slack water. At this time the stream is still running at around 2 knots. The water's momentum allows it to continue moving in the same direction despite the changes in ocean and bay levels now creating an uphill slope. Naturally this creates a force which begins to retard the flow as now it must climb slightly uphill.

This retarding force grows with the increasing slope as the ocean and mid-bay levels keep moving in opposite directions. During this uphill flow phase the frictional forces quickly drop away as the speed falls. The reverse water slope then becomes the dominant "stopping force" approaching the slack water diving window.

After the water stops briefly, it begins to reverse down the slope. The slope continues to grow, but at a slower rate since the mid-bay level is no longer changing. The increase in friction as the water speeds up is now opposing the down-slope forces. The net result is that the acceleration number, or rate of change of current with time, reaches a slight peak just after slack water. Nevertheless the rate of change during the 20-60 minute diving window can be considered as roughly constant.

I have chosen to measure these acceleration numbers at slack water in units of "knots per ten minutes". It is simply a pragmatic and sensible choice. If given in the more scientific units of "meters per second per second", the numbers would be hard to remember with three extra zeros needed. Any necessary mental arithmetic would then be more difficult.

While novice boatmen have been mocked for incorrectly saying "knots per hour" as a unit of speed, note that any form of "knots per unit time" is a perfectly valid way of expressing an acceleration value. Note also that here "acceleration" is used in the scientific sense with both a value and a sign. Depending on the combinations of the signs of both the velocity and the acceleration, it may be describing either a "slowing down" or a "speeding up" tidal steam.

The height of the hill that develops while the water is being brought to a halt varies between 12cm and 45cm depending on the particular tide cycle. This height is distributed over a distance of about 15km so the slope can't be seen with the naked eye. It is however clearly visible in the tide gauge data once these are corrected to a common datum level. This link shows some tide curves and the "uphill flow phase":-
Seven days of tide curves

It is the "hill height" at slack water that determines the rate at which the tide stream stops and reverses for each particular slack water event. This "acceleration number" not only determines how long the "diving window" is for that particular slack water, but also how vulnerable it is to having its timing pushed around by the effects of the weather.

********* Part 0 - Do I Really Want to be Here? *********

It is a difficult area to learn about, but in the long run the effort may be worth it. A diagram of tides during a large storm is included in the post. Storms greatly exaggerate weather effects and the diagram should help visualise what is being presented. Various other diagrams assist in showing how the theory is applied in practice.

Many divers may not wish to get into this topic. They can avoid the learning effort simply by always making a generous "weather allowance", and arrive at the site 30-40 minutes before the expected "drop-in" time. However long waiting periods can be frustrating and can cause distraction from the key task of monitoring the current as it decreases towards your chosen "drop-in value".

There will also always be situations when for one reason or another you can't make such a weather allowance, or might prefer not to waste that time if there are reliable ways to predict if an upcoming slack water is likely to be early, on-time, or late.

This long post is written for those who might want to better understand the factors involved. Hopefully that may result in better dive planning decisions. Due to its length, the post might best be tackled in chunks of 3 or 4 parts at a time.

********* Part 1 - The Observed Tide Curve *********

The sequence of readings from a tide gauge trace out a curve known as the "Observed Tide Curve". The readings are typically at 6 minute intervals with measures being taken so the short term variations in water level due to waves and swell are averaged out over this time period. This is not always entirely successful. In rough weather the observed tide curve from many bay tide gauges is often quite noisy with many small random height variations evident in the overall curve.

There are three main factors responsible for producing the observed tide curve:- Motion of the Moon, motion of the Sun, and the motions of weather systems down here on Earth. The first two have been determined very accurately and together with long term tide observations at a location, can be used to accurately predict the "Astronomical Tide" height at that location at any future date and time.

Those with a solid understanding of the astronomical tide may wish to skip over Part 2.

********* Part 2 - The Astronomical (or Predicted) Tide Curve *********

Despite the enormous differences in the mass of the Sun and the Moon, the difference in distance from the Earth means their ability to raise tides is roughly similar, but with the Moon's influence being 2.1 times as strong as the Sun's.

Unfortunately some enthusiastic school teachers left pupils with the notion that on Earth the Moon's gravity is stronger than the Sun's because it is so much closer. If that were true, the Earth would orbit the Moon rather than the Sun! In actual fact the Sun's gravitational pull here on Earth is 178 times stronger than the Moon's.

It turns out the tide raising forces do not depend directly on the strength of the gravitational pull of a celestial body, but rather on how much that pull differs between the near and far sides of the Earth. It is because the Moon is so much closer than the Sun that its gravitational pull, even though very much weaker, actually changes in strength by more than twice as much as the Sun's pull changes over the distance of one earth diameter.

This "Differential Gravity Field" is also the reason why two equal water bulges occur for each celestial body, one on the near side directly "under" the body, and one on the directly opposite side of the Earth.

There is a complex interplay between the two sets of tidal bulges because the Sun and Moon appear to rotate around the Earth with slightly different cycle times (24 versus 24.8 hours). This gives us roughly two Bass Strait high tides per 24hr day, with every second high tide occurring on average 50 minutes later each day. However this progressive daily delay can vary between 35 and 65 minutes later each day depending on the phase of the Moon.

Also varying with the phase of the Moon will be the amount by which the tidal forces of the Sun and Moon either reinforce, or partly cancel each other. This gives us either "Spring Tides" of a higher range at maximum reinforcement, or "Neap Tides" of a lower range at maximum cancellation. Although the net tidal forces will peak at both New and Full Moon times, there is a delay of around five days for the oceans to fully respond.

The highest tidal range and shortest slack waters therefore tend to be around 5 days after a New Moon or a Full Moon. That is at day_#5 or day_#19 of a lunar cycle. Similarly, the weakest tides, and the longest slack water times, are generally found around day_#12 or day_#26 of a lunar cycle. Divers should keep this in mind as the duration of a slack water "dive window" can vary by a factor of 3 over a complete lunar cycle.

In addition to the differences in east-west motions of the Sun and Moon causing variations in the tides, movements north and south of the equator also have a significant effect. For example as the Sun moves northward for our winter, it drags its day-side bulge further north. Its night-side bulge will move southward. As the Earth rotates, our southern coastlines are dragged through the smaller edge of the day-side bulge but closer to the middle of the night-side bulge.

This effect gives unequal sizes to consecutive high, or consecutive low tides. This is called the diurnal inequality. It also means the height range between successive highs and lows can change significantly over a tidal day. This means the duration of the diving window at each of the four slack water times will vary over a 24hr day.

The Moon's north-south wanderings further complicate the picture because its north-south cycle is rather rapid at 28.4 days, compared to the Sun's 365 days. In addition, the range of its north-south journey also changes between a latitude band of +/- 18 degrees and a latitude band of +/- 28 degrees over a very long 18.6 year cycle. The Sun's yearly latitude band limit is essentially fixed at +/- 23.5 degrees.

The interplay of the motions of the Sun and Moon give a complex astronomical tide curve with daily, monthly, yearly, and longer term variations. Further minor complicating factors are the slightly non-circular orbits of the Moon and Sun as well as a few other orbital quirks.

Despite all these complexities, future predictions of the astronomical tide can be made for certain locations many months or years in advance. Their height accuracy is around just a few millimetres.

********* Part 3 - The Residual Tide Curve *********

When the highly predictable "Astronomical Tide Curve" is subtracted from the "Observed Tide Curve", the difference produces a third curve called the "Residual Tide Curve". The residual tide curve is a grab bag of left-over positive and negative height differences that are attributable to terrestrial causes. The main contributor here is the weather.

Those weather effects range from the short term passage of squalls and weather fronts, to the usual 3 - 7 day cycle of high and low pressure weather cells, through to seasonal changes, and then to longer term trends such as the El Nino or La Nina climate cycles.

********* Part 4 - A Monster Storm *********

In late June 2014 the biggest storm in perhaps 20 years hit the Victorian coastline. The observed, astronomical and residual tide curves for Williamstown are helpful in understanding what happened.

Note that to a rough approximation, actual slack water at the Rip occurs just before the (red) observed Williamstown tide curve reaches a maximum or minimum, while the predicted slack water time is just before the (green) astronomical tide curve reaches a maximum or minimum.


Significant points during the progress of this storm are numbered in the diagram above and are discussed below. Note the dates and times in the diagram are UTC and 10 hours needs to be added to give the local times as mentioned in the text below.

#0 - The storm began on June 23rd and the residual tide at Lorne rose to +50cm over about 8 hours. At the Rip a flood slack was due shortly after the residual tide began to rise.

#1 - This flood slack was delayed by 80 minutes as the rising residual tide kept pouring more and more water into the bay.

#2 - When the ebb stream did finally begin, it was weak and lasted only 3 hours instead of the usual 6.

#3 - The following ebb slack arrived 90 minutes earlier than predicted.

#4 - A long and enhanced flood stream then followed. The residual tide stabilised at around +45cm for the next 16 hours of the storm. The streams and slack water times quickly reverted back to the predicted times, except of course that all measured heights were 45cm above the predicted levels.

#5 - At 10am on the 24th, another massive surge quickly took the residual tide to a new peak of +85cm. This gave an all time record tide height at Williamstown and flooded parts of the Southbank precinct.

#6 - Luckily the peak in the residual tide occurred somewhat after the peak in the astronomical tide and so spared the area of an extra 20cm of flooding.

#7 - The residual tide fell quickly back to +50cm not long after this and the ebb slack at the end of the outgoing stream was delayed by 140 minutes due to the extra water draining out of the bay.

#8 - At around 8pm on the 26th, the residual tide dropped sharply from +50cm to +20cm, delaying the ebb slack by 2 hours.

#9 - This extra outpouring of bay water also shortened the following flood stream from roughly 5 hours to just 3 hours.

In all the storm event lasted about 4 days after which the residual tide fell to near zero and the observed tide curve quickly returned to follow the predicted tide curve to give normal slack water timings.

In storm events like this we have a large residual tide anomaly, usually called a "storm surge", being superimposed on top of the normal astronomical tide to give a "storm tide". Note that at Williamstown, the best efforts of Moon and Sun can raise the water level by about 35cm above mean sea level, whereas large storms like this one can raise bay water levels by 85cm above normal.

The timing relationship between a storm surge and the astronomical tide is completely random, and so the severity of the resulting storm tide depends on chance coincidences between the timing of peaks in the surge, and peaks in the astronomical tide.

The same is true for the effects on streams and slacks. Had the timing of that storm been slightly different, any of the following could have occurred:-
- The maximum Rip current could have exceeded 10 knots.
- The flood slack could have been delayed by up to 5 hours
- The ebb stream could have been entirely obliterated, with water flowing in continuously for nearly 18 hours.

While the drastic effects of storm tides on beach erosion are related to the height of the residual tide, the effects of changing the strength of a tidal stream or of changing slack water timing, depends instead on the rate at which the residual tide changes with time.

In this storm the bay-wide residual tide height was changing at sustained rates of up to 9 cm/hr. The relationship between Rip current and average Bay height change rates is roughly:-

1 knot of Rip current <==> 3 cm/hr bay level change

Note this relationship is fixed for our particular combination of bay surface area and Rip cross-sectional area. It holds regardless of whether the bay level change is caused by the residual tide, or the astronomical tide, or any combination of the two.

Bay water height changes as seen in this storm correspond to a sustained "residual current" (or "weather current") component at the Rip of up to 3 knots. It is not surprising that streams and slacks are significantly altered during big storms.

The story of this storm is to reinforce three important weather effects:-

a) When the residual tide is rising, flood slacks are delayed and ebb slacks arrive early.

b) When the residual tide is falling, ebb slacks are delayed and flood slacks arrive early.

c) If the residual tide is not changing, even if it is at a very high or very low level, slack water timings are not affected.

********* Part 5 - Everyday Weather Effects *********

Although this storm was an extreme example, it turns out that even in ordinary weather the residual tide level can move around by up to +/- 4cm/hr on some diving days. This is equivalent to a "weather current" of +/- 1.3 knots.

Note that since the normal astronomical tide current, and any weather current exist totally independently of one another, the total Rip current is simply the sum of the two components, taking into account their individual signs.

So the new time of slack water in the presence of a "weather current" is simply that new time when the total Rip current is zero.

If the weather current is roughly constant, ie. slope of the residual tide curve is roughly uniform over the time of interest, then estimating the time correction is fairly straight forward. We first need to know the rate at which the astronomical Rip current changes around slack water. From this we can estimate how long before or after the predicted slack time will the astronomical current component be sufficient to exactly cancel out the weather current. Adding this time correction to the predicted slack time will give a new "weather corrected" slack water prediction.

One of the reasons for producing the "packo predictions" material I have been releasing is to provide exactly such acceleration or rate of change numbers for each slack water event. Even if you don't want to get into the details, it is a useful thing to know that slacks with the higher acceleration numbers are less vulnerable to being moved about by the weather.

Conversely the timing of slacks with low acceleration numbers is far more readily shifted about by the weather. You can save a lot of time by simply adjusting the size of your "weather allowance" to be inversely proportional to the acceleration number of the particular slack you intend diving.

The magnitude range of slack water acceleration numbers is roughly from 0.20 knots/10 mins up to 0.60 knots/10 mins. If the number has a negative sign it means the acceleration force is directed outwards through the Rip. Flood slacks have negative values as the bay level is higher than the ocean level at the time of slack water. The outward directed force is responsible for slowing the last of the flood stream before slack water and also for speeding up the new ebb stream after slack water.

Assume for example the residual tide is changing at +3cm/hr, giving a weather current of +1.0 knot. Now assume that a flood slack with an acceleration number of -0.5 knots/10 mins is approaching. That slack will be delayed by (1.0/0.5) x 10 = 20 minutes beyond the original slack water prediction.

At that new time, the outward astronomical current component will be -0.5/10 x 20 = -1.0 knot which will exactly cancel the +1.0 knot weather current component to give zero net current.

If the flood slack event had a lower acceleration number like -0.2 knots/10 min, then the time delay would be longer at (1.0/0.2) x 10 = 50 minutes. If instead the slack was an ebb slack at +0.2 knots/10 mins, then both the inward force opposing the ebb stream and the inward weather current would combine to cause the slack to arrive 50 mins earlier than the original prediction.

An important point in making an adequate "weather allowance", even if you have no idea what the residual tide is doing, is that for low acceleration slacks it should be very generous. This is because firstly the best "drop-in" time might be 20 mins before dead slack and secondly a weather shift may be say an extra 30 mins earlier (or not! ).

The rest of this post looks at how a diver can get some idea of what the residual tide curve might be doing to try to prevent some unnecessary time wasting or improve some other dive planning decisions. For example if you think you might be running late to get to the site, it may help you better decide if you should press on, or give up and divert to a closer alternative dive site that you may reach in time.

********* Part 6 - Components of the Residual Tide *********

There are several ways that the weather affects the residual tide value. However untangling the exact relative strengths of each of the components is difficult and involves some guesswork.

1) Inverse Barometer Effect: As the high and low pressure cells of our weather systems move across the oceans, the high cells press down harder on the ocean surface and the water level there drops. Under the low pressure cells the ocean surface rises. As these weather cells move through Bass Strait the sea level will rise or fall oppositely to the movements in barometric pressure. In turn this will raise or lower the bay levels relative to the normal astronomical tide curve.

An intense low pressure cell may lead to a rise in the residual tide of up to +40cm. Strong high pressure cells, which are geographically more spread out, may produce a residual tide component of around -25cm.

2) Ocean Wind Set-Up: Winds blowing across an ocean will move some of the water in the direction of the wind. If the wind is onshore, the extra water piles up against the shore and raises the water level there. If the wind is offshore, some water is blown out to sea and the inshore level falls.

These effects on the residual tide is guessed at being -15cm for strong offshore winds and up to +40cm for strong onshore winds.

3) Bay Wind Set-Up: Winds blowing across the bay also affect the bay level by reducing the level on the upwind side and increasing it on the downwind side. If the bay were a closed basin or lake, the level in the middle would remain roughly the same - as would the total amount of water.

However if the upwind "low spot" side of the bay is near the Heads, extra ocean water will enter the bay and try to fill up this low area. The wind redistributes this extra water and the average level rises everywhere. This happens for winds between west and south.

Winds between northeast and east do the opposite. They allow "built up" water near the Rip from the downwind "high spot" to leave the bay and so the average level falls.

The shape of the coastline just inside the Heads seems to suggest that northwesterly winds create a low spot near the Heads and so extra water flows in to raise the average level. Southeasterly winds do the opposite.

Guesstimates of "bay wind set-up" residual tide components are +10cm (NW to SW) down to -8cm (NE to SE). The effect of winds directly from the north or south is a little unclear.

4) Wind Helping/Hindering Rip Tidal Streams: Winds between west and south will help the flood stream and hinder the ebb stream. Both scenarios raise the bay's level above the astronomical tide predictions. Winds from north to east help an ebb stream and hinder a flood stream, leading to levels falling below the astronomical tide predictions.

Guesstimates are from -5 cm to +8 cm with the potential for higher positive values for flood streams if swells in the Rip start to break in strong SW winds.

5) El Nino / La Nina: These longer term weather effects give residual tide components of around -15cm and +15cm. Such components are roughly steady for some months and would not normally influence slack water timings.

. . continued in next post at: Part 7 - Trying to Put it all Together

DAE avatar
Forum Moderator
Posts: 58
Joined: Thu, 22 Mar 2018 8:34 am

Re: How the Weather Affects Tides and Streams

Postby packo » Mon, 26 Mar 2018 2:26 pm

Weather Post continued . . .

********* Part 7 - Trying to Put it all Together *********

How do we put all this stuff together into something useful? Well it is damn hard!

Diving the Rip back in the late 1970's we formed the notion that SW winds brought late flood slacks and SE winds brought early flood slacks. Some divers still use this today but at best it is only a part-truth. Also once the residual tide has changed to a new equilibrium value for the new weather conditions, the slack timings go back to normal and so this rule fails.

It was some years before we finally appreciated that it is not the amount of extra water in the bay that maters, but the RATE at which it either builds up or drains away.

Some divers think it is a "given", that SW winds will raise bay water levels above normal. All our senses seem to say "it seems obvious!" (onshore wind, wind blowing directly in through the Rip, etc.)

However what our senses cannot feel is the atmospheric pressure. SW winds are usually associated with a rising barometric pressure which under 1), tries to force water levels down and so drop the residual tide. So it is then a case of 1) versus 2), 3), & 4), so who wins?

The net result depends quite critically on the exact strength and path of the advancing high pressure cell and the exact strength and direction of the wind. In my long monitoring of residual tides I have seen many cases of the inverse barometer effect winning out in S to SW winds to actually lower residual tide levels, or at least largely balance out components 2, 3, & 4.

Winds with a bit more west in them, or very strong SW winds, will however usually raise the residual tide.

Note the long protrusion of Wilson's Prom into Bass Strait, plus Deal, Flinders, and Cape Barren Islands create a partial constriction across the middle of the Strait. This means that in a sense westerly winds are "onshore", and easterly winds are "offshore" as far as the levels in the western part of Bass Strait are concerned.
Update: (26/3/2018) In a recent discussion with a coastal engineer, it was pointed out that that Bay's susceptibility to high +ve surge levels in westerly gales and high -ve surge levels in easterly gales is enhanced by the "Coriolis Effect" which bends the path of moving water to the left in the southern hemisphere. Consequently during westerly gales the extra water moving into Bass Strait from the west is bent northwards onto the Victorian coast so raising the level there. In easterly gales the the extra water flow from the east is bent southward away from the Victorian coast so lowering levels.

The only scenarios with a "semi-reliable" outcome seem to be:-

NW wind: 1) up, 2) up, 3) up, 4) neutral, => usually raises the residual tide.
W wind: 1) neutral, 2) up, 3) up, 4) up, => usually raises the residual tide.
SE wind: 1) down, 2) down, 3) down, 4) neutral, => usually the lowers residual tide.
E wind: 1) down, 2) down, 3) down, 4) down, => usually the lowers residual tide.
NE wind: 1) down, 2) down, 3) down, 4) down, => usually the lowers residual tide.

Winds more directly from either the North or South often have the individual components working against one another and the result can go either way depending on subtle differences.

********* Part 8 - Hey, we are divers - not freaking weathermen! *********

Even if you become an expert in picking if the existing weather conditions will raise or lower the residual tide level, that is not quite enough. You also need to know if that change is still occurring, or has reached an equilibrium, or has begun to decline.

Remember it is the rate of change of the residual tide that determines the strength of the "weather current" component, and it is this, in combination with the "vulnerability" (inverse of acceleration number) of a particular slack water, that determines the number of minutes by which slack water will be shifted from the predicted time.

Isn't there an easier way to figure out what the residual tide is doing? Well yes there is. Obviously if we could monitor the residual tide in real-time then all the weird combinations of 1) to 5) would be done for us giving a simple final result.

Using that result we could choose the relevant rule a) or b) or c) as given earlier to figure out if the slack timing will be affected and in which direction. Putting a number on the "weather current" and noting the packo acceleration number for the slack can then give you an estimate of how many minutes earlier or later the slack will occur.

********* Part 9 - Real-Time Residual Tide Info on the Web from VPCM????? *********

The PoMC used to have both the predicted and observed tides on their website but they don't any more, and the graphs were poorly plotted and presented separately. I have put forward a case to the new VPCM body that some residual tide plots should be included in their new "wind & tides" pages. Not sure how these discussions will go.

There is heightened sensitivity about publishing such data because in bad weather these plots become "storm surge" plots. With all the controversy over the effects of the CDP on beach erosion in the bay, this is data government agencies would prefer not to be airing.

I have suggested some workarounds to VPCM so they might display either only relative (to 00:00hrs) residual tide plots rather than the absolute heights. Alternatively they could do some additional processing to publish only rate of change curves. This approach could have the advantage that results from a number of tide gauges could be averaged and filtered to try to reduce the "noise" that occurs in the raw data when the sea conditions rise above "slight seas".

Properly done this could be a great step for divers operating near the Heads. However I'm not particularly hopeful of a good outcome on getting VPCM to publish any residual tide info - but the approach was well worth making. I'll keep everyone informed of any developments and update this post if and when needed.
Update: (26/3/2018) Nothing doing here so far. Initially there claimed they couldn't do "real-time" displays because the tide gauge infrastructure is widely dispersed. I've since leaned that Melb VTS officers can bring these near real-time residual tide curves up on their consoles! I've got one or two people at the technical level left to talk to but I don't hold much hope. Looking at past data I also see that in certain weather conditions, a rapidly changing residual tide event can excite a north-south oscillation in bay levels that render the Williamstown tide residual curves "basically useless".

********* Part 10 - Low Res Real-Time Residual Tide Info on the Web from VRCA *********

Meanwhile, the Victorian Regional Channels Authority (VRCA), which looks after shipping for the Port of Geelong, does monitor two tide gauges and provide web page plots of both the predicted and observed tides. A separate plot of the residual tide is also provided. The relevant tide gauge is off Point Richards and a little north of Portarlington.

It might seem rather far from the Rip, but that is actually an advantage as the high and low tide times at this location are not that far off the slack water times at the Rip. This makes it easy to "see" approximate slack water times on the plots and look for trends in the residual tide around those times. Also the residual tide measurements from this gauge are reasonably representative of the very important large bay area north of the Great Sands.

Unfortunately there are a number of issues with this source of residual tide info:-

1) The VRCA's server has a painfully s..l..o..w connection to the internet.

2) In a rebranding exercise some time ago, a "creative person" reduced the size of the graphs.

3) The plots went off-scale in the 2014 storm. VRCA then reduced the plotting scale to even smaller plots and a very awkward calibration. The resolution is now only 1 pixel per 2 cm of height and measuring rates of change is now more difficult.

I am in discussions with the Geelong Port people about trying to double the vertical scaling of their plots and eliminate some of their over-reaction to the 2014 storm. We will have to see how that goes.
Update: (26/3/2018) I met with the Harbourmaster and VRCA CEO over in Geelong some months ago. Despite promises of "we'll see what we can do" nothing has happened. The real problem is that it is only me who is complaining about these and other defects in their real time tide displays.

Update: (4/5/2020) Well some tiny progress to report after years of nagging! The software flaw that dumped about 40 image lines as the original tide plots are copied over to the VRCA's web server for public viewing has been pretty much fixed. This means no more annoying gaps will occur at certain tide heights. However the larger issue about restoring the plot scaling to what it was pre-2014 storm (or even better) continues to get the cold shoulder from the folks at the VRCA.

I have created a URL that pulls the image of the graphs directly from the server, bypassing the image size reduction and overlay issues caused by the actual webpage code. Note that you must reload this image about every 5 minutes to see the latest data.

********* Part 11 - Decoding the VRCA's Residual Tide Plots *********

The dive-oz link for the tide plots is given below. It will load into a new browser window, which you can then move aside to keep this post visible. The image is rather large at 1920 x 980 pixels. Your browser will most likely show this at reduced scale to fit it all in the new browser window. In that window you should click "+" or whatever you normally do to allow full scale viewing of internet images.

The direct image loading URL is:- Direct VRCA tide plot link

The right half of the image concerns Corio Bay tide and its weather. It is of no interest, except perhaps to note that a plot of atmospheric pressure over time is given at bottom right. The left hand Point Richards side of the image contains wind, tide and temperature plots.

Only the "Tide History and Prediction" panel is of real interest, although please note the image date and timestamp shows in the lower left corner. Remember that although their server image is updated every 5 mins, you will need to reload from this link every 5 mins to keep your screen up to date.

The red tide plot shows the predicted astronomical tide curve, spanning roughly from -24 hrs to +24hrs of the image timestamp. The blue tide plot overlaid near the red plot shows about the last 24 hrs of the "observed (or measured) tide". The difference between these curves is the "residual tide". It is shown on a separate plot (also in blue), called the "tidal surge" in VRCA's parlance. The value of the three curves at the timestamp time is given in large text at the left of the tides panel. Unfortunately these numbers are of little value to divers.

Those parts of this (rather noisy) image than can be useful for dive planning are as follows:


Slack Water Prediction: The high points and low points of the red curve loosely correspond to the predicted flood slack and ebb slack times at the Rip. In general these max/min points will lag somewhere between 5 and 20 minutes behind slack. Even though the time graduations are in 2 hour blocks, with care these times can be read off to about +/- 15 minutes accuracy. Take off an extra 10 minutes to get rough Rip slack times. This is good enough to start planning your diving day.

Residual Tide Correction: Well here we arrive at the whole point of this thread! If the residual tide curve is level for an hour or two before the dive's entry time, then you can skip this step as no correction is required. Slack water at the Rip should occur within +/- 8mins of the "packo prediction" time.

If the curve is sloping up or down you need to try and estimate the rate of rise or fall. This can be the most difficult part. Even if the plot is smooth it can be difficult because of the awkward scaling. A straight edge such as the edge of sheet of paper can be laid on the screen with the same slope as the plot. You can then read off how many horizontal units (2.0 hrs each) it takes for the paper edge to rise or fall one vertical unit (37.5cm). With some practice, careful estimates by eye can be sufficient.

Note in the example above there is quite a lot of wave noise in the observed tide. This is transferred straight through to the residual tide curve. The estimated rising slope covering both the 21:00 flood slack and the 03:00 ebb slack, is +2.0cm/hr, indicating a "weather current" component of around +0.7 knots for most of the evening and following morning. The flood slack would have been late, while the ebb slack would have arrived earlier than predicted.

********* Part 12 - Three Worked Examples *********

Example #1:
I'm sure many of you who have persisted this far will only bother to visually check the residual tide curve, mainly to try to answer that vexed question:- Will the slack be early, normal, or late today? Remember that a rising residual tide delays flood slacks, and makes ebb slacks arrive early. A falling residual tide delays ebb slacks, and makes flood slacks arrive early. The full process of estimating weather corrections to slack water times is illustrated in this example:


Example #2:
After a storm which has pulled much extra water into the bay, the weather may rapidly improve so you may be tempted to go diving. Sometimes big storms do bring very clean southern ocean water into Bass Strait with good bay vis on the flood slack.

However be aware as levels drop back towards normal, strong "weather currents" are still possible. The following example shows one such case. Forget the fact that the affected ebb slack was well before dawn in mid winter, it is the only example I had a grab of.


This example shows care is needed in winter diving when often moderate residual levels of 20-30cm may exist. Sudden changes a far more likely to occur than on days when the observed and predicted tide curves closely overlay one another.

Note also in this example that 30 minutes before the predicted slack time, there was no hint this strong negative weather current would soon develop. Indeed at that point a small +ve weather current looked likely. Where possible, take your well protected "tech" out on the water with you so you can make decisions using the most up to date data.

Example #3:
Oh happy days! Happy days are when the residual curve is near zero and mostly flat for the whole day. On this sunny warm day there was in fact a moderate N-NE breeze 13kn in the morning, rising to 20kn early afternoon, then fading off in the late afternoon. Normally the effect of such a wind profile is to push bay levels down slightly. However the barometer was steadily falling across Bass Strait during the entire day and trying to raise levels. A happy balancing act resulted giving no significant residual tide changes for the first 3 slacks. These would have been close to their predicted times.

As the breeze faded late afternoon, the inverse barometer effect began to assert itself. This gave a slight kick up for the last slack. It is hard to see, even in the magnified insert because it has got tangled up with the bold zero residual tide line.


Note that although the numbers did turn out ok in this retrospective view, trying to pick this small late trend in sufficient time would have been difficult out on the water. In these cases knowing the various components that affect the residual tide may allow you to anticipate how a change in weather conditions may affect it.

continued in the following post at:- Part 13 - The Truly Weird Stuff
Last edited by packo on Tue, 27 Mar 2018 12:02 pm, edited 1 time in total.

DAE avatar
Forum Moderator
Posts: 58
Joined: Thu, 22 Mar 2018 8:34 am

Re: How the Weather Affects Tides and Streams

Postby packo » Mon, 26 Mar 2018 2:27 pm

Weather Post continued . . .

********* Part 13 - The Truly Weird Stuff *********

In most cases divers will be out in weather where residual tide changes occur roughly uniformly over the period of interest. This leads to weather currents of approximately constant strength (or luckily zero!). The effect of a constant weather current is a simple time-shift of the slack water, with the current-time profile remaining the same shape but just shifted in time. The dive will then seem perfectly normal except for the new timing.

However if the residual tide plot changes markedly around slack water then not only is the current-time profile time-shifted, but it may also be changed in shape. Sometimes this is for the best - slacks can be extended by a large margin if you are lucky. They can also be dramatically cut short if you are unlucky. It is not a bad safety idea if those left on the boat can keep an eye on the residual tide curve to pick up early on anything extra bad (or extra good) that might be going on below.

Here is an example of very good luck:-

Like me, many divers in this area would have occasionally heard a "weird slack story" from a fellow diver. The events they describe sound too unusual to be true and many of their listeners park these stories in the "bullshit bin" along with the "12kg cray that got away" story.

After a long study of residual tides I conclude they might not have been making up all these (tide) stories. I gather tide data from Williamstown for analysis. Unfortunately this is not real-time data. The water level there is a roughly reliable measure of how much water is in the bay and the sheltered location of the gauge gives a clean signal most of the time. Hundreds of slacks can be roughly monitored without actually being there.

The residual tide curve can have all sorts of wiggles and bumps imposed on it by the shorter term weather events such as fronts, sudden changes in wind strength, or direction, or both. These are superimposed on the longer duration events caused by the passage of high and low pressure cells through Bass Strait which generally takes from 10 to 40 hours to pass.

The overall residual tide curve can behave in rather weird ways. If an unusual event in the residual curve coincides closely with a predicted slack water time, then weird stuff will happen! - like the "super slack" on 17/1/2015.

A far more weird variation on this is also perfectly possible (someday). It is a "triple slack". Here the normal slack just completes and the water has started to reverse. A strong residual tide quirk then kicks in to slow, stop, and then reverse the new flow. This gives a second slack a little after the first one, and a second stream reversal. The predicted tide curve will eventually overpower any residual tide effects to give a third slack water, and then a third water reversal.

The best the average diver can hope to see is a sudden change in the residual tide slope just as the slack arrives with the change being in the right direction to prolong the dive window to maybe double the expected duration.

What the diver definitely does not want to see is the following bad scenario. The residual tide begins to fall quite sharply just before a flood slack is due and before divers enter the water. This fall will really put the bakes on the last of the flood stream and will bring it to a complete stop more quickly than usual.

The divers, still in the boat, are caught a little unawares. They see the rapidly slowing water and scramble to enter. They might think it is just a another "early slack". It is not. It is a potential monster in disguise! Effectively the slack can be over before it has even begun. Curvatures in both the astronomical tide and a downward bending residual tide curve can combine so the new ebb flow will accelerate at double the expected rate, shrinking the safe diving window to less than 10 minutes.

Monitoring the VRCA's tide plots while at sea might help you foresee such unpleasantness, or at least help explain what in the hell is going on if you are unlucky enough to get caught up in a weird and bad scenario.

********* Summing It All Up *********

Understanding and monitoring Residual Tides in near real-time seems to me to be a worthwhile thing for divers to do at least to some extent. Even if the curves are only glanced at to roughly gauge the direction and size of any likely weather shift some benefit will be gained.

Unfortunately the available data is of low resolution and not very comprehensive. Putting good numbers on the rate of change is awkward in the best of conditions. Hopefully VRCA, and maybe the new VPCM, can come to the party and provide timely higher resolution data that will make these weather corrections easier and more accurate.

The main points of this long post are:-

* For slack water adjustments don't try and "read" the weather - "use the plots".

* If observed and predicted curves closely overlay, no corrections are needed!

* A rising residual tide delays flood slacks and brings ebb slacks on early.

* A falling residual tide delays ebb slacks and brings flood slacks on early.

* If a moderate but steady residual curve is seen, no corrections needed - but be vigilant.

* If the curve is swerving all over the place it might be better to stay home.

* Roughly a 3cm/hr change rate <==> 1 knot of weather current at the Heads.

* Weather corrections depend on BOTH the "weather current" AND "packo acceleration value".

* Calculated time shifts > 20 mins should be increased by 20-40% as the average acceleration over these longer times will be lower than the published "peak values".

* Be aware of stronger tides around 3-5 days after Full Moon or New Moon. (day_#5 or day_#19)

* Unwise to dive the Rip Bank area if slack acceleration values are at or above 0.50 kn/10 min

* Use "packo slack times" to calculate weather shifts as official times have too many ebb slack errors in the 10-20 minute range.

If you find monitoring the VRCA's "tidal surge" curve is useful, why not send them an email to say that you do find them useful in improving your safety, BUT you would like to see them at a bigger scale, and without the "gaps" in the curves that occur from time to time.
Update: (4/5/2020) As noted earlier, the "gap problem" was finally fixed in early April. The "bigger scale" request continues to fall on deaf ears at the VRCA.

DAE avatar
Posts: 2
Joined: Fri, 16 Mar 2018 2:11 pm

Re: How the Weather Affects Tides and Streams

Postby peter69_56 » Tue, 15 May 2018 4:46 pm

Packo, thanks for the continual predictions. They have been very helpful. Keep up the good work

DAE avatar
Forum Moderator
Posts: 58
Joined: Thu, 22 Mar 2018 8:34 am

Re: How the Weather Affects Tides and Streams

Postby packo » Sun, 24 Jun 2018 4:35 pm

Hello peter69_56,

Thanks peter69_56 for the positive feedback on the "Packo predictions". It is greatly appreciated. Now I know at least one diver is using them!

Yes after the dive-Oz forum folded I did eventually take up Lloyd's offer of a space here on his resurrected Scuba Doctor forums to publish my tide ramblings. I find the SD forums a little easier to work with because they don't have those posting limitations and the nasty "spammers jail" visits that infuriated me (and you!) on the dive-Oz forums.

The down side is that this forum has a much smaller membership and reach. There seems to be about 60 new members since the resurrection this year, but hardly anyone puts up a post (except me, Lloyd, "diveshop", and "Clubdiver" -alias AB who is also a moderator here).

The low posting rate is quite a shame really and risks a possible forum shutdown in the future. Still as long as the "read numbers" stay up I don't think Lloyd will shut it down for at least a couple of years. I guess it is still very early days yet. There will also be a number of reads coming in from google search results once those who used the "packo tables" back in the dive-Oz days eventually figure out where they have moved to.

The (pre-Facebook) history from 2007 to 2009 shows the Scuba Doctor forums were very active then. I also understand the updated version of the forum code means a much reduced amount of administrator's time from Lloyd is required to keep it all running like a well oiled machine. That alone might keep the forum going even if the numbers and posting rate stay weak. Hopefully the membership and posting rate will improve so he may eventually see "the resurrection" exercise as being worthwhile.

The other down side is that I can't criticise the bad ScubaDoc tide timing advice for dive sites well inside the Bay as stridently as I used to do in dive-Oz forums. It is still a sore point with me as I have provided them with a truck load of evidence that their advice is putting people at risk, but nothing changes.

The executive of DIVA were also a major disappointment on this point. Both the president & secretary were hardened "very delayed slack" believers, despite several other committee members agreeing with me that this idea is rubbish. Unfortunately it seems nobody wants to rock the boat by actually testing the evidence and then correcting the misunderstandings. These still seem to occur in several DIVA member's dive shops.

I thought the diver "near-miss" off Rye in Dec 2017 might have shaken some of these people out of their complacency. That guy was exceedingly lucky to survive after the current swept him over 10km away from his boat and towards the Rip.

He was never in any danger of being swept out through the Rip because the current would reverse before that happened. Rather it was the danger of hypothermia after 4 hours in the water that could have finished him off.

Luckily a sharp eyed skipper from the Sorrento-Queenscliff ferry spotted him out near #2 South Channel beacon and this ended the search around 6pm. (Apparently the Search and Rescue helicopter guys did spot him 2 hours earlier, but then lost him again in the choppy conditions.)

The same sort of complacency occurs when I try to get some corrections to bad numbers in the SD GPS divest database. I had a real struggle to get the Cape Woolamai Pinnacles coords repaired. After three or four attempts to get the also very bad Lonsdale Wall coords fixed I simply had to give up. That now seems to be a lost cause. It is a real pity because the SD dive site database is a potentially an excellent diver resource once the bugs are ironed out. That is probably going to take quite a while from my experience.

I even raised again "peter69_56's" point that the Scuba Doc "Isa wreck" mark is actually to AGD66 datum and not to the WGS84 datum as stated. Charts confirm this to be the case, but again that suggested correction was ignored (along with a few others!). I thought it was a worthwhile to raise again because the bad SD info invites divers into Swan Island's "strictly no-go" restricted military area. I think this is an anti-terrorism training area - not the sort of place you would want to accidentally blunder into!

I might bring up the suggested corrections again next summer when the readership might be higher. It seems only support from other divers will get these over the line as apparently "packo credibilty" is rather low as far as the Scuba Doc crew are concerned. For now I have simply given up on this front because I have plenty of other slightly softer brick walls to bash my head against!

Update March 2019: Well another attempt in March 2019 seems to have borne some fruit. Scuba Doctor's "Lonsdale Wall" and "William Salthouse" marks have now been fixed! I'll leave the "Isa Wreck" issue alone - I suspect hardly anyone dives it, and the effort to achieve a correction is probably not worth it.

******** weather adjustments to slack water prediction times ***********

The method I gave in the original post for estimating timing corrections seems to have attracted quite a few "views". Whether they digest the whole long post or not is unknown but it least it would have highlighted the importance of this issue.

It was a real pity the VRCA hasn't come to the party and given us better resolution in the tide plots and also correct a number of significant defects in the rendering of their graphs (including getting rid of those damn data gaps!).

I made a special trip over to Geelong to address several members of the VRCA board. Despite some promises of "we'll see what we can do", nothing has changed.

It seems to be the case that when government functions are "corporatised", then nothing gets improved unless there is a direct and immediate benefit to the bottom line. Any "for the general good of the population" arguments seem to get tossed aside very quickly.

I am now also recognising that it seems that accurate corrections in good stable weather are actually more problematic than in weather where there is a definite and more or less steady weather current flowing. On some days when the residual tide plot appears "quite flat", there are in fact varying weather currents that can still alter slack times but do no show up in the plot.

For example if a +0.5knot weather current suddenly springs up a bit before a slack is predicted, it is easily capable of producing a 10-20 minute time shift of slack water. However unless such a current lasts for an hour or more, it won't always show up in the VRCA plots because these only have a 2cm resolution in the residual tide height. (a +0.5knot weather current raises the residual tide in the north of the Bay at a rate of only 1.5 cm/hr).

So unfortunately my correction method on "flat" residual tide days is not as good as I first thought it was due to these unseen, small, but variable weather currents.

These "wobbly weather currents" on otherwise good weather days have also played merry hell with me trying to confirm whether "packo" or the "official" slack water predictions are best. It is hard to be sure tar the weather effect is much less than the time differences in the two sets of predictions.

Luckily recently I did get some good conditions in late June for a few slacks which from all available evidence suggests occurred while "weather effect" was either nil, or too small to worry about.

More luckily still these days also had both ebb and flood slacks with fairly large 14 to 16 minute differences between the "packo" times and the "official" times. I'll show those results later in another post when I have completed the analysis. However preliminary results suggest the "packo" times were better ones on those those days and perhaps may even have to be tweaked a minute or two even earlier earlier.

In slightly less than idea weather it appears "wobbly weather" is quite common, with the corresponding wobbly weather currents varying between +0.5knots and -0.5 knots and occurring for just enough time to alter the slack time but not long enough for them to be detected in the plot in sufficient time. :( It seems getting weather corrections any better than +/- a dozen minutes is likely to remain out of reach when using the "residual tide curve slope method".

The only way forward is to detect these wobbly current components through real time actual current measurements and then comparing the size of the measured current with the theoretical astronomical tidal current at that time. This is a fairly complicated way to solve the problem but I will slowly chew it over and do some testing on the equipment required and the practicality of doing this.

In the meantime I will advise divers to also take note of the numerical residual tide values printed on the left of the VRCA Pt Richard's residual tide plot. These appear to have a better resolution of +/- 1cm and actually update 5 minutes ahead of the plotted points. Using a record of these numbers may help detect small trends more quickly and accurately.

Some port authorities already do real-time tidal current monitoring (eg. Lakes Entrance) but the Port of Melbourne people say real-time current measurement in the Rip are too difficult due to the often severe ocean conditions. However I think working well inside the Heads and making the proper time allowances is a possible way forward. I'll keep you all informed of any progress I make here but it is bound to be very slow going!


Who is online

Users browsing this forum: No registered users and 1 guest