The Slack Water Period

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

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The Slack Water Period

Postby packo » Fri, 28 May 2021 12:01 am

****** The "Slack Water Period" ******

One unfortunate consequence of the official "Little White Lie" is the belief that "slack water" has an extended period of no water movement. Scuba divers know this is misleading and with careful observation the current reversal can be seen to occur within just 1 or 2 minutes.

As well, the incorrect notion of no or very small forces in play, leads to the view that as the water slows down, the "slowing rate" gets less and less up to the actual slack water moment. In fact in the 30 minutes leading up to slack water the slowing rate grows significantly because of the growing reverse slope. The current speed curve therefore decreases quite abruptly to a stop, followed by a similar abrupt increase in the opposite direction.

So the reality is there is only a slack water "window". Its duration depends on what the water reversal rate might be for that particular slack water event, and what current strength is considered an acceptable "safe limit" at both the start and end of the window.

The safe current limit will depend on the activity being undertaken at Port Phillip Heads. Scuba divers might set safe limits of just plus or minus 0.7 knots (about +/- 20 metres/minute). This allows them to make headway against the current at all times within their safety window. Small yachts can accept larger current limits but must also factor in the need to travel for quite some time and distance, often at reduced headway, to escape from the danger zone in sufficient time.

Other watercraft will have their own safe current limits depending on vessel size, type, engine power, etc. They will also take into consideration the wind and swell conditions at the time to assess their safety in this potentially very dangerous area.

However the Achilles heel in all these safety considerations is that at present the official slack water predictions give only a reversal time. They don't say whether that particular slack water event will be a slowly reversing one or a rapidly reversing one. That information would improve the safety of many recreational folk operating in the vicinity of Port Phillip Heads.

Hopefully a wider appreciation of this issue among those recreational folk who operate in and around the Heads may encourage the relevant authorities to acknowledge the damage "ALWL" may be doing and amend their claims. The "Port Phillip Heads advice" offered by a host of players including, "VicPorts", Aus Hydro, Port Phillip Sea Pilots, and "BoM", needs to be amended.

Hopefully the "Rip predictions" publications could also be expanded to include predictions of the tide stream's "reversal rate" to supplement the existing "reversal time" predictions.

***** An Approximate but Useful Result *****

Note the reverse water slope force grows leading up to slack water, and continues to grow afterwards, but now as a "forward" or down slope force. However the flow's frictional drag force is always opposite to the flow direction, and increases with flow speed. These two combine so that in the period before slack water both are in the same direction but with frictional drag declining and the reverse slope force increasing. The compensation isn't perfect but it does produce a "roughly constant" deceleration force and rate.

Immediately after slack water the "slope force" (now forward in the new current's direction) continues to grow. The frictional drag force also grows as the current picks up speed but is now is in the opposite direction to the slope force. Again the compensation is not perfect but the outcome is still a "roughly constant" force, but this time an acceleration force.

The net result of all this is that during the low current window around slack water, we can use a "rough result" that for most practical purposes the water speed will change by about a fixed amount over equal time periods.

Although such outcomes are not exact, it seems to be a useful result for those who either "play around" at, or pass through Port Phillip Heads. That is why I am an advocate for publishing predictions of "reversal rates" (in knots per 10 minutes or something similar) for each slack water event alongside its predicted reversal time.

Even well inside the Heads these numbers can still be useful to recreational folk, once they are reduced by the appropriate factor. For example at Portsea Hole the rate is roughly 40% of that at the Heads.

***** How Does This New View Of Slack Water Affect Me? *****

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

Some folk who operate around the Heads already try to predict whether a coming slack water will involve a rapid or more gentle reversal, by looking at the maximum stream rates published in the tide tables. This can give some general guidance but there are so many other factors involved in determining a reversal rate that it is a calculation best done by the experts.

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

However note there can be a significant variation of reversal rates among the four slacks on any particular day. This occurs because of the daily or "diurnal" component in the tides due to the relative north-south positions of the Moon and Sun. The effect can be strong in PPB because the filtering action of the narrow Heads suppresses the semi-diurnal (or twice daily) component much more than the diurnal component.

This means the curvature of "main body" tide curves around high or low tide can vary quite a lot over 24 hours if the Sun and Moon are in the same hemisphere during that day. High or low curvatures of the "main body" tides translate fairly directly into high and low reversal rates at Port Phillip Heads.


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