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The Bay's "Balance Line"

Posted: Thu, 27 May 2021 11:51 pm
by packo
***** Port Phillip Bay's "Balance Line" *****

Many yacht skippers, fishing folk, some scuba divers, and even marine authorities subscribe to the simple idea that slack water at Port Phillip Heads after a flood tide occurs at the same time as high tide at Williamstown at the northern end of the Bay. The logic here is that "when the Bay becomes full, no more water can enter and so it will be slack water at the entrance".


Referring to the image above, it is quite true that the large "main body" zone might be "full" very close to the high tide at Williamstown.

However due to the progressive 3 hour tide delay across the "connection region", waters near the Heads have already fallen to half-tide at this time and are dropping very quickly. All the waters across the connection region will also be falling, but at somewhat slower rates.

So at the exact time the "main body" region reaches its "fullest" (or maximum water volume), the total Bay water volume will actually be falling because of the falling tide heights across the "connection region". This means the Bay has already started draining into Bass Strait with slack water already over, and the outflowing ebb tide is already underway.

***** Side Comments On the "Filling" and "Draining" of the Bay *****

These terms are not really appropriate to use in the context of the "main body" region. The tides in this region rise and fall not so much by "filling" and "draining" but rather by its waters being "squeezed" or "expanded" by water movements back and forth across the connection region. No water from the "connection region" flows into, or departs from, most of the "main body" except for a narrow strip (a few km wide) along the border between the two regions.

These very short range water back and forth flows between the two regions acts like a piston to alternately squeeze, and later expand, the volume available for the "main body" water to reside in. This in turn causes its water level to rise, and later fall. This produces the tides we see in the north.

There is very little mixing of water between the northern and southern regions and the "flushing time" for the north is around a year or more. Despite the continuous fresh water inputs from rivers and creeks in the "main body" region, it remains a marine environment only slightly less salty than the ocean. This is largely because of an almost equal balance between evaporation losses, and river inflows. In addition the small amount of water mixing that does occurs helps keep the northern salt concentration fairly stable.

***** The Volume Balance *****

At the moment of either of the two slack waters, no water is entering or leaving through the Heads. Therefore the total volume of water in the Bay must be at either a minimum (slack after Ebb), or a maximum (slack after Flood). As mentioned river inputs and evaporation losses can be ignored here because they are relatively small, and tend to largely cancel one another out.

So at slack water at the Heads, the two regions within the Bay will be gaining and loosing water at exactly the same rates. This water movement occurs simply by virtue of the residual momentum within the tidal stream that is still just running slowly from one region to the other. Slack water within the middle of the Bay occurs roughly 8 to 18 minutes after slack water at the Heads.

***** The Bay's "Balance Line" *****

At Heads slack water time, the exact location of the dividing line between the rising waters in one part of the Bay and falling waters in the other is a little uncertain. To do this we need to establish all points that have their high tide times at the same time as flood slack water at the Heads, or all points that have their low tide times at the same time as ebb slack water at the Heads.

The tide tables can help but in the real world the precise time of high or low tide for the smallish tides inside the Bay is difficult to accurately observe to within 10 minutes or so. Nevertheless the predictions do suggest the "Balance Line" lies close to the northern edge of "The Great Sands", but will move around a bit with different tidal cycles.

This view is supported by looking at the average tide rise and fall rates at Heads slack time, and the surface area ratio on either side of the line. This area ratio is about to 6 to 1 which is about the inverse of the average rise and fall rates of the two regions.

Despite some uncertainties, the "Bay's Balance Line" is a useful concept to better understand the Bay's behaviour, even if the details of exactly where it is for each tide cycle are not perfectly clear. At slack water after a flooding tide, the "balance line" joins all locations having their high tide then. At slack water after an ebbing tide, the "balance line" joins all locations having their low tide then. These diagrams show the concept:-


The "balance line" concept is one tool I am trying to use to advance the argument that some of the official ebb slack water times might be slightly too late. Many of those times seem to place the "balance line" so much further up the Bay that there is insufficient surface area to provide the necessary compensation for the strongly changing water levels in the southern part of the Bay near the Heads.

Due to weather effects on slack water timing it seems unlikely that the case can be proven by measuring slack water times on an infrequent and ad-hoc basis. It really needs continuous monitoring over several months before a statistically significant result can be extracted. This is probably beyond my level of "citizen science" capability.

In the meantime it would seem wise to "be early" where your pastime requires you to catch slack water at PPH. Remember it is pretty easy to kill a few minutes if you do arrive too early, but there is no way to "make up" any minutes if you are late on the scene!