How tides work

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mickkk
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Re: How tides work

Post by mickkk » Sat Jan 16, 2021 6:37 pm

Doesn’t explain why Darwin gets 6 plus meters of movement. And only 200 kms away at colourg they only get 1 to 1.5 m of movement.

Tides do my head in.

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Re: How tides work

Post by ronje » Sun Jan 17, 2021 6:34 am

This might help a bit Mick.
Screenshot (29).png
For the tide to get into the harbour, the water must pass firstly through the 30km wide mouth.

That same volume of water must then pass through the 6km wide restricting gap in the same period of time.

To do that, the run must run faster and also get higher.

As the water gets further in, it runs into the shallower depths of the upper harbour.

That also causes the tidal front to maintain its height even though the widening shallower harbour allows it to spread out a bit.

Result is that the vertical tide heights will be greater there than somewhere that doesn't have similar geographical features (Coburg).

I reckon that tidal run would rip through that 6km wide gap like it does at Port Philip Bay in Vic.

Also have a look at this (probably explains things better than I can).

Chapter 5 deals with examples of how local geographic features affect tidal ranges in gulfs and ports.

https://www.abs.gov.au/ausstats/abs@.ns ... enDocument
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Re: How tides work

Post by mickkk » Sun Jan 17, 2021 12:41 pm

That would make sense if it was just the harbour with big tides, but dundee, persons islands etc all have big tides.

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Re: How tides work

Post by nomad » Sun Jan 17, 2021 5:10 pm

Ron
That explains the high speed of tides but not the variation - 8m plus.
At the equator and Antarctic, tides are small no matter the size of the inlet.
There are many places with wide expanses of water but only have small variations.
Sydney harbour is huge and deep but small tides.
Ages ago, I asked the dept that gives official tide info to ask how far out the tides run etc etc
Ie If there is an 8m tide in darwin, how far out does the variation go? Ie is there a 6m tide 200 kms out? Etc
They couldn’t answer

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Re: How tides work

Post by ronje » Sun Jan 17, 2021 7:48 pm

It explains the reason for the tidal range in Darwin Harbour. Mick was comparing the Darwin Harbour tidal range to Coburg tidal range. We've sorted that out ( I think so). Mick??

I'll put up on here the actual tide gauges themselves for Darwin Harbour and Gove Harbour. Like chalk and cheese.

Happy to look at these other places like Perons and Dundee.

If there is an 8m tide in darwin, how far out does the variation go? Ie is there a 6m tide 200 kms out? Etc
They couldn’t answer


I'm guessing that you mean tidal range for variation?

No. The 8m range caused by what's happening inside Darwin Harbour is only for that area.

It doesn't automatically follow that a lesser 6m range is evident 100km out to sea.

Put a tide gauge 100 km out at sea in open water and it sure won't give a 6m range.

What it WILL give is entirely dependent on the bathymetrics of the area. Look up bathymetrics. Its the shape

of the features of the bottom which tell us what volume of water is being moved and if there's any "lumping"

of water caused by underwater bathymetric restrictions.

What info do you have on Peron's and Dundee tides?
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Re: How tides work

Post by ronje » Sun Jan 17, 2021 8:19 pm

Here are the actual tide gauges for Darwin and Gove harbours plus maps of their locations.

Note that Gove's gauge is OUTSIDE the harbour entrance and therefore hardly any restriction on tidal flow.

Gove harbour is much smaller in volume than Darwin harbour so doesn't have anything like the volume of water that has to be move. About 1/4 of the volume of water in Darwin Harbour or even less.

The bathymetrics at Gove are nothing like those at Darwin (less entrance restriction, less volume of water to move into the harbour).

Therefore the tidal range will be much less.
Screenshot (31).png
.
Darwin Harbour.xls
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Re: How tides work

Post by nomad » Mon Jan 18, 2021 1:10 pm

http://media.bom.gov.au/social/blog/167 ... australia/
its a good explanation and its got pictures!!! :twisted:

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Re: How tides work

Post by ronje » Mon Jan 18, 2021 2:31 pm

It tells the same story that I'm trying to tell ya but they're leaving a lot of stuff out.

This variation is due to the response of the ocean to astronomical tidal forces, and the shape and depth of ocean basins, bays, and estuaries, which can have a funneling effect" (these things are called bathymetrics)

The red colours (bigger tidal ranges) are in either shallower waters where the underwater bathymetrics come into play (top half of Aus) or where tidal water is trying to go through a narrow funnel (like Bass Strait) and "lumps up" in trying to do so.

The bottom half of Aus is green/orange (lower tidal ranges) BECAUSE its deeper water closer in and meets no real bathymetric obstacles.

There are many places within the "red zone" that DON'T have large tidal ranges.

North Peron and Dundee (actually Native Point) don't seem to have big tidal ranges. Sorry Mick. There are other places in the NT that have smaller tidal ranges as well.

Its the local underwater bathymetrics that determine the tidal range AT THAT PLACE.

There are only 3 standard ports in the NT that BOM has sufficient data to draw up any sort of table for. The 4th port is actually Gove but its missing the HAT (highest astronomical tide). You'll see that the HAT is missing from the template gauge I made up for Gove (above). I also found the Gove MSL from somewhere else.

To make a gauge template for a place you need to have:

1. The Mean Sea Level (MSL) at the place (that tells you how far to the bottom of the gauge); and

2. The HAT for that place (that tells you how long to make the gauge).

Note that MSL is measured as how far above gauge zero (read off the gauge) and remember that MSL = Australian Height Datum (AHD). THAT'S why its on the templates I made up for Darwin and Gove.

Note also that "gauge zero" also equals the Lowest Astronomical Tide (LAT). That was set a few years ago to prevent any tide gauges from having -ve / "minus" tide heights.

Other States have plenty of info to input to BOM to get their tides sorted out. Look at about page 114 (or close to it) on the Qld Tide Tables. THAT shows how much info Qld has supplied to BOM about tides here (including the Gulf).

Page 109 shows the different data that EVERY port (standard or secondary).

There is info about different rivers but there doesn't seem to be any single place where its brought together.

THAT place should be BOM but they don't seem too interested. Neither does the NT Govt.

So where there's a vacuum, somebody will move in (eg Willy Weather and others) trying to provide some sort of service.

And ABOVE is why "tides do ya head in" in the NT.
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Re: How tides work

Post by Lats » Mon Jan 18, 2021 7:14 pm

Tidal range has to do with distance from the continental shelf i.e. the further away you are from the shelf, the bigger the tidal range. It also has to do with distance from Amphidromic Points. They are tidal nodes with zero movement i.e. the further away from these points the bigger the tidal range

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Re: How tides work

Post by mickkk » Mon Jan 18, 2021 9:25 pm

Reckon the old tractor drivers would diss agree that Dundee has smaller tides.
I do agree that out wide of Dundee doesn’t seem run near as much as other parts of the territory.
I have just come to the conclusion that I will never totally get my head around tides. I think there is a real good understanding of tides really, no one can seem to predict the tides at the roper mouth.
And don’t get me started with the Apsley straight.

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Re: How tides work

Post by ronje » Tue Jan 19, 2021 7:37 am

Fair enough Mick.

There's a lot of publicly available info about NT tides.

The problem is that its all over the place like a dog's breakfast and nowhere have I been able to find crucial data for some of these places.

From Lats:
Tidal range has to do with distance from the continental shelf i.e. the further away you are from the shelf, the bigger the tidal range...and generally speaking that's right.

However, its anywhere that a change in bottom features occurs, that the sea "lumps up" and will cause the water level to locally increase giving greater tidal range than the norm in the vicinity of that feature(even if a long way from the continental shelf).

How much it "lumps up" in negotiating the feature will determine the tidal run around it.

Waterway users would like standardised specific tidal information about specific places.

Ports, bays and estuaries in waters inshore from continental shelf (even a long way inside) will certainly give even larger tidal ranges than those that might be expected where changes in bottom features exist.

Here's a diagram of the data needed (but mostly missing). You can find a bit on one site and a bit on another. Its for semi- diurnal tides and some in the NT & Qld aren't semi-diurnal.

Some of the info on here (gleaned from a number of various websites) is what the templates for Darwin Harbour and Gove Harbour that I put up earlier are based on.

Basically its like extracting teeth.
Screenshot (26).png
This diagram is the type and format (like that published for all other Aus states)that's needed in respect of the NT but its nowhere to be found (easily).

No list of standard ports or secondary ports and individual data for them.

Like so:
Screenshot (32).png
If u look at BOM tide info for the NT, its given in the same daily tidal format (but in separate pages) as in other states by BOM but NOT in a single document for the whole NT.

I suspect that BOM has a single "book" form for each stand port in the NT but for some reason only wants to make it available specific page by page and you can only look at each port as separate queries.

Dept of Defence also has specific tidal info but that can only be obtained by buying it.

Ditto with Aus Tidal Office.

Might enquire about what info is contained in the data from those places and invest a little if its there.

I'll put the info for Native Point and North Peron to show the tidal ranges at those gauges, Mick (but the important info (as per the above diagram) can't be easily found anywhere (not for free anyway).
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Re: How tides work

Post by ronje » Tue Jan 19, 2021 10:08 am

Thought crossed my mind is what has AFANT done about sorting out something about tides over the years?

Maybe they have and hold good info about tides.

AFANT members would have welcomed any moves by them over the years considering that tides are every bit as big a problem for members as continuing uncertainty about access to fishing places near TO land.

It was the tidal issue that brought about the Blue Mud Bay decision and THAT was a big game-changer for fishing.

I guess it really shows just how significant the dog's breakfast state of official tide data actually is.
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Re: How tides work

Post by ronje » Sat Jan 23, 2021 6:36 am

Have bought the Defence Dept's CD ROM about tides Aus wide for $40.

If its got the missing info (yet to arrive), then that'll be a good starting point.
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