tidal variations

[squire]

Was wondering if anyone knows how to calculate tidal variations in different areas other that designated rivers Ie Brisbane river Bar. Someone once told me there was a formulae for working this out. Could anyone tell me if this is true or is there another way of working it out? Regards Squire.

[Big_Muddie]

Hi Squire.

I asked this question of the National Tidal Facility at Flinders University as well as a person from the maritime division of the #QLD Dept of Transport.

Their answer, in short, was that there is no formula which can be applied to give accurate tidal variations without there being some sort of monitoring of a particular location, taking into account the shape of the place that the variation is being calculated for, the volume of water that flows there, surrounding features such as islands, sandbanks, channels etc.

I wanted some idea of variation both of tide heights and times for a spot that had no previous monitoring done. #All I finally did was take monitored spots either side of the one I wanted and applied a simple pro-rata calculation of heights and times based on the distances of my location from the 2 known ones. #They turned out to be pretty accurate, especially the times. #Very difficult to judge height accuracy though.

I was lucky in that that the place I wanted variations for had 2 relatively close monitoring points that let me do those calculations.

If someone does know of a simple formula that doesn't require all the observation data then I would like to know about it as well.

Sorry I couldn't be of any real help.


PS I had e-mails from these guys with their explanations but only recently deleted them.

[Wesley_Pang]

Squire,

Which area do you want the tides for?

I use "The Official Tide Tables & Boating Safety Guide 2003", published by Queensland Transport($6.85). Has the tides for 2 dozen standard ports from the Gold Coast Seaway, and Brisbane Bar to Mornington Island in the Gulf. The Size and bag limits are a bit out of date with the changes in Dec.

It contains the offsets for popular location from the standard ports.

Example Peel Island(secondard place) is +10 minutes for High tide and +17 minutes for the Low tide from the Brisbane Bar(standard port). I'm only usually only interested in the time of high and low tide. The guide has the information and example to calculate the predicted heights of the secondary places.

I suggest you get a copy of this book. You can get it at the local newsagent. Its much better than that small orange tide book.

Wes

[Heath]

I asked the same sort of question for the Tweed river. We would arrive at the stated low tide to head out in slack water, only to find the tide flying out. Finally worked it out that the Tweed is 2 hours behind stated tide times or for simplicity 2 hours behind the Goldcoast Seaway.

I suppose the more you visit a waterway the more you get to know it, which ultimatly helps you in the end.

[DeeGee]

Heath, I found the same thing near the mouth of the Tweed some years ago.
From memory, I seem to recall that the current flow in the river did not stop until 30-40 minutes after low tide at Point Danger and did not start to run in noticeably until nearly an hour after low water.

My guess at the time was that the tidal flow of a big river has enough momentum to maintains the actual flow for some time after the low water has been reached and the water level has started to rise again.
I have never checked this out with the experts, and in fact I had forgotten about it til I read your post.

Cheers, DeeGee.

[Heath]

Yeah, we have th Tweed pretty wired. 2 hours later than the Seaway & you have 2 hours before low or 2 hours after high to safely cross it without significant pressure waves.

One of the boys did any article in BNB about the tides working it out with water movemnt in sixths. It worked out that the most water moves in the tide during the middle 2 hours, which works out pretty damn correct.

[DeeGee]

Heath, Those are interesting figures ! I've always been wary of the bar down there, especially after seeing a couple of boats turned over one morning not long after I had come back in.

But it was not bar crossing that we were worried about when we noticed that the run out current did not coincide with the tide times.

We were fishing two or three times a month for jew in the river near the hospital. We would try to be there at least an hour and a half before the tide change, and after a couple of trips we realised that the run out continued well after the tide had bottomed out.

Cheers Deegee.

[squire]

Thanks for the info fella's I think the two nearest readings in the tide books and doing your own rough estimate works for me the best. Sometimes we overlook the obvious. thanks again squire.

[SteveCan]

Hi Guys,

As a Charter skipper said to me recently, 'You can look on the internet all you like - but nothing beats sticking your head out the window and seeing for yourself!' Tweed is a bugger to cross on the run out with the pressure waves etc - I had personally figured just over an hour difference in the tide times - so I will have to have another look next time we cross.

Heath, I notice you mentioned pressure waves - I know they are bad on the outgoing tide at Tweed - but are they a problem on the run in? We tend not to go out on days with more than 15 knots of wind or 1.5m of swell, we also try for an incoming tide and I haven't ever had a problem in those conditions...

Cheers
Steve.

[Heath]

Steve,

No problems with pressure waves on the run in. The only thing to watch for on the run in, is the sand bank on your starboard side as you round the bend to head out.