View Full Version : question on tides and flow

17-03-2007, 12:38 PM
I have been wondering for a while now the following

assuming there is a 6 hour difference between high and low tides............when does the water flow at its fastest????????the start of the flow? after 3 hours? or near the top of the tide. Furthermore does water flow faster incoming or outgoing assuming height changes are similar?

also where does the water flow faster.............in the deeper sections or near the banks???????????

NO RUN NO FUN............hhhhhhhhmmmmmmmmmmm!!!!!!!!!!!!!!!!

mitch qld
17-03-2007, 01:16 PM

We learnt a rule in Marine Studies at school last year called the rule of thwelths. This rule helps calculate tidal flow and how much water will be available at each stage of the tide. I cant remeber to much about it, but i do remeber that the rule states the tide moves the most in the 3rd and 4th hour. That is, the 3rd or 4th hour of the incoming or outgoing tide.

I dont think that the water would move to much on the top of the tide, or the bottom for that sake, as this is usually the period referred to as 'slack tide' when there is not much water movement at all.

I'm unable to answer your question about water flowing faster on the incomiong or outgoing tide (assuming the height changes are similair), as it could depend on a number of factors such as the topography of the sea floor etc. I'm sure someone else could help you out here.

Just remeber those age old rules, no run no fun and no flow no go!

Cheers, Mitch.

17-03-2007, 01:53 PM
To answer your other question, the water flows faster on the 'outside' of the bends of a river which is generally the deeper section.

17-03-2007, 02:18 PM
To answer your other question, the water flows faster on the 'outside' of the bends of a river which is generally the deeper section.

True, but it's only deeper because the tide flow is faster and not vice versa.

The middle 2 hrs of the tide is the fastest and the speed is determined by how far the tide has to travel in similar times ie. today and tomorrows tides

Sat pm Low .37 High 2.09 Sun am Low .29 High 2.56

Sun am incoming will have the fastest flow as it travels further (2.27 metres) or at least you would think so..........but................you have to take the traveling time into consideration, Saturday's incoming takes 5.45 hrs, outgoing 6.10 and sunday incoming 6.14. Can't be bothered doing the maths now, but that's the basic manner of calculating.........distance traveled and time taken to travel that distance.


17-03-2007, 02:29 PM

Just remeber those age old rules, no run no fun and no flow no go!

Cheers, Mitch.

Generally yes, but certain areas of the bay produce better at top or bottom of tide when the flow is usually very strong at other times. Curtin for example, and on king tides, the beacons also, at least in my experience.


17-03-2007, 02:52 PM
To clarify the rule of 12ths

This is an approximating method used for calculating the depth of water at a particular position where a shallow area exists and you need to know if it can be safely passed in your vessel at a particular point in time during the tidal period.

Look up the tide chart and get the lowest tide for the period and the change in height for the tide. Subtract the lowest tide from the highest tide and this gives the change in height.

in the 1st hour after the tide starts to rise, 1/12 of the change will occur
In the 2nd hour, 2/12ths will occur
In the 3rd hour 3/12ths will occur
4th hour - 3/12ths
5th hour - 2/12ths
6th hour - 1/12th

For example. Chart says depth 1m at lowest astronomical tide. The tide chart says Low = 0.85m. High = 2.0m. Change in tide = 1.15m. At high tide, you will have around 3m depth (sounding of 1m at lowest astronomical tide plus a 2 metre tide height.

But, you want to cross the shallow area at the end of the second hour of the rising tide and you want to know the depth at that time.

1m (Lowest astronomical tide depth on the chart) + 0.85 (Low tide of the day) +(1.15 *1/12) + (1.15* 2/12) = the height at the end of the second hour of the run in tide.

= 1 + 0.85 + 0.1+ 0.2 = depth at the end of the second hour of the run in tide which will be 2.15 metres. (with a bit of rounding)

At the end of the 4th hour it will be (Don't round when calculating further into the period):

= 1 + 0.85 + (1.15*1/12) + (1.15*2/12) + (1.15*3/12) + (1.15*3/12)

= 1+ 0.85 + 0.096 + 0.192 + 0.29 + 0.29

= 2.71 metres deep at the end of the 4th hour of the run in tide.

Note: This is an "approximating" method. The tides are never exactly 6 hours, and this method works best for areas of open water, eg going over a sandbank in the middle of the bay. There are delays and differences which affect everything when the tide is restricted by narrow channels, estruaries etc.



18-03-2007, 11:46 PM
For the open sea I would go for the 3rd and 4th hour as mentioned in other posts. In esturies I suspect as the area up stream being filled increases with the tide rises may have an impact on this also.

the gecko
19-03-2007, 06:41 AM
Good point Willdoe. Yesterday I was fishing upstream in coombabah ck, this spot is spot is usually high at 1-1.5 hrs later. With a high of 1.72 at the seaway at 7.39am, coming off a low of .03, I found the actual turn of the tide was around 10.50am, 2-3 hrs later than usual.

I see this happen a bit around new and full moons in esturies.

Kev, Im a bit careful with the no run no fun theory. Ive found that when there is less run, eg- like a high of .89 and a low of .52, that the flow is less, an the baitfish are easier to catch, and some predators like jacks have a field day in the easy flow around river banks.

On faster days, there is less bites in the deep, and more bites in the backeddies. The fish hang out where they have to swim less, and wait for food to come to them. Just an observation from esturies. Might be different in the bay.

Good post tho, I was thinking of asking the same thing.


19-03-2007, 07:16 AM
Double post

19-03-2007, 07:17 AM
Here's a good little tide program that may be handy. It's free for the "demo" version and gives graph curves for all hours. The main thing is getting a handle on your local spots and getting a feel for what time you can expect your high and low in relation to the Primary port listed in the tide books.

19-03-2007, 08:13 AM
Thanks for all the info guys..............I fish estuaries mostly and have decided that I should throw my baits in very close on big tide changes and get it in deeper with very little run. It appears that around 4 hours after the change is when the water flows at its fastest. I hope that by utilising this info my catch rate will improve


cheers and thanks