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The Rule of the Twelfths is very much a rule of thumb method that allows us to approximate the height of tide at a given time if we are unable to use a tidal curve to accurately predict it. It assumes that the tide flows in a more or less symmetrical pattern every six hours. It is therefore more useful in areas where the tidal curve is relatively smooth and follows this pattern.
To use the rule, we assume that the the height of tide / the tidal flow changes by:
- 1/12 th of the total range in the 1st hour,
- 2/12 ths of the total range in the 2nd hour,
- 3/12 ths of the total range in the 3rd hour,
- 3/12 ths of the total range in the 4th hour,
- 2/12 ths of the total range in the 5th hour, and
- 1/12 th of the total range in the 6th hour.
We can therefore work out what the height of tide at hourly intervals will be by adding up the number of twefths that have passed by the end of each hour and multiplying the total by the tidal range (the difference between HW and LW heights).
Example
6
SU |
0500
1100
1700
2300
|
0.9
3.1
1.1
2.8
|
- What will the height of tide be at 1500?
- Tidal Range: 3.1m - 1.1m = 2.0m
- 1500 is 4 hours after high water: 1/12 + 2/12 + 3/12 + 3/12 = 9/12 = 3/4 of the tide has ebbed out.
- Find 3/4 of the tidal range: 3/4 x 2.0m = 1.5m
- If the tide is going out, subtract from the HW height; otherwise add to LW height: 3.1m - 1.5m = 1.6m
Self-Test
2
WE |
0300
0900
1500
2100
|
1.1
3.5
1.2
3.6
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- What will the height of tide be at 1700?
Click here for the answer
HANDY HINT Don't forget that the depth of water in a particular place isn't necessarily the height of tide - you'll need to include chart datum depths! |
