Off the south east corner of Jersey the tide can go out nearly 3 miles on a large spring tide and in the Bay of Mont St Michel, the tide is rumoured to come in at the speed of a running horse at times. These huge movements of water can follow a simple rule known as the "Twelfths Rule". It is very straightforward formula but one which can prove to be very useful to know.

Essentially it is as follows:

In the first hour of the tidal sequence one/twelfth of the water moves.
In the second hour of the tidal sequence two/twelfths of the water moves.
In the third hour of the tidal sequence three/twelfths of the water moves.
In the fourth hour of the tidal sequence three/twelfths of the water moves.
In the fifth hour of the tidal sequence two/twelfths of the water moves
In the sixth hour of the tidal sequence one/twelfth of the water moves.

In Jersey we experience, at times, tides with a range of about 40 feet but let us consider what the twelfths rule means for a reasonably common 36 feet tide. In the first hour one/twelfth or three feet of water moves. By the third and fourth hours, when three/twelfths of the total volume moves, this equates to 9 vertical feet of change in an hour. In other words, for every minute during the third and fourth hours of the tidal sequence there is a vertical change in the water level of nearly 2 inches or 5 cm per minute. On flat foreshores, this means that the water can run across the sands with surprising speed, threatening the safety of any walkers in the area who are caught unawares.

An understanding of the twelfths rule will assist with knowledge about tidal flows which can only enhance safety whilst on the water.

In six hours of the tidal sequence there is nearly 12 metres of vertical movement.

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