High tide, low tide

Tidal coefficients

Tidal coefficients express the magnitude of the tide. They vary between 20 (theoretical minimum) and 120 (theoretical maximum). Those above 70 qualify the vivid waters, those lower than 70 dead waters. The tidal coefficients are related to the phases of the moon. The vivid waters always occur near the new and the full moon (the maximum coefficients generally following one or two days the full or new moon). Dead waters always occur in after the first and last quarters (the minimum coefficients usually follow the first and last quarter by one or two days)

Tidal range

The tidal range (vertical slope between open sea and low tide) can be easily calculated by multiplying the tidal coefficient of the day considered by 8.4 (for Pleumeur-Bodou) ex: coefficient 95 tidal range = 95 x 8.4 = 798 cm. It can thus be seen that for Pleumeur-Bodou, depending on the tidal coefficients, the tidal range varies from 1m70 to more than 10m.

Law of Twelfths

A simple rule can appreciate the rising or dawning of the sea. Simplifying, we can say that the sea goes up or down for 6 hours. During the first hour, the sea rises or goes down by 1/12 tidal range.

  • "2nd" "" "2/12
  • "3rd" "" "3/12
  • "4th" "" "3/12
  • "5th" "" "2/12
  • "6th" "" "1/12

This rule is extremely important. Knowing it will save you a lot of trouble if you go shore fishing. For example: by coefficient 72 tide = 78 x 8.4 = 600 cm. The sea will rise (or descend) respectively by 50 cm the first hour, 1 m the second hour, 1m50 the third hour, 1m50 the fourth hour, 1 m the fifth hour, 50 cm the sixth hour

One of the consequences of the tide, with its large variations in water level, is the establishment of particularly strong currents in high water. At Pleumeur-Bodou, the rising tidal currents (flow), bear towards the north or towards the east, the currents of downing tide (ebb tide) carry towards the south or towards the west. Take it into account for your navigation and be careful !

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