A harmonic of a main constituent, at two, three or more times its frequency, that appears when friction and depth changes make the crest of the tide travel faster than the trough. M4, with a period of about 6.2 hours, is the most common. Overtides and the related compound tides make the rise and fall unequal in length, produce double high or low waters in some ports, and drive the asymmetry that decides whether an estuary flushes or traps sediment.
M4 is a third of M2 up at the head of the estuary, no wonder the flood lasts four hours and the ebb eight.
Also heard as
- shallow-water constituent
Related terms
M2
The principal lunar semidiurnal constituent, period 12.42 hours, usually the largest part of the tide.
Ebb-dominant
Describes an estuary or inlet where ebb currents are stronger than flood currents, pushing sediment seaward.
Agger
A double high water or double low water, where the tide peaks or bottoms out twice in quick succession.
MTL
Mean Tide Level: the height midway between the mean high water and mean low water datums.