Definition
Caused by shallow-water overtide constituents, chiefly the quarter-diurnal M4 and sixth-diurnal M6, distorting the sinusoidal shape. Southampton experiences a prolonged stand or double high water, and Portland a double low water. Because the standard Rule of Twelfths assumes a sinusoid, it must not be applied at such ports; the Admiralty publishes special curves keyed to low water rather than high water for these places.
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