Falling Tide

Falling tide is the interval during which the water level decreases from high water to the next low water, also called the ebb phase of the vertical tide. In a semidiurnal regime it lasts about 6 hours 12 minutes, half the M2 period of 12.42 hours, and the rule of twelfths approximates the drop as 1, 2, 3, 3, 2, 1 twelfths of the range across the six hours. The falling tide reduces under-keel clearance and can leave a vessel aground if depth and timing are misjudged. It names the level change, distinct from the ebb stream, the horizontal flow.