Rising Tide

Rising tide is the interval during which the water level increases from low water to the next high water, also called the flood phase of the vertical tide. In a semidiurnal regime it lasts about 6 hours 12 minutes, half the M2 period of 12.42 hours, though the rise is not linear: the rule of twelfths approximates it as 1, 2, 3, 3, 2, 1 twelfths of the range across the six hours. The rising tide adds depth for entering shoal harbors and lifts a grounded vessel. It refers to the level change, distinct from the flood stream, the horizontal flow.