In Figure 1, the low tide lags or leads by 1 hr 2 min from its neighboring lines. Cotidal lines connect points which reach high tide at the same time and low tide at the same time. In most locations the "principal lunar semi-diurnal", known as M 2, is the largest tidal constituent. A separate amphidromic system is created by each periodic tidal component. There can still be tidal currents since the water levels on either side of the amphidromic point are not the same. At the amphidromic points of the dominant tidal constituent, there is almost no vertical change in sea level from tidal action that is, there is little or no difference between high tide and low tide at these locations. Īmphidromic points occur because interference within oceanic basins, seas and bays, combined with the Coriolis effect, creates a wave pattern - called an amphidromic system - which rotates around the amphidromic point. It was first discovered by William Whewell, who extrapolated the cotidal lines from the coast of the North Sea and found that the lines must meet at some point. The term derives from the Greek words amphi ("around") and dromos ("running"), referring to the rotary tides which circulate around amphidromic points. As such, the concept of amphidromic points is crucial to understanding tidal behaviour. The tidal range (the peak-to-peak amplitude, or the height difference between high tide and low tide) for that harmonic constituent increases with distance from this point, though not uniformly. The amphidromic points are the dark blue areas where the lines come together.Īn amphidromic point, also called a tidal node, is a geographical location which has zero tidal amplitude for one harmonic constituent of the tide. The white lines are cotidal lines spaced at phase intervals of 30° (a bit over 1 hr). The M 2 tidal constituent, the amplitude indicated by color. For the former stadium in Houghton, Michigan, see Dee Stadium.
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