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8.3.2: Tides in Coastal Regions, Tidal Currents

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    85138
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    The tide amplitude depends on several factors, such as the relative positions of Moon and Sun, the geographic location, and a few others. In the middle of big Oceans the amplitude of the tide (the difference between the highest level and the lowest level) may reach almost 1 meter. However, near the continental coastal areas the situation may change, sometimes even dramatically.

    As noted before, the tidal waters can be thought as a wave traveling around the world, with the two bulges acting as “crests”. The propagation of this wave would be smooth, if there was an ocean all around the globe. Which is the case only in the southern latitudes, south from the southern tips of South America and Africa, and the coasts of Antarctica. More to the north, the “global tidal wave” encounters continental coasts.

    Let’s first think of an ordinary ocean wave on a day with average weather. Far away from the coast the wave doesn’t look very impressive – it quietly “rolls” towards the shoreline. But when such waves get really close to the shore, to the region where the water is getting shallow, the waves start “mis- behaving”, they grow in height, and if they finally encounter not a beach, but a vertical rocky obstacle (there are many such locations along Oregon Coast), then the waves end their lives with spectacular “explosions”, with thunder- ous noises and fountains of foam. The much less common (fortunately!) tsunami waves exhibit a somewhat similar behavior. The wavelength of ordinary ocean waves is only 30 - 50 meters. Tsunami waves, in contrasts, have an extremely long wavelength, form 10 to several hundred kilometers. But their amplitude at deep waters is surprisingly small – one foot or even less. However, when they encounter shallow water, they grow in height and nay produce a surge which is ten meters, or over several tens of meter high.

    There is not a close analogy (fortunately!), but at least a distant analogy between the behavior of ordinary waves and tsunami waves, and of the behavior of the tidal bulge when it arrives from deep water areas and approaches a coast. The forces responsible for the creation of ordinary ocean waves and tsunami waves are not the same as those underlying the creation of a tide bulge. Nonetheless, the latter shows similar tendencies as the former in the shallow-water regions. But the tidal bulge is “decisively better behaved”, it grows in height gradually, over the period of hours, and only by a factor or two or so. But, well, it’s not only so simple. If the tidal wave encounters a funnel-shaped geographical objects, such as, e.g., bays, straights, or river estuaries, they may have a “focusing effect” and drive the tide much higher. For instance, the highest tide in the world (15 meters) occurs in the Bay of Fundy at the eastern coast of Canada.

    Bay of Fundy pier at high and low tide
    Figure \(\PageIndex{1}\): The high tide and the low tide in the Bay of Fundy (source: Wikimedia Commons).

    In fact, the pattern of tidal effects over the globe is influenced not only by shapes of coastal lines, but by a number of other factors. In addition to areas of extra-high tide, there are also many “no-tide” areas, called amphidromic points. One can find out about other peculiar tide phenomena from this YouTube clip

    or from another YouTube in which there is more about the peculiarities of the Bay of Fundy: the tide over there is record high, but there are no two, but only one high tide per day. may be

    The map showing the tide heights is pretty complicated. Since it continually changes, it’s most instructive to watch a “dynamic map”, shown, e.g., in this YouTube clip.

    Whenever water moves, there are always currents. The simplest tides that alternately raise and lower the water level at your favorite beach also pro- duce currents, perpendicular to the shore line, called rectilinear, or reversing currents. The current flowing towards the shore is called the flood current, and that carrying water away from the coast is called the ebb current. Those currents are not very strong (a tidal ebb currents should not be confused with offshore ripe currents which are produced by a different mechanism).

    But if tidal waters flow into a closed body of water such as a bay, a fjord, a lagoon or a natural harbor, through a relatively narrow inlet or canal. In such circumstances the tidal current in the inlet/outlet area may become really strong. One known such example located in the Oregon’s nearest- neighbor, the State of Washington, is Puget Sound , a complex system of interconnected waterways and basins with three outlets to the Pacific Ocean via the Strait of Juan de Fuca – one major exit (Admiralty Inlet ) and two minor, Swinomish Channel and the Deception Pass. It’s interesting that the water flow through the Deception Pass is only about 2% of the total tidal exchange between Puget Sound and the Strait of Juan de Fuca, but the speed of the current in the narrowest part of the outlet, under the bridges, may be as high as 8 knots (about 15 kilometers/hour).

    The passage to the sea narrows strongly at Deception Pass where there is a bridge over the water
    Figure \(\PageIndex{2}\): Deception Pass – the tidal currents are the strongest under the bridge (source: Wikimedia Commons).

    Another famous example of an extraordinarily strong tidal current is the Saltstraumen Maelstrom in Norway. It forms in a 3 km long, and only 150 m wide channel connecting a large Skjerstadfjorden fjord with a smaller one, Saltfjorden – the latter being widely open to the sea. The current in channel attains a speed as high as 37 kilometers/hour, nearly 2.5 times higher than the current speed in Deception Pass. Norwegian people insist that it’s fastest tidal current in the world!

    The fact that tidal phenomena can be very diverse in areas not too far apart can promote the creation of currents. For instance, one of the fastest tidal currents in the world exists in the Cook Strait, the narrow see passage separating the two island of New Zealand. Not far from the country’s capital, Wellington, is the strait’s narrowest part in which the current attains a speed up to 4 knots. It may not look impressive at the first glance, compared with the 8 knot speed of the current in Deception Pass, or the 20 knots in the Saltstraumen Maelstrom. But let’s keep in mind that the total width of the two channels at Deception Pass is about 200 meters, and the width of the Saltstraumen Maelstrom channel is 150 metes – while the width of the Cook Strait in its narrowest part is 20 kilometers wide, about 100 times wider than the Deception Pass channels.

    Map of the Cook Strait between the North and South Islands of New Zealand showing how it narrows from east to west
    Figure \(\PageIndex{1}\): New Zealand, the Cook Strait

    There are, of course, many other places all over the globe where strong tidal currents occur. Especially, in areas where there are many bays, peninsulas, and islands.


    8.3.2: Tides in Coastal Regions, Tidal Currents is shared under a CC BY 1.3 license and was authored, remixed, and/or curated by Tom Giebultowicz.

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