Highest Tides in the World
The Highest Tides on Earth Occur in the Minas Basin!
The tides on Earth are strongly influenced, in addition to astronomical factors, by the sizes, boundaries, and depths of ocean basins and inlets, and by Earth's rotation, winds, and barometric pressure fluctuations. Tides typically have ranges (vertica high-to-low) of a metre or two, but there are regions in the oceans where various influences conspire to produce virtually no tides at all, and others where the tides are greatly amplified. Among the latter regions are the Sea of Okhotsk, the northern coast of Australia, the English channel, and in Canada at the Ungava Bay in northern Quebec and the Bay of Fundy between New Brunswick and Nova Scotia. The tidal ranges in these regions are of the order of 10 metres.
The highest tides on Earth occur in the Minas Basin, the eastern extremity of the Bay of Fundy, where the average tide range is 12 metres and can reach 16 metres when the various factors affecting the tides are in phase (although the highest tides occur typically a day or two after the astronomical influences reach their peak).
The primary cause of the immense tides of Fundy is a resonance of the Bay of Fundy-Gulf of Maine system. The system is effectively bounded at tis outer end by the edge of the continental shelf with its approximately 40:1 increase in depth. The system has a natural period of approximately 13 hours, which is close to the 12h 25m period of the dominant lunar tide of the Atlantic Ocean.
Like a father pushing his daughter on a swing, the gentle Atlantic tidal pulse pushes the waters of the Bay of Fundy-Gulf of Maine basin at nearly the optimum frequency to cause a large to-and-fro oscillation. The grestest slosh occurs at the head (northeast end) of the system. Because Earth rotates counterclockwise in the Norhern Hemisphere, the tides are higher in Minas Basin (Wolfville-Truro area) than in Chignecto Bay (Amherst-Moncton area).
Although it is the gravitation of the Moon and Sun that raises the tides, the energy in the churning waters is extracted from the rotational energy of Earth spinning on its axis. Near Annapolis Royal, Nova Scotia, a tiny portion of this energy is being converted into commercial electrical energy in the only tidal power plant in the Western Hemisphere. The peak output of the Annapolis Basin generator is 20 megawatts, about 1% of Nova Scotia's electrical power capacity.
Tidal friction both lengthens the day and increases the size of the orbit of the Moon. The day is lengthening by about 1 second every 50,000 years, imperceptible on a human time scale, but of profound significance to Earth's rotation over a few billiion years. If the Sun does not first incinerate our planet, in the distant future there will come a day that is as long as the lunar month (each then equal to about 40 present days) and a more distant Moon will stand stationary in the sky, as does Earth now in theh lunar sky. But this situation will not endure, for solar tides will still be present and will cause the Moon to approach Earth once more.
Dramatic Views of the Tidal Rise and Fall
Wolfville is located on the southern shore of Minas Basin, the northeast arm of the Bay of Fundy. The Bay of Fundy is noted for its large tides, and it is on Minas Basin that the highest tides on Earth occur. This is truly a wonder of the world and should be experienced by any visitor to the Wolfville area.
Well before low tide is reached, Wolfville's small harbour is literally empty. Four km west of Wolfville, at the Port Williams bridge and wharf, the large vertical range of the tides may be seen to better advantage (a parking lot is located on the Wolfville side of the bridge). Dramatic views of the tidal rise and fall are also available town of Hantsport (at the 16 km east of Wolfville in the foot of William Street).
Views of the vast areas of sea bottom uncovered by the falling tide may be had at Evangeline Beach and at Avonport Beach, both about 8 km east of Wolfville. In late July and August, the extensive intertidal flats in these areas are visitied by hundreds of thousands of shorebirds on their annual migration from the Arctic to South America. The crustaceans and worms in these mudflats provide a rich source of food for these birds which then fly for three to four days non- stop to South America. Flocks of shorebirds are best seen within two hours of high tide when they are spectacularly concentrated along the upper limits of the beaches.
Large areas of the original flats now lie behind man-made dykes. This conversion of tidal flats into rich farmland began with Acadian settlers in the seventeeth century. Today long dykes and thousands of hectares of productive level fields may be seen in the vicinity of Grand Pre, Wolfville, Port Williams and Canard.
Near the mid-point of an incoming tide, a tidal bore may be seen tumbling upsteam in some of the rivers which flow into Minas Basin (e.g. the St. Croix and Meander rivers near Windosr, and the Shubenacadie and Salmon rivers near Truro). A bore forms where the incoming tide pushes its way upstream against the outgoing freshwater flow of the river.
Perhaps the most awesome display of the tides on our planet occurs at Cape Split, on the southern side of the entrance to Minas Basin (Cape Split may be reached by a pleasant two-hour walk along a popular hiking trail from the village of Scots Bay, which is a 30-minute drive north of Wolfville). Here at the time of the mid-point of an incoming tide, for a considerable distance the forest on the towering cliffs is filled with a hollow roar produced by the turbul;ence of the waters surging over the submarine ridges below. The currents exceed 8 knots (4m/s), and the flow in the deep, 5 km-wide channel on the north side of Cape Split equals the combined flow of all the streams and rivers of Earth (about 4 cubic kilometres per hour). Three hours later the spectacle pauses, and then begins flowing in the opposite direction.
The Mystery in the Moon
Tides were known to the ancients, but an understanding of their origin came only three centuries ago with the publication of Issac Newton's Principia. Tides originate in the fact that the force of gravity decreases with distance from a massive body. The Moon exerts a force on the Earth, and Earth responds by accelerating toward the Moon; however, the waters on the side facing then Moon, being closer to the Moon, accelerate more and fall ahead of Earth. Similarly, Earth itself accelerates more than the waters on the far side and falls ahead of these waters. Thus two aqueous bulges are produced, one on the side of Earth facing the Moon, and one on the side facing away from the Moon. As Earth rotates on its axis beneath these two bulges, the rise and fall of the oceans results. If Earth had not rigidity, the entire planet would flex freely in the same fashion, the ocean bottoms would rise and fall too, and there would be virtually no water tides. The very existence of the tides indicates that on a time scale of several hours, our planet displays considerable rigidity.
Although the Sun exerts a gravitational force 180 times as strong as does the Moon on Earth, because the Moon is so much closer, the variation in Moon's force across Earth's diameter is about 2.2 times larger than the variation in the Sun's force. As noted above, it is this variation that produces tides, thus the pair of bulges raised by the Moon are considerably larger than the pair of tidal bulges get in and out of step, combining in step to produce "spring" tides (no connecton with the season) when the Moon is new or full, and out of step to produce "neap" tides when the Moon is at first or last quarter. Another factor having a substantial influence on tidal ranges is the elliptical shape of the Moon's orbit. Although the Moon is only 9 to 14% closer at its close point to Earth (perigee) than at its far point (apogee), because the variation in its gravitational force varies inversely as the cube of its distance (the force itself varies inversely as the square of the distance), the Moon's tidal influence is 30 to 48% greater at perigee than at apogee. In the Bay of Fundy the perigee-apogee influence is greater than the spring-neap influence. Although the variation of the Moon's distance is not readily apparent to observers viewing the Moon directly, to observers near the shores of Minas Basin, the three to six metre increase in the vertical tidal range makes it obvious when the Moon is near perigee, clear skies or cloudy!
The Best Places on Earth to See the Highest Tides
Quick Facts on Our Tides
(Text written by Dr. Roy Bishop)