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What is a Tidal Bore? - Definition, Cause & Wave

Instructor: Matthew Bergstresser

Matthew has a Master of Arts degree in Physics Education. He has taught high school chemistry and physics for 14 years.

Every day on Earth, there are two high tides and two low tides, but not every location experiences a tidal bore. In this lesson, we will learn what a tidal bore is, and what causes it.

Surfing in Canada

Who has ever heard of surfing in Canada? When you think of surfing, you think of tropical locations with beaches, not places away from the ocean where the average high temperature during the summer is less than 80° F! The surfing I'm talking about is in Moncton, New Brunswick; it is not your typical surfing on ocean waves. It is surfing a tidal bore in a river!

Tidal Bore

Rivers generally flow from higher elevations to the ocean or seas. A tidal bore is a situation where the flow of water from the river into the ocean reverses, and the tide pushes water up the river.


Tidal bore in Haining, China in 1908
tidal_bore


Tidal bores occur every day, but are not common all over the planet. Special conditions are required for one to occur. Let's go through the conditions that cause tidal bores.

Geography

The river where a tidal bore occurs must not be too deep, and its estuary, or mouth of the river, needs to be relatively shallow and fairly wide compared to the inland part of the river to produce a sort of funneling effect. The estuary acts as the wide part of the funnel, channeling water into the narrower river, where the water rises to compensate for this influx of volume.


Klamath River estuary shows the funnel effect required for tidal bores. No tidal bores are experienced here because the other conditions required for one are not present.
funnel


Tides

In addition to regional geography, the local tidal swing in a location must be larger than average, approaching around 20 feet. This means the ocean level changes 20 feet between low and high tide.


Difference between high and low tide
tide_difference


For example, the Bay of Fundy near Eastern Canada has extreme tidal differences.


Boats at rest on dry land during low tide at the Bay of Fundy
Bay_of_Fundy


Tidal waves are often confused with tsunamis. Tsunamis are large wavelength waves generated by massive disruptions in ocean water levels, such as earthquakes or landslides. Tsunamis are rare occurrences, with large, violent waves.

On the other hand, tidal waves are generally gentle and slow moving. Tidal waves occur every day and are due to the gravitational effects of the moon.

The earth and the moon have a very strong gravitational pull between them. The side of Earth facing the moon is closer to the moon and feels a stronger gravitational pull, which causes large bodies of water in line with the moon to rise higher.


Two high and two low tides
tides


Opposite sides of the earth are nearly 8,000 miles apart at the equator, so why does the water also bulge on the side facing away from the moon? The answer lies in inertia. Since the earth wobbles, it causes this bulge of water on the far side of the earth that the weaker gravitational pull cannot overcome. In addition to wobbling, the earth is rotating, causing the bulge of water to move in a wave around the planet, which is known as a tidal wave.

Random Factors

The level of water in a river is not constant. The amount of precipitation it receives affects its water level. Also, if the river is large enough to handle large ships, the water level fluctuates significantly with the presence of the ships.

Wind is also a factor in the equation. Rivers near Anchorage, Alaska experience tidal bores, and if the wind is out of the West, the tidal bore will arrive earlier than estimated by the high tide time. Easterly winds will make the tidal bore late. Wind speeds can also change the magnitude of the tidal bore.

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