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Weird Science: Extreme Tidal Ranges

NGSS Crosscutting Concepts:

The Bay of Fundy, located in Canada between the eastern Maritime Provinces of Nova Scotia and New Brunswick, has one of the world’s largest tidal ranges. High tide can be as much as 16 meters (the height of a three-story house) higher than low tide (SF Fig. 6.14 A). There are two high tides and two low tides in the bay every day; over 100 billion tons of water flow in and out of the bay twice a day. It is very important for people living near the Bay of Fundy to know the tides if they want to fish from shore, go tidepooling, or go boating along the coast.

<p><strong>SF Fig. 6.14.</strong> (<strong>A</strong>) Difference between high and low tides in the Bay of Fundy</p>


<p><strong>SF Fig6.14.</strong> (<strong>B</strong>) The Bay of Fundy’s highest tidal heights are located at the narrowest point in the funnel-shaped bay.</p>

There are two main reasons for the Bay of Fundy’s huge tidal range: geography and physics. First, the Bay of Fundy is V-shaped and tapers from its mouth to its head like a funnel. Because the head is so much narrower than the mouth, when the tide enters the bay, the water gets compressed. As it is progressively constricted horizontally, it rises up vertically (SF Fig. 6.14 B). Secondly, the water in the bay forms a standing wave. Think of the Bay of Fundy like a big swing. Imagine you are on the swing, rocking your legs back and forth. If you rock too fast or too slow, the swing will hardly move and you will never get very far off the ground. However, if you rock the swing at just the right frequency, throwing your weight forward and backward at just the right time, you will soon be swinging quite high off the ground. If the tidal frequency matches the resonant frequency of a given bay or inlet it can result in very large tidal ranges. Resonant frequency is the natural frequency of vibration (in this case waves), determined by the physical parameters of the object (in this case the bay). The waves in the bay oscillate, which means that they rise and fall at a continuous rate. The time period of the natural sloshing of the water in the Bay of Fundy, the oscillation period, is about 12.5 hours. This is nearly the same time it takes for one lunar tidal cycle, which results in the amplified tidal ranges in the bay.

 

The Atlantic ocean basin has a natural oscillation period of about 12.5 hours and thus amplifies the tide twice a day. However, the Pacific ocean basin has a natural period of about 25 hours and so its shores have an amplified tide only once a day.

 

<p><strong>SF Fig. 6.15.</strong> Satellite image of the Strait of Gibraltar and western Mediterranean Sea</p><br />

Unlike in the Bay of Fundy, in the Mediterranean Sea the oscillation period does not align with the tide frequency and water can only pass into the sea from the Atlantic ocean basin through a narrow opening, the Strait of Gibraltar (SF Fig. 6.15). The Mediterranean Sea’s tidal range is only a few centimeters on average.

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Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.