Difference Between Tidal Resonance And Tidal Bores
Scientists have determined that these great differences in tidal range are caused mainly by differences in the shape and size of bays, gulfs, or other parts of the ocean basins. The Gulf of Mexico, they point out, has a comparatively narrow mouth and wide shoreline. The tidal waters that enter the mouth are spread thin along the shoreline. Gulfs and bays with similar shapes should have similarly small tidal ranges.
The Bay of Fundy is V-shaped. Water entering its wide mouth from the open ocean is squeezed into less and less space as it moves into the head of the bay. Therefore a large tidal range might be expected along the shoreline. But shape appears to be only part of the explanation of the enormous tidal range, for many other V-shaped bays do not have large tidal ranges. The rest of the explanation is related to something called tidal resonance.
Perhaps the best way to understand resonance is to think of making waves in the bathtub. Suppose you start a wave at one end. It travels to the other end of the tub, is reflected, and comes back. Now suppose with careful timing you make the second wave. You make it at the very instant the first wave starts to move away from you again. The second wave moves in time with the first—this is resonance. The first and second waves reinforce each other. If you keep on making new waves that move in time with the others, the movement of water gets bigger and bigger.
Very much the same thing happens in the Bay of Fundy. The bay has a natural period of oscillation of about 12 hours. This means that it takes 12 hours for a wave to travel the full length of the bay and back again. But the tide-making forces also have a period of about 12 hours. Resonance occurs, and great quantities of water move into and out of the bay.
In some parts of the world tides from the oceans are funneled into the mouth of a river. Under certain conditions this action creates a huge wall of water that moves quickly up the river. The wall of water is called a tidal bore. Four very famous tidal bores are those on the Severn River in England, the Amazon in South America, the Tsientang Kiang in China, and the Petitcodiac, which empties into the Bay of Fundy.
A tidal bore may occur where there is a large tidal range in the open ocean. Or it may occur where the shores of the bay funnel large amounts of tidewater toward the mouth of the river. When the high tide nears the river, it runs into shallows. It also meets water being carried down by the river. Often the result is that the tide is held back. The water builds up and reaches a considerable height. Then it forces its way up the river at great speed. The tidal bore on the Amazon is sometimes 16 feet (4.9 meters) high and 1 mile (1.6 kilometers) wide. At the time of a spring tide the Tsientang bore can be 25 feet (7.6 meters) high.