Page 536 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

500 part III The earth–atmosphere interface
by the far side, where inertial forces are slightly greater. Because of inertia, as the nearside water and Earth are drawn toward the Moon or Sun, the farside water is left behind because of the slightly weaker gravitational pull. This arrangement produces the two opposing tidal bulges on opposite sides of Earth.
Tides appear to move in and out along the shoreline, but they do not actually do so. Instead, Earth’s surface rotates into and out of the relatively “fixed” tidal bulges as Earth changes its position in relation to the Moon and Sun. Every 24 hours and 50 minutes, any given point on Earth rotates through two bulges as a direct result of this rotational positioning. Thus, every day, most coastal loca- tions experience two high (rising) tides, known as flood tides, and two low (falling) tides, known as ebb tides. The difference between consecutive high and low tides is considered the tidal range.
Spring and Neap Tides The combined gravitational effect of the Sun and Moon is strongest in the con- junction alignment—when they are on the same side of Earth—and results in the greatest tidal range between high and low tides, known as spring tides (Figure 16.6a). (Spring means to “spring forth”; it has no relation to the season of the year.) Figure 16.6b shows the other align- ment that gives rise to spring tides, when the Moon and Sun are at opposition—on opposite sides of Earth. In this arrangement, the Moon and Sun cause separate tidal bulges, as each celestial body affects the water nearest to it. In addition, the left-behind water resulting from the pull of the body on the opposite side augments each bulge.
When the Moon and Sun are neither in conjunction nor in opposition, but are more or less in the positions shown in Figure 16.6c and d, their gravitational influ- ences are offset and counteract each other, producing
a lesser tidal range known as neap tide. (Neap means “without the power of advancing.”)
Tides also are influenced by other factors, includ- ing ocean-basin characteristics (size, depth, and to- pography), latitude, and shoreline shape. These factors cause a great variety of tidal ranges. For example, some locations may experience almost no difference between high and low tides. The highest tides occur when open water is forced into partially enclosed gulfs or bays. The Bay of Fundy in Nova Scotia records the greatest tidal range on Earth, a difference of 16 m (Figure 16.7). For tide predictions in Canada, see waterlevels.gc.ca/eng/ data#s1.
Tidal Power The fact that sea level changes daily with the tides suggests an opportunity: Could these predict- able flows be harnessed to generate electricity? The answer is yes, given the right conditions. Bays and es- tuaries tend to focus tidal energy, concentrating it in a smaller area than in the open ocean. Power generation can be achieved in such locations through the building of a dam, called a tidal barrage, that creates a difference in height by holding water at flood tide and releasing it at ebb tide. The first tidal power plant was built on the Rance River estuary on the Brittany coast of France in 1967 using this method of power production. The tides in the La Rance estuary fluctuate up to 13 m, providing an electrical-generating capacity of a moderate 240 MW (about 20% of the capacity of Hoover Dam; Figure 16.8a). The first tidal power generation in North America also uses a tidal barrage, at the Annapolis Tidal Generating Station in the Bay of Fundy in Nova Scotia built in 1984. Nova Scotia Power Incorporated operates this 20-MW plant.
Tidal power generation can also be achieved through the use of tidal stream generators, underwater
(a) Flood tide at Halls Harbour, Nova Scotia (on the Bay of Fundy coastline).
▲Figure 16.7 Tidal range. [Bobbé Christopherson.]
(b) Ebb tide at Halls Harbour.