Chapter 11: Tides (Concept Check and "Thinking Critically" Study Questions)

Though they move through all the ocean, tides are referred to as shallow-water waves. How can that be?

Remember a deep-water wave is, by definition, in water deeper than half its wavelength. The wavelength of seismic sea waves usually exceeds 100 kilometers (62 miles). No ocean is 50 kilometers (31 miles) deep, so seiches, seismic sea waves, and tides are always in water that to them is shallow or intermediate in depth; their huge orbit circles flattening against a distant bottom always less than half a wavelength away.

What tidal patterns are observed on the world's coasts?

Some coastlines experience semidiurnal (twice daily) tides: two high tides and two low tides of nearly equal level each lunar day. Others have diurnal (daily) tides: one high and one low. Coastlines with mixed (or semidiurnal mixed) tides have successive high tides or low tides of significantly different heights, caused by blending of diurnal and semidiurnal tides. Figure 10.13 shows an example of each tidal pattern.

What is a spring tide? A neap tide?

Spring tides occur when Earth, moon, and sun are aligned (Figure 10.11a). Neap tides occur when Earth, moon, and sun form a right angle (Figure 10.11b).

In general terms, how is the pull of gravity between two bodies related to their distance?

The pull of gravity between two bodies is proportional to the masses of the bodies but inversely proportional to the square of the distance between them.

Where is electrical power being generated from tidal movement?

There are major tidal power stations in France on the estuary of the river Rance and on the Annapolis River in Nova Scotia.

From what you learned about tides in this chapter, where would you locate a plant that generated electricity from tidal power? What would be some advantages and disadvantages of using tides as an energy source?

Tidal power has many advantages: Operating costs are low, the source of power is free, and no carbon dioxide or other pollutants are added to the atmosphere. But even if tidal power stations were built at every appropriate site worldwide, the power generated would amount to less than 1 percent of current world needs. And, of course, this method of power generation is not free of trade-offs. The dam and electrical generators can be damaged by storms, and the large finely-made metal valves and vanes at the heart of the plant are easily corroded by seawater. Computer simulations have suggested that installing a dam would change the resonance modes of a bay or estuary and therefore the height of the tide wave. Studies also suggest that sensitive planktonic and benthic marine life would be disrupted and even that increased tidal friction would cause a minute decrease in the rate of Earth's rotation. Though residents and mariners would certainly oppose such a project, a tidal dam beneath San Francisco's Golden Gate Bridge could generate more electrical power than any existing tidal generating station. It has the prime requisites: The narrow entrance to the Bay, combined with the large volume of water passing back and forth through the strait every day, would make such a site nearly ideal for this purpose. The esthetic, navigational, and structural difficulties presented by such a project mitigate very heavily against its successful completion.

What are the most important factors influencing the heights and times of tides? What tidal patterns are observed? Are there tides in the open ocean? If so, how do they behave?

Tidal range (high-to-low-water height difference) varies with basin configuration and location. In small areas such as lakes, the tidal range is very small. In larger enclosed areas such as the Baltic or Mediterranean seas, tidal range is also moderate. Tidal range is not the same over a whole ocean basin—it varies from the coast to the centers of oceans. Tides near the centers of ocean basins exist, but they tend to be small in magnitude. The largest tidal ranges occur at the edges of the largest ocean basins, especially in bays or inlets that concentrate tidal energy because of their shape. Tidal range at the apex of a funnel-shaped sea, gulf, or bay can often be extreme. In the Bay of Fundy near Moncton, New Brunswick (Canada), tidal range is especially wide: up to 15 meters (50 feet) from highs to lows! The northern reaches of the Sea of Cortez east of Baja California have a tidal range of about 9 meters (30 feet). Tide waves sweeping toward the narrow southern end of the North Sea can build to great heights along the southeast coast of England and the north coast of France. The shape of the basin also has a strong influence on the patterns of tides. Because of basin resonances, some coastlines experience semidiurnal (twice daily) tides: two high tides and two low tides of nearly equal level each lunar day. Others have diurnal (daily) tides: one high and one low. The tidal pattern is called mixed if successive high tides or low tides are of significantly different heights through the cycle.

How is a forced wave different from a free wave?

Tide waves are called forced waves because they are never free of the forces that cause them. In contrast, after they are formed, wind waves, seiches, and tsunami are free waves -- they are no longer being acted upon by the force that created them and they do not require a maintaining force to keep them in motion.

How does the latitude of a coastal city affect tje tides there - or does it?

Look at Figures 10.6 and 10.7. Imagine an Earth without continents and with an ocean of uniform depth. Notice that tides would be more extreme near the equator and less extreme toward the poles. Adding the resonance effects of continents and ocean basins, however, alters this orderly relationship. Extreme tides can be found at nearly any latitude outside the Arctic and Antarctic circles.

What is a meteorological tide?

Meteorological tides are weather-related alterations to predicted tidal cycles, such as those associated with the storm surge of tropical cyclones.

Zones of marine organisms can usually be seen along rocky shores. How might tidal patterns result in this sort of differential growth?

Within the intertidal zone, organisms are exposed to varying amounts of emergence and submergence. The animals and plants sort themselves into horizontal bands based on the amount of exposure they can tolerate. Each distinct zone is an aggregation of animals and plants best adapted to the conditions within a particular narrow habitat.

Are tides always shallow-water waves? Are they ever in "deep" water?

Because of their immense wavelength, tides can never be in "deep" water (that is, water deeper than half the wavelength), even though their crests may traverse abyssal depths.

What is a tractive force? How is it generated?

The combined outward-flinging force of inertia and inward-pulling force of gravity are called tractive forces. Gravity and inertia don't always act in exactly the same balanced way on each particle of Earth and moon; the tractives forces are the net strength and direction that result when the two forces are combined (Figure 10.5). The key to understanding tides is to imagine Earth turning beneath bulges of water formed by tractive forces.

How does the equilibrium theory of tides differ from the dynamic theory?

The dynamic theory correctly treats tide waves as shallow-water waves. As Earth turns, landmasses divert, slow, and otherwise complicate the movements of tidal crests. This interference produces different patterns in the arrival of tidal crests at different places.

How does basin shape influence tidal activity? What's a tidal bore?

The largest tidal ranges occur at the edges of the largest ocean basins, especially in bays or inlets that concentrate tidal energy because of their shape. If the basin is narrow and restricted, the tide wave crest cannot rotate around an amphidromic point and simply moves into and out of the bay. In other cases, arriving tide crests stimulate natural oscillation periods of around 12 or 24 hours, resulting in extreme tides. A tidal bore is a steep wave moving upstream generated by the action of the tide crest in the enclosed area of a river mouth.

What celestial bodies are most important in determining tides?

The position and proximity of the moon makes the most important contribution to tidal patterns. The sun's influence on the tides is only 46% that of the moon's.

Are there tides in the open ocean?

Tidal crests rotate around amphidromic points - "no tide" points in the open ocean. Because of the shape and placement of land masses around ocean basins, the tidal crests and troughs cancel each other at these points.

Why isn't tidal power being developed more aggressively?

Tidal power plants can be damaged by storms and corroded by seawater. Computer simulations have suggested that installing a dam would change the resonance modes of a bay or estuary—and therefore the height of the tide wave. Studies also suggest that sensitive planktonic and benthic marine life would be disrupted.

Can astronomical and meteorological tides interact?

Arrival of a storm surge on top of a high tide can be especially devastating to coastal regions. Some of the astonishing destructiveness of Hurricane Katrina in 2005 can be attributed to the arrival of the surge (and wind-driven masses of water) coincidentally with a high tide.

What body generates the strongest tractive forces?

Because of its proximity to Earth, the moon generates the strongest tractive forces.