Discuss the role of tides and wave energy in coastal landform development.

We begin our exploration of coastal landforms by examining the daily sea-level changes caused by tides. Tides are important to the study of coastal processes because they allow the energy of waves to influence a greater vertical range of coastline than it would otherwise.

If you have ever visited the ocean, you may have noticed the effects of tides. Tides are the rise and fall of sea level caused by the gravitational effects of the Moon and the Sun. At low tide, you may have seen algae and barnacles clinging to slippery rocks. At high tide, these organisms are submerged.

In a classical sense taken from astronomy, “tides” are distortions of the shape of one body by the gravitational pull of another body. The gravitational pull of the Moon and the Sun distorts Earth’s atmosphere, its oceans, and the body of the planet itself. For our purposes, however, the term “tide” will be used to refer to the twice-daily changes of sea level in coastal areas.

Tides are caused by the tide-generating force created by the gravitational pull of the Moon and Sun as well as by the centrifugal force created by Earth’s and the Moon’s revolution around a common center of mass within the Sun-Earth-Moon system. These forces create two bulges of seawater on Earth. The tidal bulge beneath the Moon is larger than the one on the opposite side of the planet (Figure 19.2).

The tide-generating force of the Moon is greater than that of the Sun because the Sun is nearly 400 times farther away from Earth than the Moon. Lunar tides, those caused by the Moon, are roughly twice as high as solar tides, those caused by the Sun.

Sometimes, tides are unusually high or low. When Earth, the Moon, and the Sun are aligned along the same axis, the tide-generating forces of the Moon and the Sun act together, and high tides called spring tides result. When the Moon is positioned at a right angle between Earth and the Sun, the Moon’s pull acts perpendicularly to the Sun’s, creating a lower neap tide (Figure 19.3). Other factors, such as windiness and how close the Moon is to Earth, also influence tide levels.

Because of Earth’s rotation on its axis and the revolution of the Moon around Earth, the daily rhythm of the tides runs on a schedule of approximately 24 hours and 50 minutes. This means that for 6 hours and 13 minutes, the tide is coming in as a flood tide, and for the next 6 hours and 13 minutes the tide is going out as an ebb tide. This pattern repeats twice daily on most coastlines. High tide occurs when the flood tide has peaked, and low tide occurs when the ebb tide is at its lowest. The exposed shore area below the high-tide line is called the intertidal zone.

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If Earth’s surface were perfectly smooth, without wind, and covered only with water, the tidal range—the difference between low tide and high tide levels—would be the same for any given latitude at all locations. However, differences in bathymetry, windiness, and the positions and shapes of coastlines affect the height of tides and create uneven tidal ranges across the oceans (Figure 19.4).

In some regions, tides are barely noticeable, while in others, their effects dominate the coastline. The average tidal range on Earth is 100 cm (36 in). The world’s greatest tidal range occurs in the Bay of Fundy in eastern Canada (Figure 19.5), where the maximum tidal range is 16.8 m (55 ft).

Waves result as wind energy is transferred to the ocean’s surface by friction. If there were no wind, the oceans would be glassy smooth. Only infrequent wave-generating forces, such as earthquakes (see The Human Sphere in Chapter 14) and coastal landslides, would make waves.

As waves move across the surface of the open ocean, they give the appearance that the water is flowing, but that is not the case. A floating object on the ocean’s surface, such as a bottle, moves in a circular pattern as the waves pass beneath it. The bottle does not move forward horizontally with the waves. Individual water molecules, like the bottle, trace a circular path. Waves that cause this circular movement are called waves of oscillation.

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Because most waves are organized in long ridges, the rotating circles in a wave of oscillation are better pictured as connecting together to form long cylinders. The radius of the topmost cylinder at the surface is equal to the wave height. Waves are composed of a series of cylinders that are stacked vertically. With increasing depth, the diameter of the cylinders decreases. The movement of water ends at the wave base, at a depth equal to about three-quarters of the wavelength (the distance between wave crests) (Figure 19.6).

In the open ocean, wave height depends on the speed of the wind, the duration of the wind, and the fetch: the distance of open water over which the wind blows. Slow winds over a short fetch, such as an inland lake, produce small waves. Fast and sustained winds blowing over the open ocean with a long fetch generate large waves with long wavelengths, called swells. Swells, which are generated only in the open ocean, can have wave heights of 2 to 10 m (6 to 33 ft) or more and wavelengths of 40 to 500 m (130 to 1,600 ft) or more.

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As a wave approaches the coastline, the wave base comes in contact with the seafloor (see Figure 19.6). As the wave continues into shallower water, friction with the seafloor slows the base more than the top and as a result the wave crest grows higher. The wave crest also travels faster than the wave base, causing the wave to shear in an elliptical motion. In this type of wave, called a wave of translation, water does move forward horizontally in the direction of wave movement. Eventually, the wave crest collapses, forming a breaker (see Figure 19.6).

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On a beach, breakers form in the surf zone. As breakers collapse on themselves in their forward momentum, they create a swash: a rush of water up a beach. The slope of the beach will limit how far the swash can travel up the beach. The water flows back down the beach as backwash and returns to the ocean.

As waves approach the shoreline, they bend so that their orientation is roughly parallel to the shoreline. This process is called wave refraction. Even where the coast is uneven, with rocky prominences called headlands and concave sandy beaches called embayments, waves refract around the headlands and maintain a mostly parallel approach to the shoreline (Figure 19.7).

Although wave refraction causes waves to approach a beach nearly parallel to the shoreline, most wash ashore at a slight angle. This angled approach of waves creates a longshore current, which flows parallel to the beach in the direction of wave movement. Longshore drift is the movement of sediment down the length of the beach in the direction of wave movement (Figure 19.8). The longshore current and longshore drift transport sand, both on the beach and beneath the water in the surf zone. The surface layers of the sand are always moving down the length of the shore.

Picture This discusses rip currents, short-lived jets of water that form as backwash from breakers flows back into the sea.

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