Читать книгу Waves and Beaches - Kim McCoy - Страница 18

Waves range in size from the short ripples in a pond to the great storm waves of the ocean and the tides, whose wave length is half the distance around the Earth. In order to be able to discuss such widely varying kinds and sizes of waves, it is necessary to agree on a standard set of names for the parts of a wave (see figure 3).

The principal parts are defined as follows:

Crest:	The high point of a wave.
Trough:	The low point of a wave.
Wave height:	Vertical distance from trough to crest.
Wave length:	Horizontal distance between adjacent crests.
Wave period:	The time in seconds for a wave crest to traverse a distance equal to one wave length.

There is a direct relationship between wave period and wave length, but wave height is independent of either.

Waves are classified according to their period; most range from less than one second to minutes (tsunamis) to hours (tides). Each undulation of each wave changes sea level for a characteristic period of time. Occurrences measured in years such as El Niño–Southern Oscillation (ENSO) bring storm waves, and the thousands of years–long period Milankovitch cycles (Earth’s tilt and orbit patterns) affect sea level. The wave spectrum diagram (see figure 4, page 28) shows that the waves in the ocean are distributed among several major types, each with its characteristic range of periods and influence on sea level.

Beginning near the left side of the spectrum with the very short-period waves, we have in order: ripples, with periods of fractional seconds; wind chop, of one to four seconds; fully developed seas, five to twelve seconds; swell, six to twenty-two seconds; surf beat, of about one to three minutes; tsunamis, of ten to twenty minutes; and tides, with periods near twelve or twenty-four hours. Thus, there are many kinds of waves, each generated and developed in a special way.

FIGURE 3: The parts of a wave. The period of the wave is the time in seconds for two successive crests to pass a fixed point, such as a piling. Wave height is from crest to trough.

Note that all the water waves just mentioned are called gravity waves because, once they are created, gravity is the force that drives them, by attempting to restore the original flat-water surface.

Each gravity wave is made up of two parts: the crest that rises above the average sea level and the trough that extends below it. As a group of waves moves over the surface of the water, each crest seems to be forever attempting to overtake the trough ahead, fill it in, and restore equilibrium. The wave source, whatever it was, worked against gravity.

One special form of wave not driven by gravity is possibly the most abundant kind of wave on the sea. The first tiny ripples that a light breeze raises on a glassy sea surface, or on the slopes of larger waves, are called capillary waves—capillary because they are controlled by surface tension and respond to the same forces that cause water to rise in capillary (very small diameter) glass tubing. The capillary force inside a small glass tube is stronger than gravity, so the water moves slightly upward.

FIGURE 4: The ocean-wave spectrum extends in time from seconds to millennia. Each wave changes sea level during its own period. The most difficult waves to predict and quantify are shown in orange.

Surface tension is a property of liquids that makes them want to pull together and act as though they were covered by an elastic film. This force pulls water molecules together (cohesion) and determines the shape of each capillary wave and form of every drop of water. Capillary waves are often 1 or 2 mm high, a few centimeters from crest to crest, and are usually seen in groups of a dozen or so. In the sea, they arise when the drag of moving air stretches the surface and wrinkles the uppermost thin layer of water where there is a more systematic alignment of the molecules of water. Thus the size, slope, and velocity of these tiny waves are governed by the elasticity or tension of the surface film.

As might be expected in a phenomenon dominated by surface tension, the wave crests of capillary waves are rounded rather than peaked. A rise in the sea temperature will cause surface tension to reduce and viscosity to decrease, and these changes affect how waves are formed. Unlike gravity waves, the shorter wave lengths move faster. Capillary waves give way to the development of ripples, also caused by minute pressure differences, which are at the beginning of our wave spectrum and lead to the growth of larger waves.

GRAVITY WAVES Long-period swell can travel great distances, far beyond where the waves were created. These gravity waves cross oceans with little loss of energy before reaching land. Raglan Township, New Zealand. Rambo Estrada

CAPILLARY WAVES Capillary waves are the smallest of waves, formed by the wind and objects, which stretch and wrinkle the water’s surface only for it to be pulled back by surface tension. Kim McCoy

The simultaneous existence of so many kinds and sizes of waves on the surface of the ocean, coming from different sources, moving in many directions, and changing inexplicably from day to day, made it difficult for us to learn the ways of waves.

For example, see what happens when you toss several pebbles into a pool of water (pictured on page 31). The impulse generates a series of similar waves that move outward in all directions. The simple circular pattern is clear until the first waves reach shore and are then reflected backward. Now the pattern is not so simple, for the wave fronts of the returning waves interfere with the outgoing waves. The two sets of waves form curious patterns with diamond-shaped high points where crests coincide. As the reflections from the other sides of the puddle are added, the interference pattern becomes very complex. For a few moments there is a hopeless jumble of high points moving in all directions, and then the whole surface flattens back to mirror-like calm. You could perform this seemingly simple experiment a hundred times and still not clearly understand what happened.

Waves radiate outward from their source until encountering other waves or objects. YAY Media AS/Alamy Stock Photo

In the ocean, however, the situation is far more complicated. First, the source of the waves is rarely an impulse at one point—usually it is a gusty wind blowing over a broad area that creates very irregular wave shapes. Second, waves change in character as they leave the generating area and travel long distances. Third, usually several sets of waves with different periods and directions are present at the same time. Fourth, waves are greatly influenced by the undersea topography (frequently called bathymetry). When they approach shore and move into shallow water, the wave fronts bend and the waves break, expending their energy in foam and turbulence. Plain and simple, any questions about the manner in which waves are born, develop, travel, or die cannot be answered easily by casual observers. In fact, ocean waves are so hopelessly complex that thousands of years of observations produced only the obvious explanation that, somehow, waves are raised by the wind. The stronger the wind, the bigger the waves, of course.

The description of the sea surface remained in the province of an anonymous poet, who found it “… troubled, unsettled, restless. Purring with ripples under the caress of a breeze, flying into scattered billows before the torment of a storm and flung as raging surf against the land; heaving with tides breathed by the sleeping giant beneath.” A fanciful but quite useless description of the wave spectrum.

Now, after more than a hundred years of scientific work, including concentrated efforts since the 1940s, most of the major features of waves and their causes can be satisfactorily explained in mathematical terms and reproduced experimentally. Theoreticians use complicated equations and occasionally the study of waves slips into the hands of those who have never been to sea. Here we will resist such complexity and remain descriptive when possible.