Waves

The commonest cause of surface waves is the action of wind on the surface transmitting energy to the water and setting it in orbital motion (Figure 8.4). The size of waves depends upon the speed of the wind, the length of time during which it blows and the uninterrupted distance over which the wind acts on the water (the fetch). Surface waves do not mix the water to any great depth. Their motion falls off sharply with depth, and at a depth equal to half the wavelength of the waves the water is virtually still.

When waves move into shallow water, their advance becomes slowed and the waveform changes when the depth becomes less than half the wavelength. Friction with the bottom causes the shallower parts of the wave to be slowed more rapidly than deeper parts. This is called wave refraction. It causes wave fronts which

Extreme high water of spring tides (EHWS)

Range of spring tides

Extreme high water of spring tides (EHWS)

Range of spring tides

Range of neap tides

12.4 hours

Figure 8.3 Some of the standard terms for the tidal levels of the shore, often used to describe the zonation of littoral organisms.

Range of neap tides

Spring tide, curve

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12.4 hours

Chart datum

Figure 8.3 Some of the standard terms for the tidal levels of the shore, often used to describe the zonation of littoral organisms.

approach the coast obliquely to slew round and change direction in shallow water so that they reach the shore nearly parallel to the beach (Figure 8.5).

Consequently, waves converge upon headlands, but diverge and spread out within bays. The energy transmitted by the waves is therefore concentrated on promontories, and is correspondingly reduced along equivalent lengths of coastline between the headlands (Figure 8.6).

The slowing of wave advance in shallow water reduces the wavelength. As the wave crests become closer together the wave height increases and the fronts of the waves become steeper. Where the water gets progressively shallower near the shore, the waveform becomes increasingly distorted until eventually the waves

Direction of movement of wave

Figure 8.4 Profile of an ocean wave. The circles and arrows show the direction of movement of the water in different parts of the wave, and how this orbital movement decreases with depth.

Figure 8.5 Diagram of wave refraction as waves approach shallow water obliquely.

Figure 8.6 Diagram illustrating the concentration of wave energy on promontories. The energy of the wave fronts between A-B and C-D becomes concentrated by wave refraction onto the short stretches of shore A'-B' and C'-D'. Within the bay the energy of wave front B-C becomes spread out around the shoreline B'-C'.

Figure 8.6 Diagram illustrating the concentration of wave energy on promontories. The energy of the wave fronts between A-B and C-D becomes concentrated by wave refraction onto the short stretches of shore A'-B' and C'-D'. Within the bay the energy of wave front B-C becomes spread out around the shoreline B'-C'.

Figure 8.7 Changes of waveform on entering shallow water. At a depth of about half the wavelength, the waves become closer and higher. With decreasing water depth, the fronts of the waves become steeper until they are unstable and their crests topple forward.

become unstable, their crests overtake the troughs and topple forwards as they break. The energy of the wave motion is then translated into the energy of a forward-moving mass of water (Figure 8.7).

8.2.1 The effects of waves on beaches

The up-rush of water formed by a wave breaking on the beach is known as the 'swash' or 'send' of the wave. Part of this water percolates down through the beach and the remainder flows back over the surface as the 'backwash'. The effect of breakers on the shore may be destructive or constructive. Destructive breakers are usually formed by high waves of short wavelength; for example, those arising from gales close to the coast. As these waves break, they tend to plunge vertically or even curl seawards slightly, imparting little power to the swash, pounding the beach, loosening beach material and carrying some of it seawards in the backwash. Constructive waves are more likely to be low waves of long wavelength, sometimes the swell from distant storms. These waves move rapidly shorewards and plunge forwards as they break, transmitting much power to the swash and tending to carry material up the beach, leaving it stranded.

The continual transmission of energy from waves to shore gradually modifies the coastline, either eroding the beach by carrying away the beach material, or adding to the beach by deposition. In any sequence of breakers, there may be waves derived from many different sources which combine to form many different heights and wavelengths, some destructive and some constructive. The condition of the shore is, therefore, a somewhat unstable equilibrium between the two processes of erosion and deposition. The balance varies from time to time, and differs greatly in different regions. Where the shore is exposed to very violent wave action, erosion usually predominates. The waves break up the shore, fragmenting the softer materials and carrying them away. Harder rocks are left exposed and are gradually fractured into boulders, making the coastline rocky and irregular. Strong currents may assist the process of erosion by carrying away finely divided material. But materials carried away from the shore in one place may be deposited as beach material in another, and where the major effect of the waves is deposition, the beach is made up largely of pebbles, sand or mud.

Beach construction

Beaches consist of a veneer of beach material covering a beach platform of underlying rock. In very sheltered situations the beach material may rest on a gentle slope of rock virtually unmodified by wave action, but in wave-washed localities the beach platform has usually been formed by wave erosion. Where the land is gradually cut back, both cliff and beach platform are formed concurrently (Figure 8.8). As the beach platform becomes wider, waves crossing it lose power, and erosion of the cliff base is reduced. The seaward margin of the beach platform is itself sometimes subject to erosion by large waves breaking further out, so the cutting back of the beach platform and the cliff base may proceed together. Cliff erosion is caused largely by the abrasive action of stones, sand and silt churned up by the water and hurled against the base of the cliff, undercutting it until the overhanging rock collapses. Where the rocks are very hard, they may not be appreciably worn away, but can be cracked along lines of weakness by sudden air compression in holes and crevices when waves strike the cliff. This leads to

Figure 8.8 Beach section to illustrate erosion of the cliff to form the beach platform and veneer.

falls of large pieces of rock, and produces a boulder-strewn coastline on which the beach platform is often quite narrow. The range of tidal movement has a considerable bearing on the rate of erosion because the greater the depth of water covering the beach platform at high tide, the more powerful the waves that can cross it to erode the cliff base.

Various sources may contribute to the materials covering beach platforms; for example, fragments derived from erosion of adjacent cliffs, or churned up from the sea-bed, or eroded from the edge of the beach platform, or carried along the coast from other places by currents or beach drifting (see below), including material carried into the sea by rivers. In sheltered regions, finely divided material carried in suspension in the water may be deposited on the beach as sand, mud or silt.

Beach drifting and grading

When a wave breaks, the swash may carry stones or smaller particles up the beach. Waves which break obliquely on the shore carry materials up the beach at an angle, while the backwash and any particles it contains run directly down the slope of the beach. Consequently, each time a wave breaks obliquely, some of the beach material may be carried a short distance sideways (Figure 8.9). This process, known as beach drifting, can move huge quantities of material over great distances. Drift of sediments along beaches is also assisted by longshore currents generated by oblique waves.

Beach drifting has a sorting effect on the distribution of beach materials. Where cliffs are exposed to strong wave action, the shore is usually strewn with boulders too large to be moved by the waves. Boulders gradually become broken up, and the fragments may then be carried along the shore in a series of sideways hops. A short distance from the original site of erosion, the beach is likely to consist mainly of large stones which only the more powerful waves can move. Further along the shore the particle size of the deposit becomes progressively smaller, partly because the pebbles gradually wear away as they rub one against another, and partly because smaller particles are more easily transported.

Figure 8.9 The movement of pebbles along a beach under the influence of oblique waves.

Waves also exert a sorting action on the grade of material deposited at different levels of the beach, due to the difference in energy of swash and backwash. The swash has the full force of the wave behind it, but the backwash merely flows back down the beach under the influence of gravity, and contains less water than the swash because some is lost by percolation through the beach. The swash may move large stones up the beach but the energy of the backwash may be insufficient to carry them down again. This often leads to an accumulation of large pebbles at the back of the beach, causing the slope of the beach to become steeper towards the land until a gradient is reached at which stones begin to roll down under their own weight. The smaller particles of sand and gravel are more easily carried down by the backwash, and this has a grading effect, depositing coarse material at the back of the shore and progressively smaller particles lower down. Around the British Isles there are many beaches with steeply sloping shingle at the higher levels, and flatter sand or mud on the middle and lower shore.

Drifted beach material accumulates alongside obstructions crossing the shore; for example, headlands, large rocks or groins. The purposes of erecting groins is to limit beach drifting by causing pebbles and sand to become trapped between them. Wave action then builds the beach into high banks between the groins, preventing the beach material from being carried away, thus protecting the land against erosion.

8.2.2 Tsunamis ('tidal waves')

Earthquakes, volcanoes, movements of the sea floor and submarine escapes of gas may sometimes generate immense disturbances within the oceans which cause huge waves of unusual form. The description 'tidal waves', previously widely used in this connection, is a misnomer, because these exceptional waves have no association with tides. The Japanese name tsunami is now generally given to this phenomenon. Such waves can have devastating effects if they strike the coastline.

The term 'tidal wave' was probably applied because the first sign of an approaching tsunami is usually a rapid change of sea level, often appearing to be a small rise of the tide quickly succeeded by a sharp fall to an abnormally low level for about 15-30 minutes. This is followed by an enormous wave suddenly rising out of the sea, often with the appearance of a near-vertical wall of water several metres high, rushing forward at great speed to overwhelm the land along low-lying coasts. The water level then falls for a period up to about an hour, when another wave of even greater size may occur. A succession of these huge waves sometimes extends over several hours, and those in the middle of the sequence usually have the greatest size, which may be as much as 30-40 m in height.

Tsunamis occur mainly around the shores of the Pacific, where they have caused terrible devastation and loss of many thousands of lives. They have also been experienced in the Indian Ocean, the Mediterranean and even the North Atlantic. Many people died in Portugal in 1755 when a tsunami swept up the river Tagus. Around 7000 years ago, a tsunami struck the east coast of Scotland and penetrated 4 kms inland. It was caused by a massive landslip off the coast of Norway (MacGarvin, 1990). One of the worst ever occurred in 1960. A very strong earthquake in southern Chile initiated tsunamis that affected not only the Chilean coast but also the Hawaiian Islands and especially Japan, over 10 000 miles away from the origin.

Such extraordinary waves at the coastline are the result of ocean disturbances quite different from wind-induced surface waves. The submarine upheavals which cause tsunamis generate deep-water waves of low height but great wavelength and velocity. Wind waves seldom have wavelengths greater than 300 m or speeds of more than 100 km/h. The wavelengths of tsunamis range between 150 and 1000 km with speeds of 400-800 km/h. The wave height of tsunamis in deep water is only a few metres and, because the wavelength is so long and the period between wavecrests often an hour or more, they are virtually undetectable in the ocean. But when they move into shallow water the velocity of the waves becomes sharply reduced, the wave height rises steeply as the crests become crowded together, resulting in a series of catastrophic waves breaking at intervals of about 15-60 minutes.

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