Читать книгу Flowers of the Coast - Ian Hepburn - Страница 11

If the beach along a fairly straight coast is examined, it will usually be noticed that if shingle is present it is at or near the top. Sand and finer particles occur lower down, and usually the fineness of material increases seawards. The cliffs behind the shingle may be of any kind, and are not necessarily the source of the shingle. After a severe storm much or all of the beach material can be removed, and the platform on which it rests exposed. In the succeeding normal weather, the beach will gradually accumulate again. Even after an ordinary blow the beach may be combed down, so that coarse and fine material are much more mixed.

When waves break, beach material, coarse and fine, is churned up. There is often some order and arrangement in this movement. If the waves are approaching the shore at right-angles, the pebbles and small stones move up and down the beach. The waves break and send up the beach sheets of water called the swash or send, which carry material upwards. Some of the water of the swash percolates into the beach, some returns to the sea as the backwash. This is nearly always less powerful than the swash, but in its deeper parts can move a good deal of material. If, however, the waves approach the beach obliquely, so also may they advance up it, and stones and sand are not merely carried upwards, but also sideways. When the swash dies out, the backwash returns directly down the slope, and any material moved by it travels in the same direction. Thus on open coasts on to which the waves come obliquely, there is a great deal of lateral displacement of beach material. This process is called beach-drifting, and is of the utmost importance. Its effect is often seen where groynes or breakwaters are built athwart the beach to hold material travelling along it by this process. The beach on one side of a groyne is usually higher than on the other, although often after a storm from a different quarter the high and low sides may temporarily change places.¹

The waves also have a sorting effect, and drive the stones to higher parts of the beach. This process can often be seen in action while waves are breaking on a beach of mixed material. There is still another important factor. Around our coasts there is usually a noticeable difference in the level of the water at low and high tide. On the open coast the range seldom exceeds twenty feet, but in bays and gulfs it may be more. The highest rises and the lowest falls occur about the times of new and full moon. At the half-moons the difference between high and low water is small. Suppose it is now the moon’s first quarter, and the weather generally fair. The waves at both the morning and evening tide will reach to much about the same level. But in succeeding days the high waters will rise higher and the low waters will fall lower with each succeeding tide until the time of full moon.² What effect will this have on the beach? If marked pebbles have been scattered near the water line at the time of the moon’s first quarter they will be seen not only to have moved along the beach if the waves are oblique, but to have been pushed up it by waves at each successive tide, and have gathered near the top, usually in an existing ridge. During the subsequent fall of tide-level after full moon, the pebbles are left stranded, and they may only just be reached again at the next period of springs at the time of new moon.

Thus, if nothing else happened, the pebbles might remain at the beach top for ever. But two other factors are likely to affect them. First, the swing of the tides from neaps to springs is only part of a larger swing that shows itself in particularly high tides near the equinoxes, and sometimes at other times of the year. Secondly, if a severe storm attacks a coast, especially at a period of big tides, shingle may be either swept far above its normal level or dragged in large quantities down the beach. An exceptional storm may overtop the highest beaches. The effect of ordinary storms is plain along any shingle beach, since the seaward face is frequently marked by minor ridges parallel to its length. These are either the heights reached by the last high tides, or the limits of recent storms.

Quite apart from these up-and-down and lateral movements of some beach material, vast quantities of finer stuff are moved alongshore by a different process. A bather on a sandy shelving beach in ordinary weather and in water three or four feet deep notices the lifting effect of the waves: the sand is at the same time somewhat disturbed about his feet. If he allows himself to float off such a beach, he notices that the tidal current carries him one way or another along it. The sand stirred up by the waves may, when a tidal current is running with any speed, also be carried sideways for a short distance. Picture this process during a tidal cycle, and during rough and stormy conditions, and it is at once apparent what vast quantities of fine material can be carried along a beach.

Current-action, however, only takes place under water and below the zone of wave-break. It operates on the higher parts of the beach only at or near high tide; on the lower parts, on an open coast the current may run one way at or near low water, and the opposite way at about high water. Two things at least follow—first, in the deeper water, material may move in different ways at different stages of the tide, and whether there is a balance of movement will depend upon the relative strengths of the flood and ebb currents. Secondly, on the parts of the beach covered only at high water, the movement of material is likely to be in one direction only—that of the current at the time of high water. The resultant process is called long-shore drifting. With beach-drifting it is of the utmost importance in the study of shoreline phenomena.

A wave breaks when it enters water the depth of which is approximately half its wave-length. Thus on a shallow coast, big waves break farther out than do small ones. When breaking offshore, the waves—just as on a beach—drive material up in front of them, so that sometimes ridges of sand and shingle are built some distance from the original shore. If a shingle ridge of this sort attains a fair degree of stability, it becomes an outer beach along which beach-drifting can take place. Hence the ridge may lengthen and become what is called an offshore bar (see here). If the process continues a lagoon-like expanse may be enclosed between it and the old shoreline. Offshore bars are seldom unbroken for long distances, since there are often gaps through which the tide enters and leaves the lagoon, in which marsh development is favoured.

If the supply of shingle is great, and if the lateral transport along a coast is marked, the shingle can accumulate in great forelands like Orfordness, Dungeness, the Crumbles, and the shingle ridges off the Culbin Sands and other parts of the Moray Firth shore. At an early stage a ridge is built. After a time the new shingle coming along the coast shallows the sea floor off the first ridge, so that the waves build another in front of it. This may go on until a whole series of such ridges is formed. It is often noticed that at one part of a shingle foreland the ridges run out to sea in such a way that it is clear they are suffering erosion, whereas at another part new shingle is accreting and being built up into ridges. A study of any big shingle foreland will illustrate this process, but there are few more striking examples than the shingle formation known as the Bar, near Nairn. Fig. 1 shows that it is composed of a number of individual ridges, the north-eastern ends of which are being eroded, whereas growth is continuous at the other end. In short, the whole structure is slowly shifting along the coast.

Along the south-west facing side of Dungeness there are many ridges running directly out to sea, and obviously at one time they continued for some distance. Erosion has cut them, and the material thus provided has travelled round the point of Dungeness and gradually helped to build the numerous ridges forming Denge Beach. Erosion is constantly taking place on the one side, accretion on the other.

FIG. 1.—The Bar (from Steers, Geogr. Journal, 1937).

The shingle that composes the banks and beaches comes partly from the erosion of cliffs, partly from boulder clay and other materials on the sea floor, and largely from glacial and gravel deposits, from which it has been swept by rivers in past times. Along our east and south coast it is mainly composed of flint which originally came from the Chalk. In west coast and Scottish beaches, the percentage of local rocks is far higher, and flint may be quite absent.

In its lateral travel alongshore, shingle often builds ridges or embankments, running across the mouths of rivers and inlets. Nearly all rivers are to some extent obstructed by a bar composed of shingle or sand, or both. When a bar is growing across a river mouth, the unattached end extends very much in the same manner as does a tip heap. At first the bar may be wholly below water; it gradually grows up to the surface. But whether above or below water, the free end tends to be turned inwards as a result of wave action. Many bars of this sort have on their landward side laterals or recurved ends (see here). Some bars grow forward, later turn inwards, and, after a time, grow forward again. It is easy to give general reasons for this—e.g. wave-attack in a storm—but it is extremely difficult to be precise. If the bar is obstructing a river, its form will depend in part on the power of the river to keep its mouth clear. Small streams like those at Chideock and other places on the Dorset coast are completely dammed. In others, e.g. the Exe and Teign, the river maintains a mouth; the Exe Bar is particularly interesting since it is double. At Orford Ness the shingle has not only formed a bar, but has grown into a great foreland and deflected the river for eleven miles. Some rivers like the Spey usually keep their mouths through shingle beaches in nearly the same place, whereas others, like the neighbouring Findhorn, by no means free from violent floods, are deflected.

Scolt Head Island and other features of the Norfolk coast and the Bar off Nairn on the Moray Firth are good examples of offshore bars, and, they, too, lengthen in the same way as ordinary beaches. Since they are offshore, they can send back long lateral ridges.

In bays and other inlets, shingle beaches are usually washed up at the head, to form what are called bay-head beaches. Normal beach-drifting for any distance along an indented coast is impossible. If, however, the bays are rather wider and more open, there is a certain amount of lateral travel of shingle in them, and it gathers at their leeward ends. The distribution of shingle along the several bays between Pwllheli and Penrhyndeudraeth is most instructive. Sometimes a ridge forms across the mid-part of a bay, such as Cemlyn Bay in Anglesey.