The Sea Coast

CHAPTER 2 (#ulink_8192062e-8540-58b7-bdae-4b92316dff8b)

THE MOVEMENTS OF BEACH MATERIAL

THE MOST casual acquaintance with the coast of this country reveals the fact that lines of cliff and beach, capes and headlands standing out into deep water, bays with sandy beaches, and large expanses of shingle are common and repeated features.

In the first place it is important to realise that beaches are not permanent. That this is so is perhaps best seen during a storm which may remove the whole of a particular beach. If the place where it formerly stood is then examined it will be realised that the materials composing the beach rest on a platform cut in whatever rock may be present. Sometimes, in more sheltered places, the sands and gravels may rest on a slope or ledge of rock hardly if at all modified by marine action, but in the more open or exposed localities the platform under the beach is an erosion product. Beaches may also have the form off-shore bars (q.v.).

The sands, gravels, and coarser materials making the beaches are primarily formed by the erosion of some pre-existing rocks. It may be that the cutting of the platform itself is the main source of the material resting on it. Frequently, however, the beach material of a particular locality has migrated to that place from elsewhere. Let us see, therefore, how material moves alongshore. There are two main ways: by beach drifting and by currents, and each needs careful attention.

On an open beach it is often noticeable that waves do not break directly parallel with the trend of the coast, but approach at an oblique angle. If their action is watched, it is plain that as the wave breaks it sends rushing obliquely up the beach a mass of water called the swash or send. This water carries stones and smaller material with it, which move up the beach in the same direction as the swash. Where the swash dies away, the water that has not percolated downwards returns directly seawards.

FIG. 2 Diagrams showing the drift of shingle along a shelving beach

It, too, carries back some material, and the observer may notice that stones moved obliquely up the beach by the swash, may return directly down with the backwash. In short, any one pebble has advanced sideways from its original position along a path resembling a parabola (Fig. 2). All sand and gravel capable of being moved by the waves is similarly treated, and between low and high water a considerable extent of beach may be affected, depending chiefly upon the slope of the beach and the range of the tide. After high water much material is left stranded on the upper parts of the beach, and will remain there until the next high water. Since in most places the tides run through a fortnightly cycle it may happen that during the period from neaps to springs a great deal of material is swept to the top of the beach.

This introduces another point. Many of our beaches are sandy in their lower parts, although there may be a good deal of shingle higher up. Even on an entirely shingle beach, larger stones are often commoner on the higher parts. This follows largely from the facts already given; the swash is more powerful than the backwash, since the latter is weakened, especially on a pebbly beach, by rapid percolation. Moreover, the swash dashing up the beach can carry big stones with it—in storms even large boulders. But these stones, if in the front parts of the swash, are unlikely to retreat down the beach with backwash, partly for the reason just given, and partly because the retreating water, already lessened by percolation, has to start from zero and consequently has but little power. Of course, if the beach profile is steep, some stones may roll down again. The continuation of these movements results in the accumulation of the coarser stuff at the top of the beach. The consequence is that the shingle of many beaches is only worked upon by the waves at and near high water, and the highest shingle may be reached only in storms. On the other hand, sand and finer materials are carried seawards by the backwash. It follows that the general direction of travel of beach material by this process of beach-drifting depends a great deal on the relation and nature of the beaches to the prevalent and dominant winds. This can be readily illustrated. Along the Sussex coast the prevalent wind comes up Channel; this is also the dominant wind since it comes from off the greatest open space, or fetch, of water. Hence the beach material, in so far as this action is concerned, is moved eastwards. On the Suffolk coast the prevalent winds are the same, i.e. the westerlies, but they are certainly not dominant, and, in fact, come directly off the land and so have virtually no effect on the beaches. The important winds are those reaching the shore from the quarter between north and east, the quarter with the greatest expanse of open water. Hence these waves approach the Suffolk coast in such a way as to give a southerly movement of beach material.

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The effects of this action have often been investigated, and the rate at which this shingle moves measured. The figures on the following page will serve as an example.

It is easy to investigate this type of movement on the beach. Little, however, is known of what happens at a rather lower level. In June, 1948, an attempt was made to explore this problem along the Chesil Beach. A diver either walking in from the beach or let down from a boat and walking toward the zone of breaking waves was able to notice the effect of the waves on the stones which are completely submerged. Unfortunately, in rough weather the observations are impossible and in this first attempt we had to be content with generalities and noticing how to overcome difficulties if the experiment can be repeated. Enough, however, was seen to make it clear that under the waves in which the diver could operate with safety, there was a to-and-fro motion of pebbles. When the waves were approaching rather more obliquely there was some lateral translation of the stones. There seemed little doubt that waves can and do move pebbles below water level even in fairly calm weather. In rough weather it is probable that the waves can lift the pebbles off the bottom,

(#ulink_9a60fe8e-f29f-5dfc-bdeb-e285c1afbeda) if they are not too big, and that (see below) they may be carried laterally while “afloat” in the tidal current. However, much yet remains to be verified.

The motion and effect of wind waves die out rapidly downwards. In a wave the movement of the particles of water is as shown in Figure 3, and the downward loss of power is rapid.

(#ulink_c0a2ff71-6252-57e2-937a-a1131bf6c31a) On the bottom there may be only a to-and-fro motion. A rough but useful test of what happens on a sandy foreshore can easily be made when bathing. If you stand in water about four or five feet deep when ordinary calm weather waves are running, you will find that as the wave passes you, you are lifted up a little. At the same time (Fig. 3) you are “bent” shorewards when the crest passes, and in the opposite. direction during the passage of the trough. The general effect is visible when a wind is blowing over a field of corn in ear. The whole field appears as if it were waving, but any individual stalk and ear is not displaced in the sense of being torn out of the ground, but the ear of wheat undergoes a motion something like the water particles in a wave. A similar kind of motion applies to you if you stand firm when you are bathing. At the same time you may notice that the lighter sand is stirred up around your feet.

TABLE I

SCOOT HEAD ISLAND

If you allow yourself to float, and let the water take you as it will, you may be carried along, slowly, but roughly parallel with the beach.

FIG. 3 Wave motion

It is the tidal current which is doing this, and these currents vary in direction with the state of the tide. At any given locality they can be investigated, but on an open beach it may happen that with a rising tide the movement is, say, eastward, whereas on a falling tide it is in the opposite direction. If you continue your observations while bathing, and if you peer down a little under the water, you will note that the sand is disturbed by the waves about your feet, and that individual grains so stirred up may be carried a short distance one way or the other by the tidal current. The disturbed particles soon fall to the bottom, but the next wave may stir them up again, and another lateral jump takes place. In rough weather this action takes place on a much greater scale. The particles “jump” sideways, or if you like, move laterally in a saltatory manner. If the tidal current is moving fast, it may carry material with it just as a river does. In both of these ways—beach-drifting and tidal currents—vast amounts of sand and silt are carried. It will be appreciated that current action, which can go on at all states of the tide, is quite distinct from the beach-drifting that takes place on the beach itself. What is more, since tidal currents along a stretch of coast may flow in opposite directions at different times, the balance of movement may be quite small. Often, however, the flood current, i.e. the current associated with a rising tide, is the more powerful.

To current action of this sort, impossible effects are occasionally ascribed. A current which can move an individual pebble in a smooth trough is by no means necessarily capable of moving the same pebble over a sand surface, and still less of moving it from a group of similar stones. The mass of pebbles in a sense behaves as one large one. Hence, a five or six knot current is not necessarily moving shingle over which it may be flowing. If, however, the shingle were put into motion in some other way, it might continue to move for some time. The method by which a current causes the movement of material on the sea floor can take place in one of two ways. Either, the current causes a direct thrust against any particle projecting above the general level, or, because of swirls and eddies a downward motion of the water is caused. It is for this reason that large stones lying on the bottom are often direct aids to erosion. It is therefore important to know the bottom speed of a current; it may be quite different from the surface speed, and the surface and bottom currents, even in shallow water, may run in opposite directions. Assuming that the direction of movement is the same at all depths in moderately deep water, the speed close to the bottom is about 85 per cent of the surface speed. On the other hand, anomalous factors may arise.

Owens found that where sand exists in quantity all currents up to 2·5 feet per second, or 1·7 miles per hour, are ineffectual in moving shingle, whereas at about 2·5 feet per second the current suddenly acquires the power of moving stones up to nearly 3 inches in diameter over a sandy bottom. Complications naturally occur if the bottom is not smooth, and in nature it seldom is. Owens’ conclusion, with which I fully concur as a result of my own observations, is that “… since the sea bottom is nearly always irregular, and stones are seldom perfect spheres, the effect of currents alone, unless of exceptional velocity, is chiefly limited to the transport of fine matter such as sand and mud.”

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Before turning to various types of beach, two other factors in wave action need attention. It is perhaps too great a simplification to assert that, even for our present purpose, waves are of two sorts, but it is helpful to note the suggestion of Lewis,

(#ulink_dfc1cc41-e90e-5a5b-b308-ec519a05ff09) who differentiated between constructive and destructive waves.

FIG. 4 Destructive (A) and Constructive (B) waves (After V. W. Lewis)

It has long been realised that some waves erode while others build up the beach. Lewis pointed out that in destructive waves the power of the backwash is large relative to the swash, which, in this type of wave, spreads up the beach rather lifelessly, even though the amount of water in it may be considerable. These waves have a marked orbital motion, and plunge down vertically or even curl seawards a little, into the backwash of the receding wave. Their energy is thus expended, and the swash is left with little power. Constructive waves break more regularly, and with a frequency of five to eight a minute. Since the breaker plunges less vertically, more energy is transmitted to the swash, which is swift and effective, even if the volume of water is less than in destructive waves. The backwash is weaker because the swash has spread over a bigger area, and has therefore lost more by percolation; also the greater amount of friction lessens the effect of the backwash. The length of constructive waves is greater than that of destructive waves. The nearly vertical plunge of a destructive wave is induced by the great mass of water brought to the beach by its predecessor, thus allowing the new wave to travel farther inshore, and plunge vertically on to a stretch of water without any extensive swash. These factors intensify the backwash. In a constructive wave, the orbital motion is more elliptical, and Lewis thinks that they break at an earlier stage in their orbit than do destructive waves (Fig. 4).

The action of waves on beaches needs much more attention, but certain generalisations may be made, although future work may lead to some modification of them. It is often noted after storms or strong onshore winds that beaches have been combed down and their profile flattened. In normal and calm weather, the profile is steepened. Storm waves are more likely to result from gales near to, or not far distant from, the shore, and are not to be confused with a ground-swell, the product of a distant gale. Storm waves are high in proportion to their length and so they may resemble destructive waves. There are, however, still many uncertainties. Big storm waves, although they comb down a beach, also throw up shingle to its highest parts, or even over its crest, and big swell waves may have a destructive effect because friction and percolation will then have less relative effect in weakening the great volumes of swash and backwash, and the effect of gravity may thus make the backwash dominant.

Further complications arise because two or even more sets of waves may approach a beach at the same time, or because smaller waves may be superimposed on larger ones. There is much scope for intelligent observation of the effect of waves on beaches, and anyone staying by the sea can contribute to the solution of the problem if he will note carefully the weather conditions, especially the strength and direction of the wind, the height and general appearance of the waves, the rate at which they break (i.e. so many per minute), and, as far as possible, the effect they are having on the beach. Any simple statement of the matter is impossible. Destructive waves may begin to work on a beach profile built up by constructive waves, or vice versa, or perhaps a swell may run in and break upon a beach formed by storm waves, or possibly some peculiar local conditions may exist. However, the effects of waves on beaches may be watched and can often be measured, so that any carefully made series of observations is of value.

Some of our beaches do not run parallel with the general trend of the coast, but turn at a slight angle from it, so that they align themselves rather more perpendicularly to the main waves which break upon them.

(#ulink_39efa20a-a6e7-53f4-b208-0adb62980bf8) Blakeney Point, for example, turns outwards from the coast and runs to the north of west for most of its length; the bends in Hurst Castle spit at the mouth of the Solent may be similarly related to the winds and waves playing on it. In Cardigan Bay, Morfa Dyffryn and Morfa Harlech turn away from the trend of coast, and their seaward sides are orientated approximately at right angles to the dominant winds (Fig. 27). The same phenomenon may be traced in numerous other examples. If the prevalent and dominant winds are from the same, or nearly the same, direction, conditions are fairly simple, but if the prevalent winds and waves come in from a different quarter from that of the dominant ones, complexities arise. The speed and direction of beach-drifting may become variable, and often much extra material is added to the foreshore of the growing spit or beach.

Beaches, then, are constructed features, and the part above water is wholly the result of wave action, although currents may affect the lower part. They may form at the foot of cliffs, and may fringe the coast for many miles. In more indented shores, beaches are common at the head of bays and gulfs, for example, Mount’s Bay, St. Bride’s Bay, the Duddon Estuary, Holborn Bay and Sinclair’s Bay in Caithness, and in the numerous smaller inlets and coves on the coasts of Pembrokeshire, Cornwall, and Devon. In the north of Scotland there are some excellent examples, including Sandwood Bay, Torrisdale Bay, and Farr Bay. But around the exposed coast between many of these re-entrants there is often no beach at all. The material worn from the cliffs, or sometimes that which has travelled from a greater distance, is swept up the inlet and gathers as a beach. Since the upper parts of narrow gulfs are generally sheltered, the beach need not necessarily rest on a platform of erosion, but may merely gather in the shallow head waters. Sometimes it is partly built of fluviatile deposits if a brook flows into the inlet.

Beaches in a gulf or loch may form not at the head, but at some intermediate point and run out athwart its course. There are some excellent examples in south-west Ireland; one of the best examples in Great Britain occurs at the mouth of Inverness Firth (Fig. 33), where Ardersier Point and Chanonry Point have grown outwards and almost towards one another from opposite banks. In Cemlyn Bay, in Anglesey, there is a good example of a bay bar which has impounded a tidal lagoon. There are some excellent examples in Shetland.

Much of the sand and other material that forms a beach is obtained from the sea floor. Waves break in water the depth of which is approximately half the wave length. In stormy weather when big waves are running, the sea floor may be churned up well offshore, and if it consists of suitable material can afford a great supply of detritus. It is difficult, for example, to account for the apparent renewal of the beaches in places like Scolt Head Island unless the new matter is assumed to come from the sea floor which was at one time covered, at least in part, with boulder clay. That beaches can be formed wholly from offshore material in this way is clearly proved in parts of Scotland. Near Claigan on Loch Dunvegan (Skye) there are two unusual beaches formed of the glistening white remains of lime-secreting organisms which live on the adjacent sea bed. Since the fragments have some resemblance to broken coral, the beaches are often called coral beaches. Whilst this is not correct, the name serves to distinguish these beaches from those formed of sand and gravel. The corallines, when they break up, produce great quantities of a coarse calcareous sand, which is spread over the sea bed and eventually washed up to make the beaches.

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When the sea bed deepens gradually, it often happens that except in calm weather waves break some way beyond low-water mark of ordinary tides. These waves erode the sea floor where they break, and may throw some of the eroded material up on to the uneroded part in front. If this process is continued, a ridge or bar of sand, shell, and perhaps shingle may be built. It is called an offshore bar. In the early stages of its evolution it is likely to change form rapidly, or to be washed away only to be built again later on. Once, however, it attains a reasonable degree of stability it will, in course of time, be pushed up above the zone of normal wave-break. At low water much of its seaward side may be exposed, and under favourable conditions the wind may carry sand to the bar. Sooner or later seeds reach it, take root and produce tufts of grass. Once this has happened further growth of the bar is much more likely. Perhaps the original bar may become a continuous dune ridge. It is likely that Scolt Head Island and several other similar features originated in this way (see Fig. 22).

Once a beach has formed either alongside a cliff or as an offshore bar, or in any other locality where there is room for it to develop, it will probably extend one way or the other as a result of beach-drifting. This process is important, not only physiographically, but economically since it largely accounts for the sanding or silting up of many harbours. The general effects are seen in many places, but in few are they better exemplified than in Sussex and Suffolk, The Arun, Adur, Ouse, and Cuckmere, are all deflected eastward by shingle ridges. In Suffolk, Orford Ness, and the smaller spits at the mouths of the Deben and Orwell are of the same nature. Spits grow rather like an embankment or tip heap. Beach-drifting gradually sweeps material along them and, at their ends, adds to the dump. Since the process is fairly continuous, it follows that the accumulated deposits may be conspicuous. On the other hand a spit growing out in this way across a river mouth, or across a bay or other re-entrant in the coast is easily attacked by the waves, especially those that approach it from a direction contrary to that of its growth. For this reason the free end of the growing spit is often turned or bent inwards, and in rather more severe weather large parts of it may be completely cut off.

Changes of this kind are common, but we do not know the precise causes. We know that in a certain storm such and such a spit was shortened, and we may make a shrewd guess as to what may have happened. But precisely what happened and why is quite another matter. Similarly, it is a well-known fact that many sand and shingle beaches, but especially shingle ones, are not merely a simple line of stones, but have running back from them branch ridges called laterals, or recurved ends. Figs. 25 and 26, show this. Each lateral ridge was at one time the distal or free end of the growing spit. Later, for some reason or other, conditions caused the main beach to lengthen again, and to continue to do so until another lateral was formed. Figures 25 and 26 show that the laterals often meet the main ridge at high angles, even at right angles. The newer laterals, especially those we may see forming over a period of years, may bend gently round. Hence, something has happened to bring about the abrupt junction of the older laterals with the main ridge. The main beach has been over-rolled on to them as is indicated in Fig. 24. A direct proof of this inrolling can sometimes be found if marsh deposits are exposed on the foreshore. These deposits can often be shown to have originated inside the spit, which because of the occasional overtopping by big waves has been rolled landwards. A misleading impression of stability is given if the main ridge is now dune covered.

In another chapter some of the coastal districts in which these lateral ridges are well developed are analysed. In this, it is emphasised that each such ridge was built by wave action, and was therefore at one time the outer ridge, and it follows that if one ridge lies in front of, or cuts across another, it is the newer one. Thus by carefully mapping the ridges, and noting their relationships one with another, the evolution of the whole structure of a shingle foreland may be investigated. Difficulties occur, especially in places where the ridges are in groups, but the groups themselves separated from one another. It is, for example, far from clear just how the several groups which are truncated by the present beach ridge on the southward facing part of Dungeness are related to one another and to the formation of the whole foreland. Reconstructions that have been made are quite possible, even probable, but they remain hypothetical (see here (#litres_trial_promo)).

It may also be remarked that Benacre Ness has altered greatly in form since the current Ordnance maps were published. The Ness point is now considerably farther north, and the southern end near Benacre Broad has suffered severe erosion. To balance this, there has been accumulation at or near the sluice.

(#ulink_74b779e1-bb73-59bf-8f1f-477bb68e9de1) At the northern part of Kessingland and again at the southern end of Aldeburgh, new groynes have recently been built. In both cases they have been successful in collecting some beach, and it is interesting to note that at both places, perhaps more markedly at Kessingland, a reversal of beach drift seems to have taken place. As a rule the beach is piled up on the south side of the groynes, pointing to a northerly movement. This seems to be quite local, but no ready and adequate explanation is forthcoming. It may also be remarked that Benacre Ness has altered greatly in form since the current Ordnance maps were published. The Ness point is now considerably farther north, and the southern end near Benacre Broad has suffered severe erosion. To balance this, there has been accumulation at or near the sluice.

(#ulink_e5724072-6050-5ef5-9bab-6e233b30edd5) It is well known that large cobbles on which seaweeds grow can be carried long distances in this way. The remark in the text applies to bare stones and cobbles.

(#ulink_91469cb7-00f1-5d37-918d-ada8c8471445) Approximately the relation between the movement of the particles and the depth is: “If L is the length of the wave, the movement of the particles from the surface downwards decreases one half for each 1/9 L of depth; i.e., at a depth of 1/9 L the movement is 1/2 that at the surface; at 2/9 L it is 1/4 the movement at the surface, etc.” (R. S. Patton and H. A. Marmer, The Waves of the Sea, in Physics of the Earth; V, Oceanography, Bull. Nat. Res. Council, Washington, 1932.)

(#ulink_fe710b85-2d50-58ac-a62d-a74bf302eb28) J. S. Owens and G. O. Case, Coast Erosion and Foreshore Protection, Ch. iii, 1908.

(#ulink_fd87e6b3-1eea-50fb-8a16-1edd0c400953)Geogr. Journ. 78, 1931, 131.

(#ulink_bfee6cbc-d60d-512a-ab0b-03bf451f9332) W. V. Lewis, Proc. Geol. Assoc., 49, 1938, 107.

(#ulink_fd50463d-e8a3-5ddd-a205-3f311061cb4f) See D. Haldane, Trans. Edin. Geol. Soc. 13, 1931–38, 442. The nullipore is a robust form of Lithothamnium calcareum, Aresch, and at low tide can be seen growing on the spit connecting Lampay Island with the shore. Nullipore sands also occur at Loch Bracadale, Morar, Arran, Lower Loch Fyne, South Bute, and Cumbrae.

CHAPTER 3 (#ulink_abf97d48-0aba-53c7-8249-4251c9844b68)

EROSION AND ACCRETION:

EVIDENCE OF COASTAL CHANGES