Chesil Bank Research Articles

UK meteotsunamis: a revision and update on events and their frequency

<scp>UK</scp> meteotsunamis: a revision and update on events and their frequency

The Evolution of the Barrier Beaches between Fairlight and Hythe

ACOASTLINE can be studied in several different ways, but whichever way is chosen it is a great help if the writer of any paper knows about a considerable length of coastline so that the part with which he is concerned can be assessed in relation to that on either side of it. This afternoon we are concerned with shingle coasts, and differences between, e.g. the Chesil Bank and Dungeness are profound. Mrs Eddison has been studying Dungeness and Romney Marsh in detail and her paper, which gave rise to this meeting, adds greatly to our knowledge of that complicated structure. Dr Carr has made himself an authority on the Chesil Bank. Both Dungeness and Chesil are no longer unspoiled features and their origins are not directly comparable. But now much greater account is paid to Flandrian changes of sea level, and this is perhaps the main factor which emphasizes the difference between those papers written in the last 20 years and those of earlier date. Gilbert, W. V. Lewis and Balchin all recognized changes of level in Dungeness, but were unable to relate them to recent work. Dr Jolliffe has given a somewhat different paper of much interest about the options available to the next planner. He considers the options in general and also in relation to the Chesil and Dungeness.

THE LERRETS OF CHESIL BANK

THE LERRETS OF CHESIL BANK

Underwater pebble grading of Chesil Bank

This paper describes the examination of pebble grading below low-water mark off Chesil Bank, Dorset, using skin-diving techniques. It is found, contrary to some of the statements in previous literature, that there is no reversal of size grading of pebbles below low water compared with that present on the beach, but a series of zones are shown to exist parallel to the Bank which appear to be related both to its evolution and to present-day wave action. The zones are thought to be best defined under constructive wave conditions, and suggestions on their relationship with the Bank are put forward.

Sea Waves and Beach Cusps

Although the formation and destruction of beach cusps is manifestly associated with the action of waves breaking on a beach, there are few studies known to us in which the sea waves have been recorded continuously at the time that the movement of material is taking place. This paper describes briefly some observations and experi? ments that were carried out in two places on the south coast of England, at Tide Mill near Seaford, and on Chesil Bank, to try and relate the formation of the cusps to the characteristics of the waves. A model-scale experiment performed at the Hydraulics Research Station, Wallingford, is also reported. These investigations were stimulated in the first place by the idea that the construc? tion of the cusps might be due to the occurrence of 'edge waves', that is to say a type of wave motion that can occur in the neighbourhood of a sloping beach (Stokes, 1846; Lamb, 1932). In that case one would have expected a simple relation between the period of the waves, the slope of the beach and the average cusp-spacing (defined as the mean longshore distance between adjacent cuspidal promontories). The first series of observations recorded here showed that no such simple relation exists. On the contrary it was found that (over the range of parameters explored) a close correlation exists between the cusp-spacing and the height of the waves which form them, consistent with earlier findings of Johnson (1919); and an even closer correlation exists between the cusp-spacing and the width of the swash zone. The observations to be described also appear to have some bearing on the mechanism of cusp formation. The relative power of the backwash to erode material from the 'bays', for example, is dependent on the relative amounts of upwash and backwash. The backwash tends to be reduced by any percolation of water into the beach. Hence the permeability of the material forming the cusps is of some importance. The experi? ments show that the permeability in the bays is very much less than that on the cusps; and that this may be ascribed to the presence of an impermeable mixture of material close to the surface of the bays. In all our observations such an impermeable layer was present within a few inches of the beach surface, and it seems likely to us that this is a condition tending to promote instability of the beach profile. Our observations were made during the summer of 1956; the unusual delay in presenting them has been due to our sense of their incompleteness. Recently, however, some interesting model experiments have been performed by Mr. N. C. Flemming in the Department of Geography at Cambridge University. Taken together with Mr. Flemming's experiments, our own results have perhaps a greater interest. But though we possess this advantage we have tried to present our conclusions here as far as possible in the same light as we saw them at the time of their completion. We are indebted to the late Mr. W. V. Lewis, and to Brig. R. A. Bagnold and Dr. P. H. Kemp for valuable discussions. Apparatus.?For surveying the beach profile above the waterline, an ordinary sur? veyor's pole, chain and level were used. One observer carried the pole, to which was attached one end of the chain, halted at varying distances down the beach, while a sight was taken from a fixed point above the high-tide level. To survey the profile below the waterline the pole was carried out farther into the surf. During cold weather a frogman's suit was found useful. In the experiments on Chesil Bank, the so-called 'giraffes' designed by members of the Department of Geography at Cambridge were also used. Each 'giraffe' consists essentially of a graduated vertical pole, sunk firmly into the beach, to which is attached a sliding vertical shaft with a broad conical 'foot'. The sliding shaft may be raised by an observer on land, by means of a wire and pulley. To take a reading, the shaft is first raised clear of beach material and then allowed to fall so that the foot rests on the surface of the beach. The level of the top of the shaft is then

Miniature Spits and Embankments on a Lake-Shore, Iceland

A SERIES of miniature spits and embankments examined during a Cam? bridge expedition to Iceland in 1937 si ulated to a high degree their greater counterparts round the British coasts. It seems therefore that scale reductions of 10 or 100 to 1 do not materially affect the mechanism of spitbuilding. The lake occupied a mountain-girt hollow in the eastern foothills of Snaefell, East Iceland. It is shown in Fig. 1, taken from a plane-table survey made by Lieut. J. N. Jennings. The bare steep slopes yield large quantities of sand and gravel, and extensive delta flats encroach on the lake. Abundant supplies of shore material are thus made available, and a variety of depositional shore-forms have resulted. The most striking shore feature was the storm beach ridge (Plate 1) which extended the full length of the eastern shore. It was terraced on its lakeward side (Plate 2) by as many as eight lesser beach ridges. It reproduced both in plan and profile countless features possessed by the far greater storm beach ridges round the British Isles, such as the Chesil Bank (Plate 3). The highest ridge rosje 15 inches above lake-level, and the width of the embankment varie'd from 17 to over 30 feet. The back of the bank was corrugated by a series of shingle tongues, formed, like their counterparts at Chesil, Dungeness, etc, where storm waves had broken through the embankment locally, and washed tongues of shingle down the landward side. The profile of such embankments is now largely understood, and can be closely examined in the wave trough of the Cambridge Physiographical Laboratory, on a scale little smaller than that .shown in Plate 2. The close relationship between the height of the embankment and wave size is shown by the heights (in inches) given in Fig. 2. The size of the waves depends on the strength of the wind, and the stretch of water over which it blows. To produce the largest breakers the waves must not be very oblique to the shore or their power is dissipated over a wider extent of shore, and energy also seems to be lost in swinging round to break onshore. Depth of water immediately offshore is also important, but this is chiefly determined by the dominant (or largest) breakers, being deeper where the breakers are large than where they are small. The bank was highest (15 inches) at its southern end where it faced north-west, towards the greatest stretch of water and the maximum exposure to wind. Farther north the bank gradually

Wave-Action and Shore-Lines

AN article by Mr. W. V. Lewis on the “Evolution of Shore-line Curves” (Proc. Qeol. Ass., 49, Pt. 2; 1938) is a valuable contribution to a well-worn subject. The author's position is fundamentally that of Sir Henry de la Beche, founder of the Geological Survey, who a hundred years ago stressed the importance both of the longshore drift of material by prevalent waves and the independent action of storm or dominant waves in piling up embankments of detritus in the direction of the greatest force, sometimes at right angles to the shore. In later years opinions changed, and explanation of the evolution of coastal outline was purely on the action of longshore and eddy currents. Latterly, however, there has been a revival of the wave-action hypothesis due largely to the findings of the Boyal Commission on Coast Erosion and Afforestation and the work of Steers, Johnson, and others. Mr. Lewis proceeds to examine in detail various sections of coastline in Britain on the Moray Firth, Cardigan Bay, Bristol Channel, Isle of Purbeck and Chesil Bank, and suggests that shore curves, far from being explicable only by reference to longshore currents, yield in certain cases readily to analysis in terms of the direction of approach of both dominant and prevalent waves, especially the former.

IV.—Pebble Ridges

As a contribution to our knowledge of the formation of these very interesting “natural embankments of the sea,” I may point to a little bay in Anglesea, immediately westward of the Bryn Ddu Limestone quarries on the north-east coast of Anglesea, about two miles and a half westward of Puffin Island. This little bay is not more than about a furlong across, and may be roughly described as semicircular in form, lying nearly due west to east from point to point. From being in miniature as it were, the ridge can be readily studied, and it is very striking to see how, commencing in the westward as a beach, it gradually rises into a ridge having very steep sides. No less remarkable is the way in which the stones increase in size as the ridge does in height. At the west end it may be described as composed of Limestone pebbles, with here and there a boulder, while at the east end it is built up almost entirely of large limestone boulders and blocks, many containing from one to two cubic feet and some more. Intermixed there are boulders from the size of the closed hand and upwards. The larger blocks are sub-angular and rounded, and no doubt get gradually worn down smaller and rounder, until they become “boulders.” by being moved about, by the sea; but some on the other hand, being thrown over on to the back of the bank, cannot be further affected by the waves. In this ridge, as I have pointed out is the case with the Chesil Bank, the stones follow the law of the bank itself, the largest being collected to form the highest part of the bank, which in both occurs where the wave-action is most intense.

The Chesil Bank

THE letter of your correspondent, Col. Greenwood (vol. xi. p. 386), has only now been brought under my notice.

Origin of the Chesil Bank

IN your report (vol. xi. p. 299) of the paper on this subject by Prof. Prestwich, read at the Institution of Civil Engineers, are these words:—“The large dimensions of the bank he attributed to the great accumulative and small lateral action of the waves.” Why, then, does not so general a cause form hundreds of such banks? Why is “the great accumulative action of the waves” confined solely to the Chesil Bank, and particularly to the Portland end of it? Because the travelling of the pebbles is towards Portland, which checks the travelling, and so allows of “accumulation” exactly as a groin does. This is the simple “open sesame” of the secret; and if we could build groins as large as Portland, every one of them would “accumulate” a bank of precisely the same conditions as the Chesil Bank. If the pebbles travelled from Portland, as the professor thinks, that end of the bank should be the lowest; it would be perpetually robbed by the waves. But it is the highest—forty-three feet; and the Abbotsbury end, to which he supposes the pebbles to travel, should be the highest, but it is the lowest—scarcely more than half the height, twenty-three feet; while at Bridport there should be a still higher bank, for the professor makes the pebbles travel from east to west there and meet the pebbles which had “travelled from the opposite direction, viz., from west to east” But at Bridport there is not a single pebble, but only blown sand.

Why are the Largest Stones Found at the East End of the Chesil Bank?

Why are the Largest Stones Found at the East End of the Chesil Bank?

On the Chesil Bank

On the Chesil Bank

Why are the Largest Stones found at the East End of the Chesil Bank?

Why are the Largest Stones found at the East End of the Chesil Bank?

The Chesil Bank

The Chesil Bank

VI.—Some Remarks on the Formation of the Chesil Bank

The Paper “On the Formation of the Chesil Bank,” which appeared in the GEOLOGICAL MAGAZINE for October last, is prefaced by a statement that the authors (Messrs. Bristow and Whitaker) do not propose to enter into the question of the way in which the shingle was heaped up. But the way in which the shingle was heaped up is intimately connected both with the formation of the Chesil Bank and with its position in advance of the shore line, and, if rightly understood, any speculations on the waste of the coast behind the bank by sub-aerial denudation are superfluous.

The Chesil Bank

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On the Formation of the Chesil Bank

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I.—On the Formation of the Chesil Bank, Dorset

We do not propose to enter into the questions of the source whence the pebbles of the Chesil Beach are derived, nor of the way in which they are heaped up; these and other like matters having been almost exhaustively treated by Mr. J. Coode, to whose paper we refer the reader for a detailed account of the bank. The subject with which we propose to deal is simply the cause of the formation pf a long shingle-bank, separated from the mainland by a strip of water. This has not been noticed at length by any writer, as far as we know, though three theories of the origin of the bank have been brought forward.

On the Formation of the Chesil Bank, Dorset

On the Formation of the Chesil Bank, Dorset

DISCUSSION. DESCRIPTION OF THE CHESIL BANK, WITH REMARKS UPON ITS ORIGIN, THE CAUSES WHICH HAVE CONTRIBUTES TO ITS FORMATION AND UPON THE MOVEMENT OF SHINGLE GENERALLY.

Keywords BANKS, SHINGLE, BARS, FORMATION, TRAVEL, MOVEMENT, PEBBLES, BEACHES, COASTS, WAVES, CURRENTS, CLAYS, KIMMERIDGE CLAY, GEOLOGY CHESIL BANK, PORTLAND, DORSET UK... Show All