Sea Waves and Beach Cusps

Abstract

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