Size Grading Along A Pebble Beach: Chesil Beach, England

Abstract

ABSTRACT The paper discusses aspects of the grading of a pebble beach, Chesil Beach, Dorset, England. Because of the prevailing conditions, grading is especially well developed along this beach. The principles included apply elsewhere. The initial samples were taken in July, 1965, from the surface along 23 lines of section extending over a distance of 26 km. The number of sites per section varied between 3 and 11 and included the main Beach Crest, and High and Low Water Mark. The same section lines were used for monthly sampling at High and Low Water Mark on alternate series of Spring Tides until July, 1966. Samples were also obtained from a number of boreholes through either the beach ridge or the back slope of the beach, and from the Raised Beach nearby. Generally 500 pebbles were analyzed. The initial samples were measured for long diameter, short diameter, and weight, but both monthly and borehole samples were examined for long diameter alone. The data were computed for mean, standard deviation, and standard error. The results show that Low Water samples are frequently bimodal; that certain landward samples are of a different population to the adjacent Beach Crest and seaward face, and that a higher proportion of coarser material is found near, but not necessarily on, the present Beach Crest. The profile along the beach suggests a marked inflection of pebble size both along the Beach Crest and at High Water towards the eastern end, and greater variability toward the west. Monthly variations at High Water Mark suggest rapid lateral transport from time to time. The tendency towards bimodal distributions at Low Water, and the presence of different size ranges there compared with the rest of the adjacent beach, reflects both current beach processes and mode of origin. Boreholes sited on the Beach Crest indicate fine material at and below present-day Low Water Mark. There is a suggestion of grading below the present-day beach level The Raised Beach samples fall within the range of the present-day beach. The significance of these results is discussed.