An analysis of some factors involved in the depth of disturbance of beach sand by waves

Abstract

A study of the depth of disturbance of beach sand by waves indicates that breaker height is the dominant factor in the mid- and probably low-tide zone; slope in the higher high-tide zone. Depth of disturbance figures are much higher than those reported by King (1951), in Great Britain but agree with Otvos' (1965), in the U.S.A. Similarly no difference was found between erosional depths of fine versus coarse sand again confirming Otvos' (1965) work. Newly deposited sand found on top of the coloured sand in the sedimentation stations was mainly uniform in structure, dual sedimentation units, if occurring at all, being more common at mid-tide stations and on beaches where there was a greater range of sediment. A new method was devised for detecting those areas on the foreshore where experiments were set up, based on the principle of a change in frequency noticed when an oscillator passed over a metal plate buried in the beach face. This, on an open beach face devoid of any external features, enabled the precise experimental location to be found. From statistical analysis of over 150 samples, skewness values were found in the main to be typically negative, whilst kurtosis values tended to indicate a difference in modality between the various sedimentation units obtained. Standard deviation figures showed that moderate sorting was typical for bottom units, whilst upper and uniform units seemed to be moderately well sorted. The results obtained indicated that 82% of the factors causing depth of disturbance of sand at mid-tide position could be attributed to breaker height, slope and wave period whilst at higher high-water mark only 61% of the disturbance was due to these three factors. The latter result was possibly due to the many complex swash zone processes which were not investigated during these experiments.