MORPHOLOGICAL CHANGE AND SEDIMENT DISTRIBUTION OF A MACROTIDAL GRAVEL BEACH, LOE BAR, CORNWALL

The field measurements presented in this paper describe the morpho-sedimentary evolution of Loe Bar, at a medium time scale, over various wave and tide conditions. A lack of research in this area has been attributed to the difficulty in deploying instrumentation on gravel beaches, often in high-energy environments, while traditional grain size estimation techniques such as sieving is considered arduous and time consuming . To combat this, a digital image grain size estimation technique was employed, providing a much broader spatial coverage. Surveys along two cross-shore transects, one in an embayment and one on a horn, were studied consistently over 42 tides. As gravel beaches are considered effective forms of coastal protection due to their hydraulic roughness, two periods of contrasting hydrodynamic conditions were selected to aid the analysis. In terms of morphology, the embayment and horn were both characterised by a high tide berm which was dictated by tidal elevation, while large beach cusps were situated along the bar. During the calm conditions both the embayment and horn experienced a net beach volume loss, especially at the lower portion of the swash zone, while an overall decrease in grain size was measured. This has been attributed to the larger antecedent wave conditions which would have promoted onshore transport of coarser grains while increasing beach volume. The storm conditions showed the embayment to experience a larger net beach volume surplus (6.5m3m-1>), primarily in the lower swash zone, while the horn measured an overall net beach volume loss, with more that 6m3m-1 lost during the peak wave conditions. This may be attributed to complex beach cusp feedback mechanisms with the sediment from the horn transported down gradient with swash motion. Grain size appeared to correspond relatively well with increasing grain size measured at the embayment, yet grain size at the horn showed little variation