Relationships between grain size, size-sorting, and foreshore slope on mixed sand - shingle beaches

Abstract

The variation of mean grain size (Mz_φ), sorting (σ_I), and beach face slope over mixed-sand shingle beaches at two areas along the east coast, South Island, New Zealand, it analysed in order to determine the nature of the relationships between these three variables. The study is confined to mean grain sizes between 0.25 and 16.0 mm, and the sorting and slope values associated with this range of sizes. Trend analysis of the data has been performed by fitting curves with the aid of a computer. As distinct from the linear average relationship betweeen size and slope distinguished by many workers, it is demonstrated that for given beaches curvilinear trends in size/sorting are reflected in slope/size relationship. Thus, the best sorted sediments are found to be associated with the steepest gradients on the curves, while zones of poor sorting are associated with “plateaus” in the size/slope trend. It is suggested that size is the primary control of sorting trends in the sediments examined whilst hydraulic effects (wave action, etc.) contribute to variability, or spread of the data around the trend. Since size and sorting exert a primary influence on beach face slope through permeability, it is further suggested that, for the study beaches at least, trends in the size/slope relationship clearly reflect the characteristic local distributions of size and sorting.