Understanding and predicting the temporal variability of sediment grain size characteristics on high-energy beaches

Despite representing a key parameter when modelling morphology or sediment transport, surface sediments are often assumed homogenous, with grain size temporally constant. This contribution uses a 6-year data set of monthly sediment samples to quantify the observed variability in intertidal beach sands at four energetic, macrotidal locations (North Cornwall, UK). Changes in grain size and sorting were related to periods of high-steepness storm waves promoting a relatively rapid coarsening and an improvement in sorting and low-steepness swell waves, a fining and a reduction in sorting. These temporal changes in intertidal grain size were coherently linked to the disequilibrium in wave steepness, with peak coarsening occurring when the instantaneous wave steepness conditions vastly exceeded a temporally evolving antecedent time series. Using this concept, a simple model is proposed that provides skilful predictions of the unseen variability in sediment grain size (average r2 = 0.86, p < 0.01) and sorting (average r2 = 0.75, p < 0.05), at all four sites.