GEOMETRY AND GRAIN-SIZE SORTING OF RIPPLES ON LOW-ENERGY SANDY BEACHES: FIELD OBSERVATIONS AND MODEL PREDICTIONS

Abstract

There are very few field measurements of nearshore bedforms and grain-size distribution on low-energy microtidal beaches that experience low-amplitude, long-period waves. Field observations are needed to determine grain-size distribution over nearshore bedforms, which may be important for understanding the mechanisms responsible for ripple development and migration. Additional nearshore field observations of ripple geometry are needed to test predictive models of ripple geometry. Ripple height, length and sediment composition were measured in the nearshore of several low-energy beaches with concurrent measurements of incident waves. The distribution of sediment sizes over individual ripples was investigated, and the performance of several models of ripple geometry prediction was tested both spatially and temporally. Sediment samples were collected from the crest and trough of 164 ripples. The sand-sized sediment was separated from the small amount (generally <3%) of coarser material (>2 mm) that was present. Within the sand-sized fraction, the ripple crests were found to be significantly coarser, better sorted and more positively skewed than the troughs. Overall, the troughs were finer than the crests but contained a greater proportion of the small fraction of sediment larger than 2 mm. The field model of Nielsen (1981) and the model of Wiberg & Harris (1994) were found to be the most accurate models for predicting the wavelength of parallel ripples in the nearshore of the low-energy microtidal environments surveyed. The Wiberg & Harris (1994) model was also the most accurate model for predicting ripple height. Temporal changes in ripple wavelength appear to be dependent on the morphological history of the bed.