Simulation of grain-size abundances on a barred upper shoreface

Abstract

Using the barred upper shoreface of Long Island, New York, as an example, this study provides a semi-quantitative demonstration of the concept that the distribution of grain sizes in a closed depositional system is the product of the reworking and redistribution by the physical process and the availability of the sediment source. In the particular case of Long Island, the physical process is represented by the agitation of sediment grain sizes by the shoaling waves under the average fair-weather wave conditions as a first-order approximation. A nearby subaerial morainal deposit represents the drowned glacial deposit on the shoreface as the sediment source. The probability of abundance of four representative sediment grain sizes across the upper shoreface of the study site was simulated. The shoaling wave field over a segment of the upper shoreface of Long Island was first simulated by a numerical wave and circulation model using deepwater incident waves under the conditions of the average fair-weather, remote ocean swells, and a local storm. The model results were then used to estimate sediment entrainment across the upper shoreface of the study site by using an empirical formulation developed for sediment movement under progressive waves. The probability of the occurrence of each grain size on the upper shoreface is characterized by a non-dimensional retention index. The results indicate that the fair-weather wave climate has the highest probability for sediment retention on the seabed. The cross-shore distribution of a grain size, which is represented by the probability of abundance (abundance index), can be simulated by multiplying the probability of occurrence (retention index) as a result of the differential wave agitation with the availability (grain-size frequency distribution) of the source sediment deposit. The grain sizes representing the fine sand group have the best fit between the observed and simulated distributions, suggesting a group of grain sizes that is in dynamic equilibrium with shoaling waves. For the rest of the grain sizes, the combined effects of shoaling waves and other secondary hydrodynamic processes are important in determining their cross-shore distributions.