Selective Longshore Transport Rates of Different Grain-size Fractions within a Beach

Abstract

ABSTRACT Measurements were obtained at El Moreno Beach, Mexico, of the longshore transport rates of different grain sizes found within the beach sand. The rates were determined through the use of fluorescent sand tracers, but unlike previous studies of selective grain sorting, the entire grain-size distribution was tagged with only one color. By sieving the grid samples obtained downdrift from the tracer injection location into their several size fractions and counting the number of tracer grains in each, one obtains transport patterns for each of the individual grain-size fractions. The first moment of the tracer concentration about the injection line yields the mean longshore transport distance and thus the mean transport velocity for each grain-size fraction found in the beach sand. The measurements show that the coarsest sand grains found within the beach, the fraction centered at 1.19 mm, moved alongshore the fastest with a mean advection rate of 0.31 cm/sec. This rate is about four times faster than grains of diameter 0.30 mm. The results substantiate the conclusions arrived at by Evans in 1939 on Lake Michigan beaches that the finer sand grains swash high up the beach face where they move alongshore more slowly than the coarser grains that remain near the breaker zone and are transported by stronger longshore currents.