SIZE AND SHAPE SORTING OF A HURRICANE WASHOVER DEPOSIT, MURRELLS INLET, SOUTH CAROLINA

The size and shape sorting of a suspension transported sand resulting from a hurricane washover event was studied. A transect of samples was collected from the Pleistocene upland, across the intertidal marsh to the present beach at Murrells Inlet, South Carolina. Standard sieve analysis was used to determine sand size distribution and percent silt-clay. Fourier grain shape analysis was performed on 400 grains per sample for each of the three most abundant half phi size fractions.

Grain size analysis reveals the expected pattern of decreasing proportion of coarser grains across the marsh behind the washover fan. However, this trend continues only 65 meters beyond the terminus of the washover fan. Distinct from the washover sand is the landward most marsh. It had the largest mean size and its size distribution was distinct from the samples. However, the intertidal creek bottom sand could not be distinguish from the washover sands samples based on their size distributions.

Fourier grain shape revealed that in only the 1.5 phi size class was the second harmonic (elongation) significant. The second harmonic of the 1.5 phi sizes clearly shows the difference between the washover sand, the Pleistocene upland sand and the sand transported by tidal currents in the tidal creek system. The washover sand decreased in proportion from 100% at its source on the beach to less than 60% in the samples on the marsh surface. There is a continued decrease in proportion of the beach source along the direction of transport across the marsh surface. Replacing the beach source shape type in the marsh washover samples is the shape type found in the Pleistocene upland sand. That shape type is the most elongated of the three source types. This indicates that as the suspended sand was transported beyond the terminus of the washover fan, not only were larger grains settling faster from the suspension, but the more spherical 1.5 phi grains were also settling faster, thus enriching the proportion of the more elongated grains further from the beach. This trend is not found in higher harmonics of the 1.5 phi nor in the smaller size fractions. It can be concluded that smaller grain sizes and smaller scale departures from sphericity in larger grains had little to no effect on suspension transport along a transport path of only a few hundred meters during a washover event.