Digital Grain Size Analysis of a Mixed Sand and Gravel Beach

PENTNEY, R.M. and DICKSON, ME., 2012. Digital grain size analysis of a mixed sand and gravel beach. Journal of Coastal Research, 28(1), 196-201. West Palm Beach (Florida), ISSN 0749-0208. A new technique to estimate the properties of sediment from digital images is investigated. Sediment was obtained from mixed sand and gravel beaches in Hawke Bay, New Zealand. Samples were manually sieved, and grain size distributions were compared to those obtained from digital images. Digital image analysis provided variable estimates of the grain size distribution (R-2 = 0.21-0.74), with much poorer estimates obtained when images contained grain sizes beyond the range of sediments imaged for the calibration catalogue. A reasonable approximation of the mean grain size of a sample was returned from digital analyses when compared with the mean grain size obtained from sieving (R-2 = 0.68), provided all sediment sizes within images fitted within the calibration catalogue. This paper discusses limitations of the technique when applied to mixed sand and gravel beaches, and possibilities for overcoming these (e.g., increasing sampling quantity). It is clear that a deeper understanding of the morphodynamics of mixed sand and gravel beaches requires methods capable of detailing the dynamic fluctuations of grain assemblages. Digital image analysis provides a very promising alternative to laborious and time-consuming manual sieving, and it is hoped that this paper provides a useful additional data set to help in the further refinement of methods for digital image analysis.