Semiautomatic Digital Clast Sizing of a Cobble Beach, Nantian, Taiwan

Surface sediment data is scarce for beaches that are made of material close to the cobble size range, partly because of the difficulty of direct sampling for sediment size analysis. An object-detection tool for semiautomatic analysis of clast geometry, originally developed for riverbeds, was applied to the coarse-clastic beach of Nantian, Taiwan. Comparison of the software with digital point counts for the common size percentiles D-5, D-16 , D-30, D-50 , D-70 , D-84 , and D-95 indicated an average coefficient of determination of 0.990 over 10 test pictures. The surface sediment was digitally sampled at three cross-shore locations along the beach: near the shoreline, on the steep part of the beach face, and on the beach crest. Clast-size distributions, including clast area and elongation ratio, were computed from the geometry of 1000 clasts for each location and the cross-shore variance was captured. The combination of sediment characteristics with topographic measurements showed significant relations between bidimensional size and elongation of the sediments and morphology of the beach. The method can only access the clast geometry that is visible on pictures and cannot describe the full tridimensional shape. Despite this limitation, semiautomatic digital sizing is well suited to coarse-clastic beaches and has the potential to increase the understanding of coastal sedimentology over a wide range of sediment sizes.