Optical granulometry of coarse aggregates and sediments

Abstract

In order to determine the granularity of the material, image analysis may be used instead of traditional methods such as sieving. Data on granulometry is important in the water management sector for several practical applications, such as calculation of sediment transport capacity in watercourses, design of hydraulic structures, or modeling of debris flows. The WipFrag and Basegrain programs were tested on the case study of the Belca rockfall and the Sava Dolinka River’s gravel bar near the village of Mojstrana. In both cases, multiple images were taken from different heights and sieve analysis was performed. The results of both programs were compared with the results of the sieving analysis. The results showed that WipFrag yielded more comparable results with the sieving analysis than Basegrain. WipFrag gave slightly better results in the case of the Belca rockfall than in the case of the river gravel bar. The Basegrain program, on the other hand, produced similarly comparable results in both case studies. In most cases, both programs underestimated the grain size compared to the results of the sieving analysis. In some cases, the relative differences were close to 100%. On the other hand, the selected statistical test did not show a statistically significant difference between the results of the sieving analysis and the results of the image analysis in both of the programs. Optimal image analysis results were obtained from images taken from the height of about 1.5-2 m, which means that we suggest that photos of aggregates, fluvial sediments, and erosion material should be taken from a height of approximately ten times the maximum grain size.