Grain size of gravel: recent progress on sampling, analysis and calculation

Gravel beaches serve as both important recreational destinations and natural barriers against wave erosion. In the context of global climate change and sea level rising, the use of gravel for beach nourishment has become an increasingly favored option for soft shoreline protection solutions. However, sampling and analyzing the grain size of gravel have always been challenging in this field of research. This challenge not only hampers the accurate prediction of gravel beach morphodynamics but also adds uncertainty to designs of beach nourishment projects. Here, we present three recent progresses in grain size sampling and analysis of gravel sediments. (1) Minimum sampling mass for sieving method. By assuming a normal distribution and considering the standard deviation and the grain size of the coarsest particle, we derived a method for calculating the minimum sampling mass, for instance, a gravel sample with a standard deviation of 4.0 φ and the coarsest grain size of 64 mm would require a minimum sampling mass not exceeding 48 kg. (2) Digital photograph-based automatic grain size and shape analysis technique. We have developed hardwares for measuring gravel grain size and shape and have written corresponding software, namely the Gravel Grain Size and Shape Automatic Measurement System (GraSSAMS). This system caters to both laboratory and fieldwork requirements, enabling measurements of particles within the range of 1–330 mm. (3) Grain size parameter calculation for samples analysed with a comprehensive approach. When applying a comprehensive approach (combining laser and sieving methods) to analyze the grain size of gravel-sand-mud mixtures, we observed overlap between the results of the laser and sieving methods. To address this issue, we propose two algorithms, both of which yield highly consistent results. Finally, we discuss two questions: (1) Is there a standard technique for gravel grain size analysis? (2) How will technology for automated identification of gravel grain size and shape evolve in the future? Exploring these questions can help to better understand the morphology and evolution of gravel beaches.