GraSSAMS: A new instrument designed for the determination of grain size and shape of sand-gravel-sized sediment

Natural coasts define the interface between land and ocean and constitute a natural barrier to protect humans and infrastructure from waves and storm surges. In the context of global sea level rise and coastal erosion, beaches, especially sand-gravel beaches, can achieve better marine energy reduction at the expense of a smaller spatial buffer zone (i.e., the lateral range of the coastal zone), meaning that nature-based coastal reconstruction and protection are becoming increasingly important. The need for beach replenishment and coastal protection necessitates fast, accurate, portable, and wide-range gravel grain size measurement technology to provide technical support and data for the designing and monitoring of artificial gravel beaches. However, existing grain size analysis methods for gravel-sized sediment are either inefficient (such as sieve analysis) or have a limited range of sizes (such as dynamic image analyzers, up to −3 φ), or have difficulty in eliminating errors caused by gravel overlap (e.g., remote-sensing in situ image analysis). In this study, a Gravel Grain Size and Shape Automatic Measurement System (GraSSAMS) was designed to analyze grain size and shape parameters of gravel-sized sediments. The mean diameters of three types of coins and some Zirconia balls measured using this method were compared with those measured by a vernier caliper. The results show that the GraSSAMS method has good accuracy and yields an acceptable relative error. The grain sizes of 67 field gravel samples were analyzed using both the GraSSAMS method and sieve analysis. The irreducible random error between the two methods is 1.41 φ. In addition, a Camsizer XT and GraSSAMS were used to measure samples with a grain size range of 1∼8 mm (0 φ ∼ -3 φ), respectively. The results of the two methods show good correspondence, with an irreducible random error of 0.31 φ. GraSSAMS was also tested in the field on several gravel beaches. Results revealed that GraSSAMS could complete the surface gravel measurement of a 0.5 m × 0.5 m quadrat within 8 min, and the analysis results avoided the fabric error involved in remote-sensing (photographic) measurement. Based on these various results, we propose that GraSSAMS can be used to analyze the grain size and shape of sand-gravel sediments in the size range of 1∼330 mm (0 φ ∼ -8.37 φ) in a laboratory as well as in the field. Compared with traditional methods, GraSSAM has the advantages of accuracy, high efficiency, and portability and can be used as an alternative for grain size analysis of gravel sediments, with broad application prospects to gravel beaches and fluvial deposits worldwide.