The grain size gap at gravel-sand transition in beach sediments

The grain size of sediments is one of the most crucial sedimentary parameters. In riverbeds, the surface sediments often lack the transitional grain sizes between sand and gravel (i.e., 1～10 mm), a phenomenon widely referred to as the "grain size gap." Whether this is also the case for beach sediments remains a fundamental yet unexplored question. Through the analysis of mean grain size data from 228 beach locations globally, we revealed that beaches also exhibit a grain size gap of sand-gravel transition, ranging from −3.5 ϕ to −1 ϕ (2 to 11 mm). Beaches have a high probability of being either gravel beaches with mean sediment diameters of several tens of millimeter, or sand beaches with mean sediment diameters of several hundred micrometers, mirroring the findings in riverbeds. Potential reasons for this absence include particle abrasion and selective transportation.