Dimensionless argument: a narrow grain size range near 2 mm plays a special role in river sediment transport and morphodynamics

The grain size 2mm is the conventional border between sand and gravel. This size is used extensively, and generally without much physical justification, to discriminate between such features as sedimentary deposit type (clast-supported versus matrix-supported), river type (gravel bed versus sand bed), and sediment transport relation (gravel versus sand). Here we inquire as to whether this 2mm boundary is simply a social construct upon which the research community has decided to agree or whether there is some underlying physics. We use dimensionless arguments to show the following for typical conditions on Earth, i.e., natural clasts (e.g., granitic or limestone) in 20C water. As grain size ranges from 1 to 5mm (a narrow band including 2mm), sediment suspension becomes vanishingly small at normal flood conditions in alluvial rivers. We refer to this range as pea gravel. We further show that bedload movement of a clast in the pea gravel range with, for example, a size of 4mm moving over a bed of 0.4mm particles has an enhanced relative mobility compared to a clast with a size of 40mm moving over a bed of the same 4mm particles. With this in mind, we use 2mm here as shorthand for the narrow pea gravel range of 1-5mm over which transport behavior is distinct from both coarser and finer material. The use of viscosity allows the delineation of a generalized dimensionless bed grain size discriminator between "sand-like"and "gravel-like"rivers. The discriminator is applicable to sediment transport on Titan (ice clasts in flowing methane/ethane liquid at reduced gravity) and Mars (mafic clasts in flowing water at reduced gravity), as well as Earth.