Sediment particle size is represented by nominal, sieve, or fall diameter due to different measurement techniques used. The nominal diameter is often measured for boulders, cobbles, and coarse gravels. The sieve diameter is measured by sieving for fine gravel and sand particles. The fall diameter is measured by settling for silt and clay. The sieve diameter is about 0.9 times the nominal diameter for naturally worn sediment particles in the range of about 0.2 to 20 mm (U.S. Interagency Committee, 1957), and the fall diameter is about 0.85 times the nominal diameter for naturally worn sediment particles finer than 0.1 mm (Wu, 2024). The formulas of Zhang (1961), Cheng (1997), and Wu and Wang (2006) use nominal diameter for all sediment size ranges from clay to boulder. However, different representative diameters are used in several other existing formulas. For example, the curves of the U.S. Interagency Committee (1957) and the formulas of Zanke (1977), Soulsby (1997), and van Rijn (1984) approach the Stokes law of spheres in the laminar settling regime; thus, fall diameter should be used the representative diameter when these methods are used for clay and silt particles. Nominal diameter should be used in these methods for coarse particles, whereas sieve diameter should be used in the formulas of Zanke (1977), Soulsby (1997), and van Rijn (1984) for sand particles. In addition, the formula of Ahrens (2000) is based on sieve diameter. The author reviewed this inconsistency and suggested how to choose representative diameters for different formulas of sediment settling velocity. Details can be found in the book “Sediment Transport Dynamics” newly published by Wu (2024).
