Dear Doc Hydro: It seems like there are many different ways to classify particle sizes. Is there a particle size classification standard that is widely accepted?

High particle size variability is a common trait of many natural rivers. The range of particle sizes in a river may span five orders of magnitude. Consequently, the properties of an individual particle are largely irrelevant, and the behavior and characteristics of groups of particles takes on increased significance. For this reason, it is both necessary and convenient to group sediments into different size classes.

As a result, numerous classification systems have developed. Most are essentially arbitrary and can be found in the engineering, geologic, and fisheries literature. Most are based on the metric system; however, a few early classification approaches used English units.

The table on the following page shows some of the most commonly used particle size classification systems and allows ready comparison among them. Note that there is a remarkable degree of similarity between the various systems especially with respect to size class breaks. Most differences are related to nomenclature. This can be a problem, especially when people use the same words but mean something different.

The most common classification system used in stream studies in the United States is that proposed by the American Geophysical Union (AGU) (Lane, 1947). While there is no official standard in the strict sense, the vast majority of professional practitioners use the AGU system.

The AGU system, which is an extension of the Wentworth scale, is widely used because size classes vary by doubling of the lower class (e.g., 2, 4, 8, 16, 32, etc.) and the sizes correspond closely to United States standard sieve mesh openings.

The actual size of particles may be based on:

(1) sieve diameter (the size of a square opening through which the particle will just pass),
(2) sedimentation diameter (based on the terminal settling velocity of a sphere), or
(3) the nominal diameter (the diameter of a sphere of the same volume as the given particle).
In practical application, the nominal diameter of large particles (generally greater than 2 mm) is estimated by measuring the intermediate axis (neither the longest nor shortest of three mutually perpendicular sides of a particle).

Particle dimensions are normally expressed in millimeters. However, in sedimentology literature, particle size (D) is often expressed in phi (f) units where they are derived from the equation:

f = -log2D (mm) = -3.3219 log10D(mm).

The minus sign was introduced so that sand sizes would have positive numbers. The phi index is useful because it normalizes particle size distributions so they can be analyzed using parametric statistics and plotted directly on arithmetic graph paper.

For detailed particle size classification, many users classify particles into 1/2 f unit classes which increase by the Ö2 (e.g., 2, 2.6, 4, 5.6, 8, 11.3, 16, 22.6, 32, etc.) to achieve added precision.

Doc Hydro encourages use of the AGU system for consistency among those studying stream channel characteristics and systems.

The following table was originally developed by staff of the USGS Hydrologic Laboratory, Denver, Colorado, and modified for this presentation.