Scientific Investigations Report 2008–5093
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2008–5093
Table 7. Critical shear stress by particle-size classification for determining approximate condition for sediment mobility at 20 degrees Celsius.
[Modified from Julien, 1998, table 7.1. Sediment mobility for a given particle size occurs when the bed shear stress exceeds the critical shear stress. This only determines whether or not a given particle size is mobile. Critical bed shear stress (τc) calculated from equation 4 using particle diameters from this table. Abbreviations: φ, phi scale where φ = -log2 (diameter in mm); mm, millimeter; N/m2, Newtons per square meter]
| Particle classification name |
Ranges of particle diameters |
Shields parameter (dimensionless) |
Critical bed shear stress (τc) (N/m2) | |
|---|---|---|---|---|
| φ | mm | |||
| Coarse cobble | -7 – -8 | 128 – 256 | 0.054 – 0.054 | 112 – 223 |
| Fine cobble | -6 – -7 | 64 – 128 | 0.052 – 0.054 | 53.8 – 112 |
| Very coarse gravel | -5 – -6 | 32 – 64 | 0.05 – 0.052 | 25.9 – 53.8 |
| Coarse gravel | -4 – -5 | 16 – 32 | 0.047 – 0.05 | 12.2 – 25.9 |
| Medium gravel | -3 – -4 | 8 – 16 | 0.044 – 0.047 | 5.7 – 12.2 |
| Fine gravel | -2 – -3 | 4 – 8 | 0.042 – 0.044 | 2.7 – 5.7 |
| Very fine gravel | -1 – -2 | 2 – 4 | 0.039 – 0.042 | 1.3 – 2.7 |
| Very coarse sand | 0 – -1 | 1 – 2 | 0.029 – 0.039 | 0.47 – 1.3 |
| Coarse sand | 1 – 0 | 0.5 – 1 | 0.033 – 0.029 | 0.27 – 0.47 |
| Medium sand | 2 – 1 | 0.25 – 0.5 | 0.048 – 0.033 | 0.194 – 0.27 |
| Fine sand | 3 – 2 | 0.125 – 0.25 | 0.072 – 0.048 | 0.145 – 0.194 |
| Very fine sand | 4 – 3 | 0.0625 – 0.125 | 0.109 – 0.072 | 0.110 – 0.145 |
| Coarse silt | 5 – 4 | 0.0310 – 0.0625 | 0.165 – 0.109 | 0.0826 – 0.110 |
| Medium silt | 6 – 5 | 0.0156 – 0.0310 | 0.25 – 0.165 | 0.0630 – 0.0826 |
| Fine silt | 7 – 6 | 0.0078 – 0.0156 | 0.3 – 0.25 | 0.0378 – 0.0630 |
U.S. Department of the Interior | U.S. Geological Survey
URL: http://pubs.usgs.gov/sir/2008/5093
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