Stokes Settling Velocity — Reference
Terminal settling velocity of discrete particles, and the surface-overflow-rate basis for sedimentation basins, forebays, and grit chambers.
Settling Velocity by Regime
| Regime | Particle Re | Applies to |
|---|---|---|
| Stokes (laminar) | < 1 | Silt, clay, fine sand |
| Transition | 1–1000 | Medium–coarse sand |
| Newton (turbulent) | > 1000 | Gravel, large grit |
Particle Re = ρ·vs·d/μ. Ss = ρs/ρ (quartz sand ≈ 2.65). Water at 20°C: μ = 1.00×10−3 Pa·s, ρ = 998 kg/m³.
Typical Settling Velocities (quartz, 20°C water)
| Particle | Diameter | vs (approx.) |
|---|---|---|
| Coarse sand | 1.0 mm | ~100 mm/s |
| Medium sand | 0.5 mm | ~50 mm/s |
| Fine sand | 0.1 mm | ~6–8 mm/s |
| Very fine sand | 0.05 mm | ~2 mm/s |
| Silt | 0.01 mm | ~0.08 mm/s |
| Clay | 0.001 mm | ~0.0008 mm/s |
Fine silt/clay settle far too slowly for practical basins — they require flocculation or filtration. The d² dependence is why removing the fine fraction is so hard.
Sources: Camp, T.R. (1946), "Sedimentation and the Design of Settling Tanks," Trans. ASCE. Metcalf & Eddy, Wastewater Engineering. Reynolds & Richards, Unit Operations and Processes in Environmental Engineering.
Related cheat sheets and tools
Use vs with the settling velocity tool and check detention time for the basin. Protect the basin inlet/outlet with riprap. For full RUSLE sediment load and BMP/forebay sizing across a watershed, see HydroComplete.