Grit Channel Velocity Calculator
Grit settles out while organics stay suspended only in a narrow velocity band. Check your channel velocity and detention time at current flow.
Grit Channel Check
| Item | Value |
|---|
How the calculation works
Formulas
Flow per channel (cfs) = total flow (cfs) ÷ channels in service; 1 MGD = 1.547 cfs
Cross-section (ft2) = width (ft) × depth (ft)
Horizontal velocity (ft/s) = flow per channel (cfs) ÷ cross-section (ft2)
Channel volume (ft3) = width × depth × length × channels
Detention time (min) = volume (gal) ÷ flow (gpm); 1 ft3 = 7.48 gal
Flow per channel (cfs) = total flow (cfs) ÷ channels in service; 1 MGD = 1.547 cfs
Cross-section (ft2) = width (ft) × depth (ft)
Horizontal velocity (ft/s) = flow per channel (cfs) ÷ cross-section (ft2)
Channel volume (ft3) = width × depth × length × channels
Detention time (min) = volume (gal) ÷ flow (gpm); 1 ft3 = 7.48 gal
| Velocity (ft/s) | What it means |
|---|---|
| Below 0.75 | Too slow — organics settle with the grit and putrefy |
| 0.75 – 1.4 | Target band — grit settles, organics stay suspended |
| Above 1.4 | Too fast — grit washes through to downstream processes |
Notes & limitations
The ~1 ft/s target and 0.75–1.4 ft/s band are standard design practice for horizontal-flow grit channels (Metcalf & Eddy; Ten State Standards). Proportional weirs or Parshall flumes hold velocity across the flow range.
- Check velocity at both average and peak flow — a channel that is fine at average flow can wash grit through at peak.
- Aerated grit chambers use a different mechanism (spiral roll, ~1–3 min detention) — this tool is for horizontal-flow channels.
- Depth changes with flow in a weir-controlled channel; enter the depth that matches the flow you are checking.
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