Concept reference

Self-Cleansing Velocity and Tractive Force

C.1Plain-language definition

“Self-cleansing” means the flow condition is intended to limit persistent deposition; it does not mean the pipe cleans every material at all times. Mean velocity is an accessible proxy. Boundary shear, or tractive force, more directly describes the drag available at the wall to move particles, but the critical shear depends on the solids and bed.

C.2Governing relation or decision logic

Mean velocity is V = Q/A in m/s (ft/s). For approximately uniform open-channel flow, boundary shear is τ = ρgRS in Pa, where hydraulic radius R is m and slope S is m/m. In a force main, review velocities at each pump combination and actual internal diameter.

C.3The failure to name

A universal copied velocity can be physically overconfident and locally noncompliant at once. Checking peak flow only misses deposition during long low-flow periods; increasing pump size to create occasional scour can worsen cycling or surge without fixing wet-well solids.

C.4Field review checklist

• Name the local criterion, its source and the flow/duration condition it addresses.
• Check minimum, normal and staged-pump velocities—not only design peak.
• For gravity flow, consider depth-dependent hydraulic radius and boundary shear.
• Pair hydraulic checks with flushing, pigging, wet-well cleaning and operating strategy where needed.

C.5Where it fits in the course

§1.3 separates sewage from potable constraints; §3.5 shows diameter trade-offs.