Sewer hydraulics — definition of self-cleansing velocity in sanitary/storm drains

Introduction / Context:
Self-cleansing velocity is a key design parameter for sewers: it is the minimum flow velocity required to prevent deposition of suspended solids and grit so that the system remains largely free of silt under normal operation. This protects capacity and reduces maintenance costs.

Given Data / Assumptions:

• Gravity flow in sanitary or storm sewers.
• Typical sediment sizes expected from domestic/urban sources.
• Designers select slopes to meet self-cleansing at a specified fraction of design flow (often near DWF).

Concept / Approach:

The critical tractive stress on the pipe invert must exceed the resisting weight of settled particles for re-entrainment. Expressed simply, if the mean velocity exceeds a threshold (commonly ~0.6–1.0 m/s depending on practice and pipe), the sewer is considered self-cleansing because settled solids are scoured and do not accumulate.

Step-by-Step Solution:

Verification / Alternative check (if short method exists):

Operational data: systems designed for V_min at DWF show fewer blockages than those that only meet criteria at peak flow.

Why Other Options Are Wrong:

“Velocity at dry weather flow” is not necessarily self-cleansing; “flushing” velocity is a maintenance operation, not a design criterion; “pressure filter” is unrelated.

Common Pitfalls (misconceptions, mistakes):

Confusing peak-flow velocity with self-cleansing; ignoring sediment characteristics when selecting V_min.

Final Answer:

velocity at which no accumulation remains in the drains