What is the Hazen-Williams C coefficient used for in water distribution pipelines?

• A. It is the friction factor in Darcy-Weisbach.
• B. It is an empirical roughness factor in Hazen-Williams equation; typical range 100-150.
• C. It is the discharge coefficient for nozzles.
• D. It is the exponent in Manning's equation.

Answer: (B)

The Hazen-Williams C coefficient is an empirical roughness factor used in the Hazen-Williams equation to estimate flow in water distribution pipes. It’s a dimensionless value that captures how pipe material, surface condition, and cleanliness affect friction losses; higher values mean smoother pipes with less friction, while lower values indicate rougher or aged pipes with more head loss for the same flow.

Typical values for clean, well‑constructed distribution pipes fall around 100 to 150. It’s not the friction factor from Darcy-Weisbach (which depends on Reynolds number and relative roughness), not a nozzle discharge coefficient, and not the exponent in Manning’s equation (Manning uses the roughness coefficient n). The Hazen-Williams constant is used specifically with the Hazen-Williams formula to describe how roughness influences flow in pipes.