The phrase 'closed conduit' flow refers to flows in pipes under pressure. Which term best describes this concept?

• A. Open channel
• B. Closed conduit
• C. Free-flow
• D. Subsurface

Answer: (B)

In wastewater hydraulics, flows inside pipes are categorized by whether the conduit is fully filled and pressurized. This is exactly what the term describes: a sealed, full pipe carrying flow under pressure. Open-channel flow, in contrast, has a free surface and is not pressurized, like water in a ditch or river. The other options don’t specify the hydraulic regime: free-flow isn’t a precise, standard label for pipe systems, and subsurface refers to location rather than whether the flow is in a closed, pressurized conduit. Therefore, closed conduit is the best description for flows in pipes under pressure.