--- title: "Pipe flow versus open channel flow" subject: "Hydraulics" chapter: 5 key: "semester_4/hydraulics:pipe-flow-vs-open-channel-flow" source: https://www.bctengineer.com/q/semester_4/hydraulics/pipe-flow-vs-open-channel-flow site: BCT Engineering --- # Pipe flow versus open channel flow Hydraulics · Chapter 5: Basics of Open channel flow - Asked in: 2079 Chaitra, 2 marks; 2076 Bhadra, 1 marks (asked 2 times) - Video: [Difference between Open channel Flow and Pipes Flow || Open channel flow || Hydraulics || Fluid mech](https://www.youtube.com/watch?v=hskfDh3vZuU) — Mukesh Kumar - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/hydraulics/pipe-flow-vs-open-channel-flow - Whole chapter: https://www.bctengineer.com/md/semester_4/hydraulics/chapter-5 ## Question Differentiate between pipe flow and open channel flow. ## Short answer Pipe flow runs full in a closed conduit, driven by pressure difference, with fixed area and may flow uphill. Open channel flow has a free surface at atmospheric pressure, driven by gravity, with variable area and depth, and flows only downhill. ## Answer **Pipe flow** takes place in a closed conduit running full, driven by pressure difference. **Open channel flow** has a free surface exposed to atmosphere, driven by gravity. | Basis | Pipe flow | Open channel flow | |---|---|---| | Section | Closed conduit, flows full | Free surface exposed to atmosphere | | Driving force | Pressure gradient (and gravity) | Gravity (slope of bed) | | Pressure at surface | Above or below atmospheric | Atmospheric (zero gauge) | | Hydraulic grade line | Piezometric head, may rise above pipe | Coincides with water surface | | Flow area | Fixed by pipe size | Changes with depth, so is a variable | | Cross-section | Almost always circular | Any shape (rectangular, trapezoidal, circular, natural) | | Roughness | Fairly constant, well known | Varies, difficult to estimate (e.g., river beds) | | Analysis | Simple, more accurate | More complex, empirical | | Flow direction | Can go uphill | Only downhill (by gravity) | | Examples | Water supply main, penstock | River, canal, partially full sewer, culvert | A pipe flowing partly full behaves as open channel flow. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What mainly drives flow in an open channel, compared with pipe flow? A. Pressure difference, same as pipe flow B. Gravity acting along the bed slope C. Pump head only D. Surface tension at the free surface Answer: B. Open channel flow has a free surface at atmospheric pressure, so it is driven by gravity along the bed slope; pipe flow is driven by a pressure gradient. (Key point: Driving force) 2. In an open channel, where is the hydraulic grade line? A. Above the water surface by the pressure head B. Below the bed C. At the water surface, where gauge pressure is zero D. At the channel centre line Answer: C. The surface is at atmospheric (zero gauge) pressure, so the HGL coincides with the water surface. (Key point: Surface pressure and HGL) 3. How does the flow area differ between a pipe flowing full and an open channel? A. Both are fixed by the boundary B. Pipe area is fixed by pipe size; channel area changes with depth C. Pipe area changes with depth; channel area is fixed D. Both vary with the pressure Answer: B. A full pipe has a fixed area, while the area of an open channel changes with the depth of flow. (Key point: Flow area) 4. A circular sewer flows partly full. How should it be analysed? A. As pipe flow, because it is circular B. As open channel flow, because it has a free surface C. As pipe flow with zero friction D. It cannot be analysed Answer: B. A pipe flowing partly full has a free surface at atmospheric pressure and behaves as open channel flow. 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