---
title: "Rigid versus mobile boundary design principle"
subject: "Hydraulics"
chapter: 10
key: "semester_4/hydraulics:rigid-vs-mobile-boundary-design-principle"
source: https://www.bctengineer.com/q/semester_4/hydraulics/rigid-vs-mobile-boundary-design-principle
site: BCT Engineering
---

# Rigid versus mobile boundary design principle

Hydraulics · Chapter 10: Flow in mobile boundary channel

- Asked in: 2077 Chaitra, 1 marks; 2075 Baisakh, 3 marks; 2073 Magh, 3 marks (asked 3 times)
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/hydraulics/rigid-vs-mobile-boundary-design-principle
- Whole chapter: https://www.bctengineer.com/md/semester_4/hydraulics/chapter-10

## Question

With respect to design principle, distinguish between rigid boundary and mobile boundary channels.

## Short answer

Rigid boundary design chooses an efficient section to carry discharge without erosion limits, only checking a minimum velocity to avoid silting. Mobile boundary design must also ensure boundary stability by keeping flow shear or velocity below critical values for the erodible material; channel shape and slope are governed by sediment properties.

## Answer

The two types of channels are designed on different principles.

| Basis | Rigid boundary channel | Mobile boundary channel |
|---|---|---|
| Boundary | non-erodible: concrete, masonry, rock | erodible: sand, gravel, silt (alluvium) |
| Main design aim | carry the required discharge with an efficient, economical section | carry the discharge **and** keep the boundary stable (no scour or silt) |
| Governing principle | continuity and a flow resistance equation (Manning, Chezy) with no limit on boundary erosion; only a **minimum velocity** (to avoid silting and weeds) is checked | the boundary is stable only if the flow stays below a threshold: **maximum permissible velocity** or **critical tractive force** (shear stress) of the bed material |
| Channel shape | best hydraulic section (minimum area for a given $Q$), side slope from construction needs | cross-section and side slope fixed by the soil (angle of repose, $K$) and stability |
| Slope | from topography and the minimum velocity | limited by the permissible velocity or shear stress |
| Sediment | not considered (clear water) | sediment transport, scour, deposition and bed forms matter; regime theory may be used |
| Boundary shape in time | unchanged | adjusts with time |

In short, rigid boundary design is a **hydraulic-capacity** problem; mobile boundary design is a **capacity plus stability** problem.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is the main design check on boundary velocity in a rigid boundary channel?
   A. A maximum shear stress from Shields
   B. Lacey silt factor
   C. A minimum velocity to avoid silting and weeds, with sizing by Manning or Chezy
   D. Critical Reynolds number
   Answer: C. Rigid channels are a hydraulic-capacity problem, checked only for a minimum velocity. (Key point: Rigid design aim)

2. What governs the design of a mobile boundary channel?
   A. Choosing the best hydraulic section only
   B. Using the steepest possible slope
   C. Ignoring the sediment
   D. Keeping flow below maximum permissible velocity or critical tractive force of the bed material
   Answer: D. The boundary is stable only if velocity or shear stays below the threshold of the material, so stability is added to capacity. (Key point: Mobile design principle)

3. Which statement correctly contrasts the two?
   A. Mobile channel section and side slope are fixed by soil stability (angle of repose, K) and sediment transport matters; rigid channel uses best hydraulic section and clear water
   B. Rigid channels are governed by the angle of repose
   C. Mobile channels are always clear-water designs
   D. Both use identical principles
   Answer: A. Rigid boundaries are unchanged and considered clear water; mobile boundaries adjust and involve sediment and stability. (Key point: Shape and sediment)

## Related in Hydraulics

- [Mobile boundary design: three approaches](https://www.bctengineer.com/question.md?key=semester_4%2Fhydraulics%3Adesign-three-methods-mobile-boundary) — Hydraulics, chapter 10
- [Pressure rise: rigid versus elastic pipe](https://www.bctengineer.com/question.md?key=semester_4%2Fhydraulics%3Arigid-vs-elastic-pipe-pressure-rise-5km) — Hydraulics, chapter 4
- [Design by permissible velocity approach](https://www.bctengineer.com/question.md?key=semester_4%2Fhydraulics%3Amax-permissible-velocity-design-procedure) — Hydraulics, chapter 10, asked 3×

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