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Rigid versus mobile boundary design principle
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.
With respect to design principle, distinguish between rigid boundary and mobile boundary channels.
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 ), side slope from construction needs | cross-section and side slope fixed by the soil (angle of repose, ) 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.
✓ Answer checked by Claude Opus 5.5 (high effort)
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