Concrete-lined canals: The approximate permissible mean velocity of water in a concrete canal should not exceed which value for general design practice?

Introduction / Context:Concrete lining increases permissible non-erosive velocities in canals by providing a smooth, erosion-resistant boundary. Selecting a reasonable upper bound for design velocity is important to balance economy (smaller section) against risks of cavitation, joint distress, or local scour at transitions.

Given Data / Assumptions:

• Well-constructed concrete lining with proper joints and bedding.
• Normal sediment load and no sharp hydraulic transitions.
• Mean (section-averaged) velocity for preliminary design.

Concept / Approach:Empirical guidance suggests that concrete-lined canals can safely carry higher velocities than earth sections. Typical upper-bound design values are around 2.5 m/sec (sometimes up to ~3 m/sec for special cases). A value of 2.5 m/sec serves as a conservative ceiling for many irrigation canals.

Step-by-Step Solution:1) Identify material: concrete lining allows higher permissible velocities.2) Compare with standard ranges: earth (0.6–1.0 m/sec), masonry (up to ~2.0 m/sec), concrete (up to ~2.5 m/sec or more).3) Select the standard approximate upper value: 2.5 m/sec.

Verification / Alternative check:Project specifications often cap velocities near transitions and structures. Many canal design manuals treat 2.5 m/sec as a practical limit to avoid maintenance problems while enabling economical cross-sections.

Why Other Options Are Wrong:

• 0.5–1.5 m/sec: overly conservative for concrete lining, closer to earth channels.
• 2.0 m/sec: acceptable in many cases, but commonly not the upper bound.

Common Pitfalls:

• Ignoring localized velocities at bends and structures which can exceed mean values.
• Overlooking cavitation risks at high velocities in undersluices or steep drops.

Final Answer:2.5 m/sec.