Lined canal section selection: Which combination best represents commonly recommended sections for small and large lined canals?

Introduction / Context:
Lining reduces seepage and stabilizes canal sections. Standard practice suggests different economical shapes for different capacities while keeping constructability in mind.

Given Data / Assumptions:

• Small canals: low discharge, narrow beds.
• Large canals: higher discharge requiring stable, hydraulically efficient shapes.
• Rounded corners reduce stress concentration and erosion.

Concept / Approach:
For small lined canals, a triangular section with a circular invert is easy to construct and economical. For larger canals, a trapezoidal section with rounded corners combines hydraulic efficiency with structural stability and ease of formwork.

Step-by-Step Solution:
Identify recommended small-canal section → triangular with circular bottom.Identify recommended large-canal section → trapezoidal with rounded corners.Therefore, the correct combined choice is options (a) and (b).

Verification / Alternative check:
Design handbooks list popular linings (concrete, brick, shotcrete) applied to these shapes due to stability and reduced eddies at corners.

Why Other Options Are Wrong:

• Rectangular with rounded corners is sometimes used but is less common for very large canals due to structural and freeboard demands.
• Single options do not capture the small/large pairing in the question.

Common Pitfalls:
Assuming a single “best” shape for all capacities; neglecting corner rounding which improves durability.

Final Answer:
both (a) and (b)