Canal design practice: A minimum freeboard of 90 cm is provided for which discharge range in the canal? (Select the most appropriate typical range used in irrigation design handbooks.)

Introduction / Context:
Freeboard is the vertical clearance provided above the full supply level in canals to accommodate wind setup, wave action, superelevation at bends, and uncertain rises in water level. Typical values are standardized against discharge ranges for safe and economical canal sections.

Given Data / Assumptions:

• Asked: discharge range that commonly warrants a minimum freeboard of 90 cm.
• Context: irrigation canal design (earthen or lined) with standard handbook recommendations.
• Objective: recall/recognize typical tabulated practice rather than compute.

Concept / Approach:
As discharge increases, wave height and velocity head effects grow, so freeboard is stepped up in ranges. Many reference tables assign about 60 cm for small distributaries, about 90 cm for medium canals in the 30–60 m^3/s range, and 120 cm or more for larger canals.

Step-by-Step Solution:
Link discharge range to standard freeboard tables.Identify the range that typically maps to 0.90 m.Select 30 to 60 cumecs.

Verification / Alternative check:
Cross-check with state PWD/Water Resources Dept. manuals that list staged freeboards (e.g., 0.6 m; 0.9 m; 1.2 m+) across discharge bands; medium canals (30–60 cumecs) usually adopt 0.9 m.

Why Other Options Are Wrong:

• 30 to 33 cumecs: Narrow slice; handbooks group 30–60 cumecs together.
• Over 60 cumecs / Over 100 cumecs: Typically require ≥1.2 m, not just 0.9 m.
• None of these: Incorrect because 30–60 cumecs is a standard mapping to 0.9 m.

Common Pitfalls:
Assuming one value fits all canal classes; ignoring added allowances at bends or windy reaches; forgetting that lining type can slightly influence adopted freeboard.

Final Answer:
30 to 60 cumecs