Sarda type fall—rectangular crest applicability: Up to what discharge is a rectangular crest commonly used before switching to a trapezoidal crest?

Introduction / Context:

Sarda falls use standardized crest shapes to pass and measure discharge in distributaries/minors. At higher discharges, a trapezoidal crest is preferred to limit afflux, maintain modularity, and ease passage of floating debris with acceptable velocities.

Given Data / Assumptions:

• Free over-fall conditions (modular flow) expected.
• Rectangular crest ratings calibrated up to a practical upper limit.
• Beyond that limit, a trapezoidal crest offers better hydraulic performance.

Concept / Approach:

Practice indicates rectangular crests are adopted for relatively small discharges; for larger flows, trapezoidal crests provide improved capacity with manageable heads and velocities. The commonly cited threshold is around 14 m³/s (14 cumecs) for switching crest type in Sarda layouts.

Step-by-Step Solution:

Verification / Alternative check:

Field practice and standard tables align with 14 cumecs as a practical demarcation, though local standards may fine-tune based on approach conditions and headroom.

Why Other Options Are Wrong:

• 6 or 10 cumecs: Too restrictive; rectangular crests function well above these flows.
• 20 or 25 cumecs: Typically high for rectangular crest without excessive head/afflux.

Common Pitfalls:

• Using a rectangular crest at high discharges causing submergence or excessive velocities.

Final Answer:

14 cumecs