Sharp-crested (narrow-crested) weir – thumb rule for thickness In a sharp-crested weir, the thickness of the crest is kept less than half of the head of water above the crest. State whether this design guideline is correct:

Introduction:
Weir discharge equations assume a sharp, effectively thin crest so the nappe springs clear and the flow approximates an orifice sheet. The thickness guideline helps maintain sharp-crested behaviour.

Given Data / Assumptions:

• Free overfall with aerated nappe.
• Head H measured upstream above the crest elevation.
• Negligible scale effects for standard laboratory/field conditions.

Concept / Approach:
Keeping crest thickness less than about H/2 limits streamline curvature within the crest and avoids broad-crested behaviour, which would alter coefficients and calibration.

Step-by-Step Solution:
1) Begin with objective: produce a sharp overfall with minimal crest interference.2) Use the empirical rule t < H/2 to classify narrow (sharp) crests.3) This ensures discharge coefficient correlations for sharp-crested weirs remain valid.

Verification / Alternative check:
Laboratory calibrations show that as t/H increases, coefficients shift toward broad-crested behaviour, increasing sensitivity to approach velocity and crest geometry.

Why Other Options Are Wrong:

• False or restrictive cases: standard rectangular and triangular sharp crests follow this guidance.
• Very thick crests behave differently and need different formulae.

Common Pitfalls:
Ignoring aeration under the nappe; air-cushion failure changes calibration even with a thin crest.

Final Answer:
True