Sharp-crested weirs: To ensure a clean, fully aerated nappe for accurate discharge measurement, the crest (plate) thickness is generally kept less than what fraction of the head over the sill?

Introduction / Context:

Sharp-crested (thin-plate) weirs require a thin knife-edge crest so that the nappe springs clear and aerates, matching standard discharge calibrations. Excessive crest thickness distorts the flow contraction and invalidates empirical coefficients.

Given Data / Assumptions:

• Sharp upstream edge maintained; crest not rounded.
• Free overfall with ventilated nappe below the sheet.
• Head over crest within typical laboratory calibration range.

Concept / Approach:

Guidelines recommend the crest thickness be small compared to the head on the sill (H). A widely accepted rule of thumb is t < H/3, which preserves the thin-plate behavior and minimizes interference with the contracted section at the vena contracta.

Step-by-Step Solution:

Verification / Alternative check:

Standard references (laboratory practice manuals) specify similar limits; many also recommend that upstream face be vertical and downstream edge beveled to sustain a clean separation.

Why Other Options Are Wrong:

• One-half or two-thirds of H are too thick and disrupt sharp-crested behavior.
• One-fourth of H is conservative but not the commonly cited threshold (question asks the general limit).
• “None of these” is incorrect because the H/3 guideline is standard.

Common Pitfalls:

• Allowing burrs or rounding at the crest, which alters discharge coefficient more than thickness alone.

Final Answer:

One-third of the head on the sill